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SDG 7

Insect rate-temperature relationships: Environmental variation and the metabolic theory of ecology
15 December 2009
Stellenbosch University

Irlich, U.M.;Terblanche, J.S.;Blackburn, T.M.;Chown, S.L.

Much of the recent discussion concerning the form and underlying mechanistic basis of metabolic rate–temperature and development rate–temperature relationships has been recipitated by the development of the metabolic theory of ecology (MTE). Empirical tests of the theory’s fundamental equation are an essential component of establishing its validity. Here, we test the temperature component of the fundamental equation of the MTE as it applies to metabolic rate and development rate, using insects as model organisms. Specifically, we test (i) whether mean activation energies, E, approximate the 0.65 eV value proposed by the proponents of the MTE and whether the range of values is tightly constrained between 0.6 and 0.7 eV, as they have argued; (ii) whether phylogenetic signal is apparent in the rate-temperature relationships; (iii) whether the slopes of the rate-temperature relationships show consistent, directional variation associated with environmental variables; and (iv) whether intra- and interspecific rate-temperature relationships differ significantly. Because the majority of activation energy values fell outside the predicted range and rate-temperature relationships showed consistent directional variation correlated with large-scale climatic variation, we conclude that data from insects provide only limited support for the MTE. In consequence, we consider alternative explanations for variation in rate-temperature relationships.
Biofuels and biodiversity in South Africa
16 August 2012
Stellenbosch University

Blanchard, R.;Richardson, D.M.;O'Farrell, P.J.;van Malititz, G.P.

The South African government, as part of its efforts to mitigate the effects of the ongoing energy crisis, has proposed that biofuels should form an important part of the country’s energy supply. The contribution of liquid biofuels to the national fuel supply is expected to be at least 2% by 2013. The Biofuels Industrial Strategy of the Republic of South Africa of 2007 outlines key incentives for reaching this target and promoting the development of a sustainable biofuels industry. This paper discusses issues relating to this strategy as well as key drivers in biofuel processing with reference to potential impacts on South Africa’s rich biological heritage. Our understanding of many of the broader aspects of biofuels needs to be enhanced. We identify key areas where challenges exist, such as the link between technology, conversion processes and feedstock selection. The available and proposed processing technologies have important implications for land use and the use of different non-native plant species as desired feedstocks. South Africa has a long history of planting non-native plant species for commercial purposes, notably for commercial forestry. Valuable lessons can be drawn from this experience on mitigation against potential impacts by considering plausible scenarios and the appropriate management framework and policies. We conceptualise key issues embodied in the biofuels strategy, adapting a framework developed for assessing and quantifying impacts of invasive alien species. In so doing, we provide guidelines for minimising the potential impacts of biofuel projects on biodiversity.
Rate dynamics of ectotherm responses to thermal stress
05 June 2019
Stellenbosch University

Kovacevic, A.;Latombe, G.;Chown, S.L.

Critical thermal limits (CTLs) show much variation associated with the experimental rate of temperature change used in their estimation. Understanding the full range of variation in rate effects on CTLs and their underlying basis is thus essential if methodological noise is not to overwhelm or bias the ecological signal. We consider the effects of rate variation from multiple intraspecific assessments and provide a comprehensive empirical analysis of the rate effects on both the critical thermal maximum (CTmax) and critical thermal minimum (CTmin) for 47 species of ectotherms, exploring which of the available theoretical models best explains this variation. We find substantial interspecific variation in rate effects, which takes four different forms (increase, decline, no change, mixed), with phylogenetic signal in effects on CTmax, but not CTmin. Exponential and zero exponential failure rate models best explain the rate effects on CTmax. The majority of the empirical rate variation in CTmin could not be explained by the failure rate models. Our work demonstrates that rate effects cannot be ignored in comparative analyses, and suggests that incorporation of the failure rate models into such analyses is a useful further avenue for exploration of the fundamental basis and implications of such variation.
Thermal landscape change as a driver of ectotherm responses to plant invasions
19 July 2019
Stellenbosch University

Garcia, R.A.;Clusella-Trullas, S.

A growing body of research demonstrates the impacts of invasive alien plants on native animals, but few studies consider thermal effects as a driver of the responses of native organisms. As invasive alien plants establish and alter the composition and arrangement of plant communities, the thermal landscapes available to ectotherms also change. Our study reviews the research undertaken to date on the thermal effects of alien plant invasions on native reptiles, amphibians, insects and arachnids. The 37 studies published between 1970 and early 2019 portray an overall detrimental effect of invasive plants on thermal landscapes, ectothermic individuals' performance and species abundance, diversity and composition. With a case study of a lizard species, we illustrate the use of thermal ecology tools in plant invasion research and test the generality of alien plant effects: changes in thermoregulation behaviour in invaded landscapes varied depending on the level of invasion and lizard traits. Together, the literature review and case study show that thermal effects of alien plants on ectotherms can be substantial albeit context-dependent. Further research should cover multiple combinations of native/invasive plant growth forms, invasion stages and ectotherm traits. More attention is also needed to test causality along the chain of effects from thermal landscapes to individuals, populations and communities.
Measuring continuous compositional change using decline and decay in zeta diversity
13 February 2020
Stellenbosch University

McGeoch, M.A.;Latombe, G.;Andrew, N.R.;Nakagawa, S.;Nipperess, D.A.;Roigé, M.;Marzinelli, E.M.;Campbell, A.H.;Vergés, A.;Thomas, T.;Steinberg, P.D.;Selwood, K.E.;Henriksen, M.V.;Hui, C.

Incidence, or compositional, matrices are generated for a broad range of research applications in biology. Zeta diversity provides a common currency and conceptual framework that links incidence-based metrics with multiple patterns of interest in biology, ecology and biodiversity science. It quantifies the variation in species (or OTU) composition of multiple assemblages (or cases) in space or time, to capture the contribution of the full suite of narrow, intermediate and wide-ranging species to biotic heterogeneity. Here we provide a conceptual framework for the application and interpretation of patterns of continuous change in compositional diversity using zeta diversity. This includes consideration of the survey design context, and the multiple ways in which zeta diversity decline and decay can be used to examine and test turnover in the identity of elements across space and time. We introduce the zeta ratio-based retention rate curve to quantify rates of compositional change. We illustrate these applications using 11 empirical datasets from a broad range of taxa, scales and levels of biological organisation – from DNA molecules and microbes to communities and interaction networks – including one of the original data sets used to express compositional change and distance decay in ecology. We show (i) how different sample selection schemes used during the calculation of compositional change are appropriate for different data types and questions, (ii) how higher orders of zeta may in some cases better detect shifts and transitions, and (iii) the relative roles of rare versus common species in driving patterns of compositional change. By exploring the application of zeta diversity decline and decay, including the retention rate, across this broad range of contexts, we demonstrate its application for understanding continuous turnover in biological systems.
Cosmic ray flux at the earth in a variable heliosphere
18 August 2009
North West University

Scherer, K.;Ferreira, S.E.S.;Potgieter, M.S.;Fichtner, H.;Heber, B.

In recent years the variability of the cosmic ray flux has become one of the main issues not only for the interpretation of the abundances of cosmogenic isotopes in cosmochronic archives like, e.g., ice cores, but also for its potential impact on the terrestrial climate. It has been re-emphasized that the cosmic ray flux is not only varying due to the solar activity-induced changes of the solar wind but also in response to the changing state of the interstellar medium surrounding the heliosphere. We demonstrate the significance of these external boundary condition changes along the galactic orbit of the Sun for the flux as well as spectra of cosmic rays. Such interstellar–terrestrial relations are a major topic of the International Heliophysical Year 2007
The role of radial perpendicular diffusion and latitude dependent acceleration along the solar
20 August 2009
North West University

Langner, U.W.;Potgieter, M.S.

A numerical model, based on Parker’s transport equation, describing the modulation of anomalous cosmic rays and containing diffusive shock acceleration is applied. The role of radial perpendicular diffusion at the solar wind termination shock, and as the dominant diffusion coefficient in the outer heliosphere, is studied, in particular the role it plays in the effectiveness of the acceleration of anomalous protons and helium when its latitude dependence is changed. It is found that the latitudinal enhancement of radial perpendicular diffusion towards the heliospheric poles and along the termination shock has a prominent effect on the acceleration of these particles. It results in a ‘break’ in the energy spectrum for anomalous protons at ∼6.0 MeV, causing the spectral index to change from E−1.38 to E−2.23, but for anomalous helium at ∼3.0 MeV, changing the spectral index from E−1.38 to E−2.30. When approaching the simulated TS, the changes in the modulated spectra as they unfold to a ‘steady’ power law shape at energies below 50 MeV are much less prominent as a function of radial distances when radial perpendicular diffusion is increased with heliolatitude
Evaluation of a detailed radiation heat tranfer model in a high temperature reactor systems simulation model
31 August 2009
North West University

Van Antwerpen, H.J.;Greyvenstein, G.P.

Radiation heat transfer is a major mode of heat transfer in high temperature gas-cooled reactors (HTRs) because of the high operating temperatures. It is, however, a difficult phenomenon to calculate in full detail due to its geometrical complexity. One has to use either a numerical method or complex analytical view factor formulae. Except the difficulty of view factor calculation, a vast number of calculation elements are required to consider all interacting surfaces around a cavity. A common approximation in systems simulation codes is to connect only directly opposing surfaces with a view factor of one. The accuracy of this approximation was investigated with a finite volume, two-dimensional axial-symmetric reactor model implemented in the systems simulation code Flownex. A detailed radiation model was developed and also implemented in the Flownex reactor model. This paper also describes the analytical formulae for view factor calculation in this detailed radiation heat transfer model. The HTR-10 and the 268 MW version of the PBMR were used as case studies in which Loss-of-Flow events without SCRAM were simulated. In these simulations, the time to reach recriticality was used as an indicator of heat removal effectiveness. With the HTR-10, other non-linear phenomena in the reactor core constrained the solution process, so that the number of radiation elements had no effect on solution time, while with the 268 MW PBMR DLOFC, the use of a detailed radiation model increased solution time with 30%. With both the HTR-10 and the PBMR, the radiation model had negligible effect on the total heat resistance from the reactor, as indicated by the time elapsed until recriticality. For system simulation codes that focus on transient response of a plant, it is not considered worthwhile to use a detailed radiation model, as the gain in accuracy does not justify the increased solution time or the implementation and verification effort
Developing esco procedures for large telecommunication facilities using novel simulation techniques
17 November 2009
North West University

Van Rensburg, J.F.;Geyser, M.F.;Kleingeld, M.;Mathews, E.H.

Peak electricity demand in South Africa will exceed the available operational generation capacity in 2007. The state utility, Eskom, is addressing this challenge, inter alia, with the implementation of a national Demand-side Management (DSM) initiative. Studies in South Africa have shown that 20% of the total municipal energy is utilised in commercial buildings. Telecommunication companies own and operate a large portfolio of diverse buildings within the municipal boundaries. Energy Services Company (ESCO) analyses on these buildings showed huge savings as well as load reduction opportunities. ESCOs however face major problems in evaluating DSM projects on telecommunication facilities. To address these problems a new ESCO procedure for telecommunications facilities was developed and successfully implemented. It was proven that the new ESCO procedure is successful in solving the unique problems in performing ESCO analyses for telecommunications facilities
Quantification of oxygen capture in mineral matter during gasification
17 November 2009
North West University

Van Dyk, J.C.;Waanders, F.B.;Van Heerden, J.H.P.

It has been observed that during the transformation of minerals at higher temperatures (>1000 °C), mineral species are formed containing a high number of oxygen molecules, i.e. gehlenite (Ca2Al2SiO7), mullite (Al6Si3O15), margarite (CaAl4Si2O10(OH)2) and almandine (Fe3Al2Si3O12). Results of the coal sources evaluated in this investigation indicated significant differences in mineral elemental composition, i.e. the CaO content varied between 5 mass % and 10 mass %, the Fe2O3 content varied between 1.6 mass % to more than 5 mass %, as well as differences in the TiO2, P2O5 and MgO content. The coal sources producing the highest concentration of Ca–Al–Si species (CaAl2Si2O8 anorthite and CaAl4Si2O10(OH)2 margarite), which crystallized from the slag-liquid phase during the combustion stage, also contained the highest amount of acidic components or highest percentage of kaolinite. The highest concentration of mullite and free SiO2 after the gasification reaction (before the combustion zone), also resulted in the highest concentration of Ca–Al–Si compounds forming during the oxidation phase. The free-SiO2 in the mineral structure of the coal sources resulted then in the formation of mineral structures with Mg, Na or Ca when present in the mineral structure, to form new mineral compounds such as KAl3Si3O10(OH)2 (muscovite), Mg5Al2Si3O10(OH)8 (clinochlore), or other high oxygen molecule-containing mineral compounds. Thus, if free-SiO2 was not present after the gasification phase, and mostly taken up in the form of anorthite (due to high or higher CaO contents or Fe-contents in high Fe-containing coal sources), the concentration of Si-oxygen capture compounds are relatively low. An acceptable linear correlation between oxygen capture tendencies (increase in mineral matter content during the combustion phase) versus CaO-content was obtained with the South African coal sources evaluated. This confirmed the observations obtained based on HT-XRD and FactSage modelling. It can be concluded that the linear model to predict oxygen capture behavior from CaO-content is acceptable and can be used as a predictive tool. The SiO2 content, for example, has an inverse affect on oxygen trends up to a specific concentration of CaO in the coal. However, this model is only valid for the coal types tested (South African Highveld coal sources), and additional test work will have to be conducted for other coal types, i.e. northern hemisphere coal
The application of system CFD to the design and optimization of high-temperature gas-cooled nuclear power plants
18 November 2009
North West University

Greyvenstein, Gideon P.

The objective of this paper is to model the steady-state and dynamic operation of a pebble-bed-type high temperature gas-cooled reactor power plant using a system computational fluid dynamics (CFD) approach. System CFD codes are 1D network codes with embedded 2D or even 3D discretized component models that provide a good balance between accuracy and speed. In the method presented in this paper, valves, orifices, compressors, and turbines are modeled as lumped or 0D components, whereas pipes and heat exchangers are modeled as 1D discretized components. The reactor is modeled as 2D discretized system. A point kinetics neutronic model will predict the heat release in the reactor. Firstly, the layout of the power conversion system is discussed together with the major plant parameters. This is followed by a high level description of the system CFD approach together with a description of the various component models. The code is used to model the steady-state operation of the system. The results are verified by comparing them with detailed cycle analysis calculations performed with another code. The model is then used to predict the net power delivered to the shaft over a wide range of speeds from zero to full speed. This information is used to specify parameters for a proportional-integral-derivative controller that senses the speed of the power turbine and adjusts the generator power during the startup of the plant. The generator initially acts as a motor that drives the shaft and then changes over to a generator load that approaches the design point value as the speed of the shaft approaches the design speed. A full startup simulation is done to demonstrate the behavior of the plant during startup. This example demonstrates the application of a system CFD code to test control strategies. A load rejection example is considered where the generator load is suddenly dropped to zero from a full load condition. A controller senses the speed of the low pressure compressor/low pressure turbine shaft and then adjusts the opening of a bypass valve to keep the speed of the shaft constant at 60rps⁠. The example demonstrates how detailed information on critical parameters such as turbine and reactor inlet temperatures, maximum fuel temperature, and compressor surge margin can be obtained during operating transients. System CFD codes is a powerful design tool that is indispensable in the design of complex power systems such as gas-cooled nuclear power plants
A kinetic expression for the pyrolytic decomposition of polyterafluoroethylene
25 November 2009
North West University

Van der Walt, Izak J.;Neomagus, Hein W.J.P.;Bruinsma, O.S.L.;Nel, Johann T.;Crouse, Philippus L.

Despite the fact that the thermal decomposition of polytetrafluoroethylene has been extensively studied over the past six decades, some inconsistencies regarding the kinetic parameters, e.g. the order of the reaction, remain. Representative kinetic data are essential for practical purposes such as reactor design and scaling. In general the literature data refer to homogeneous bulk heating, whereas the case of the non-homogeneous heating of a single particle has not received attention. Data (reaction rate and pre-exponential factor) applicable to this latter case were experimentally determined from isothermal thermogravimetric analyses of the depolymerisation reaction of PTFE. The kinetic data obtained on coarse granules (800–1000 μm) are reported here. The rate law is consistent with a shrinking particle kinetic model, with chemical kinetics controlling phase-boundary movement. The mass loss rate is directly proportional to surface area. A rate law applicable to this case, and useable for geometries of arbitrary shape, is derived
A calibration neutron monitor: energy response and instrumental temperature sensitivity
25 November 2009
North West University

Krüger, H.;Moraal, H.;Bieber, J.W.;Clem, J.M.;Evenson, P.A.

Neutron monitors are integral detectors of secondary cosmic rays. Since each of them has its own detection efficiency, energy spectra cannot readily be derived from their observations. To circumvent this problem, latitudinal surveys have been conducted for many years with mobile neutron monitors to derive such spectra. Another way, however, is to use the worldwide stationary neutron monitor network, but then the counting rates of these monitors must be normalized sufficiently accurately against one another. For this reason, two portable calibration neutron monitors were built at the Potchefstroom campus of the North-West University, South Africa. When calibrations of an adequate number of the worldwide neutron monitors have been done, this paper demonstrates that the differential response functions derived from them will provide experimental data for modulation studies in the rigidity range of 1 to 15 GV. Several properties of these calibrators were investigated, in order to achieve sufficient calibration accuracy. The energy response over the cutoff rigidity range from the poles to the equator, as well as the instrumental temperature sensitivity, are described in this paper. The main conclusion is that the calibrator has a difference of almost 4% in its energy response over the cutoff rigidity range 0–16 GV in comparison with a standard 3NM64 neutron monitor. Furthermore, it is shown that not only the calibrator, but also the NM64 and IGY monitors, have fairly large instrumental temperature sensitivities. Correction coefficients for these effects are given
Investigation of the multiple-component structure of the 20 January 2005 cosmic ray ground level enhancement
25 November 2009
North West University

McCracken, K.G.;Moraal, H.;Stoker, P.H.

Worldwide observations of the cosmic ray ground level enhancement (GLE) of 20 January 2005 are used to investigate a commonly observed but poorly understood feature of this class of event. It is argued that the GLE comprised two distinctly different cosmic ray populations. The first resulted in an impulsive, highly anisotropic, field-aligned pulse with a relatively hard rigidity spectrum and significant velocity dispersion. The characteristics of the anisotropy were almost identical to those for similar impulsive increases observed during GLEs in 1960, 1978, and 1989. The p 0 g ray observations from the RHESSI and CORONAS-F spacecraft and Type III radio emissions yield a path length of 1.76 ± 0.1 AU to Earth for the first pulse. After the highest energies in the initial anisotropic pulse had passed Earth, another field-aligned but mildly anisotropic cosmic ray pulse developed slowly worldwide, exhibiting the characteristics of the conventional GLE. The risetime and anisotropy of this second population indicate substantial scattering, apparently at variance to the essentially scatter-free nature of the initial pulse. We show that the coexisting scatter-free initial impulsive increase and the diffusive character of the second pulse are consistent with the standard quasi-linear theory of pitch angle diffusion. Throughout the GLE, the anisotropy remained field-aligned, and a third maximum, seen by some stations, is shown to be due to changes in the direction of the heliospheric magnetic field (HMF). Examination of 22 large (>20%) GLEs in the historical record shows that the impulsive pulse never occurs after the commencement of the P2 pulse, indicating that the impulsive-gradual combination is not due to a chance sampling of differing scattering regions of the HMF. It is further shown that impulsive pulses, or their equivalents, have been observed in 13 out of the 15 GLEs associated with solar activity in the solar longitude range 24° –98°W, leading us to propose that the event of 20 January 2005 should be regarded as the defining example of the GLE. The observations lead us to propose two separate acceleration episodes in the typical GLE: (1) acceleration directly associated with the flare itself and located in the lower corona and (2) acceleration by a supercritical shock driven by the associated coronal mass ejection, located at 3–5 solar radii and farther in the upper corona. A one-to-one association with so-called impulsive and gradual solar energetic particle events at lower energies is proposed. On the basis of these observations, a generic model for the GLE is proposed
Discovery of a VHE gamma-ray source coincident with the supernova remnant CTB 37A
17 December 2009
North West University

Aharonian, F.;Büsching, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Aims. The supernova remnant (SNR) complex CTB 37 is an interesting candidate for observations with very high energy (VHE) γ-ray telescopes such as HESS. In this region, three SNRs are seen. One of them is potentially associated with several molecular clouds, a circumstance that can be used to probe the acceleration of hadronic cosmic rays. Methods. This region was observed with the HESS Cherenkov telescopes and the data were analyzed with standard HESS procedures. Recent X-ray observations with Chandra and XMM-Newton were used to search for X-ray counterparts. Results. The discovery of a new VHE γ-ray source HESS J1714-385 coincident with the remnant CTB 37A is reported. The energy spectrum is well described by a power-law with a photon index of Γ = 2.30 ± 0.13 and a differential flux at 1 TeV of cm-2 s-1 TeV-1. The integrated flux above 1 TeV is equivalent to 3% of the flux of the Crab nebula above the same energy. This VHE γ-ray source is a counterpart candidate for the unidentified EGRET source 3EG J1714-3857. The observed VHE emission is consistent with the molecular gas distribution around CTB 37A; a close match is expected in a hadronic scenario for γ-ray production. The X-ray observations reveal the presence of thermal X-rays from the NE part of the SNR. In the NW part of the remnant, an extended non-thermal X-ray source, CXOU J171419.8-383023, is discovered as well. Possible connections of the X-ray emission to the newly found VHE source are discussed
Optimisation of a storage facility used to effect power control in the PBMR power cycle
08 January 2010
North West University

Matimba, A.;Mathews, E.H.;Pelzer, R.

This article presents the optimization of a gas storage facility used to effect power control in South Africa's PBMR power cycle. It was shown in the article, a multitank storage facility to affect power control in the PBMR power cyclel, that a multi tank design with heat capacitance improves storage effictiveness, which could make the system cheaper. This storage facility is known as the Inventory Control System (ICS). The focus in this article is to determine an optimum number of tanks and heat capacitance that will achieve a spectfied performance for the lowest possible cost. Please note the values used in this exercise are not the actual valaes used by PBMR. However this article serves to demonstrate an approach to achieving an optimum solution for the ICS.
Exploring a SNR/molecular cloud association within HESS J1745-303
08 January 2010
North West University

Aharonian, F.;Büsching, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Aims. HESS J1745−303 is an extended, unidentified VHE (very high energy) gamma-ray source discovered using HESS in the Galactic Plane Survey. Since no obvious counterpart has previously been found in longer-wavelength data, the processes that power the VHE emission are not well understood. Methods. Combining the latest VHE data with recent XMM-Newton observations and a variety of source catalogs and lower-energy survey data, we attempt to match (from an energetic and positional standpoint) the various parts of the emission of HESS J1745−303 with possible candidates. Results. Though no single counterpart is found to fully explain the VHE emission, we postulate that at least a fraction of the VHE source may be explained by a supernova-remnant/molecular-cloud association and/or a high-spin-down-flux pulsar
HESS observations and VLT spectroscopy of PG 1553+113
18 January 2010
North West University

Aharonian, F.;Büshing, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Aims.The properties of the very high energy (VHE; GeV) γ-ray emission from the high-frequency peaked BL Lac PG 1553+113 are investigated. An attempt is made to measure the currently unknown redshift of this object. Methods.VHE Observations of PG 1553+113 were made with the High Energy Stereoscopic System (HESS) in 2005 and 2006. (1.45-2.45 μm) spectroscopy of PG 1553+113 was performed in March 2006 with SINFONI, an integral field spectrometer of the ESO Very Large Telescope (VLT) in Chile. Results.A VHE signal, ~10 standard deviations, is detected by HESS during the 2 years of observations (24.8 h live time). The integral flux above 300 GeV is 10-12 cm-2 s-1, corresponding to ~3.4% of the flux from the Crab Nebula above the same threshold. The time-averaged energy spectrum is measured from 225 GeV to ~1.3 TeV, and is characterized by a very soft power law (photon index of ). No evidence for any flux or spectral variations is found on any sampled time scale within the VHE data. The redshift of PG 1553+113 could not be determined. Indeed, even though the measured SINFONI spectrum is the most sensitive ever reported for this object at near infrared wavelengths, and the sensitivity is comparable to the best spectroscopy at other wavelengths, no absorption or emission lines were found in the H+K spectrum presented here
HESS upper limits for Kepler's supernova remnant
18 January 2010
North West University

Büsching, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Aims. Observations of Kepler's supernova remnant (G4.5+6.8) with the HESS telescope array in 2004 and 2005 with a total live time of 13 h are presented. Methods. Stereoscopic imaging of Cherenkov radiation from extensive air showers is used to reconstruct the energy and direction of the incident gamma rays. Results. No evidence for a very high energy (VHE: >100 GeV) gamma-ray signal from the direction of the remnant is found. An upper limit (99% confidence level) on the energy flux in the range $230~{\rm GeV}{-}12.8~{\rm TeV}$ of 8.6 $\times$ $10^{-13}~{\rm erg}~{\rm cm}^{-2}~{\rm s}^{-1}$ is obtained. Conclusions. In the context of an existing theoretical model for the remnant, the lack of a detectable gamma-ray flux implies a distance of at least $6.4~{\rm kpc}$. A corresponding upper limit for the density of the ambient matter of $0.7~{\rm cm}^{-3}$ is derived. With this distance limit, and assuming a spectral index $\Gamma = 2$, the total energy in accelerated protons is limited to $E_{\rm p} < 8.6$ $\times$ $10^{49}~{\rm erg}$. In the synchrotron/inverse Compton framework, extrapolating the power law measured by RXTE between 10 and $20~{\rm keV}$ down in energy, the predicted gamma-ray flux from inverse Compton scattering is below the measured upper limit for magnetic field values greater than $52~\mu {\rm G}$
HESS very-high-energy gamma-ray sources without identified counterparts
18 January 2010
North West University

Büsching, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Context.The detection of gamma rays in the very-high-energy (VHE) energy range (100 GeV–100 TeV) provides a direct view of the parent population of ultra-relativistic particles found in astrophysical sources. For this reason, VHE gamma rays are useful for understanding the underlying astrophysical processes in non-thermal sources. Aims.We investigate unidentified VHE gamma-ray sources that have been discovered with HESS in the most sensitive blind survey of the Galactic plane at VHE energies conducted so far. Methods.The HESS array of imaging atmospheric Cherenkov telescopes (IACTs) has a high sensitivity compared with previous instruments (~ in 25 h observation time for a point-source detection), and with its large field of view, is well suited for scan-based observations. The on-going HESS survey of the inner Galaxy has revealed a large number of new VHE sources, and for each we attempt to associate the VHE emission with multi-wavelength data in the radio through X-ray wavebands. Results. For each of the eight unidentified VHE sources considered here, we present the energy spectra and sky maps of the sources and their environment. The VHE morphology is compared with available multi-wavelength data (mainly radio and X-rays). No plausible counterparts are found.
Simultaneous HESS and Chandra observations of Sagitarius A* during an X-ray flare
18 January 2010
North West University

Aharonian, F.;Büsching, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

The rapidly varying (~10 min timescale) non-thermal X-ray emission observed from Sgr A implies that particle acceleration is occuring close to the event horizon of the supermassive black hole. The TeV γ-ray source HESS J1745-290 is coincident with Sgr A and may be closely related to its X-ray emission. Simultaneous X-ray and TeV observations are required to elucidate the relationship between these objects. We report on joint HESS/Chandra observations performed in July 2005, during which an X-ray flare was detected. Despite a factor of ≈9 increase in the X-ray flux of Sgr A, no evidence is found for an increase in the TeV γ-ray flux from this region. We find that an increase in the γ-ray flux of a factor of 2 or greater can be excluded at a confidence level of 99%. This finding disfavours scenarios in which the keV and TeV emission are associated with the same population of accelerated particles and in which the bulk of the γ-ray emission is produced within ~1014 cm (~100) of the supermassive black hole
An understanding of lump coal physical property behaviour (density and particle size effects) impacting on a commercial-scale Sasol-Lurgi FBDB gasifier
22 January 2010
North West University

Bunt, J.R.;Waanders, Frans B.

Thermal processes which utilize coarse coal, such as fixed-bed gasification and chain grate stoker boilers, are dependant on a stable particle size for stable operation. During coarse coal utilization, thermal fragmentation of lump coal (upon heating) produces hydrodynamic effects (pressure drop fluctuations) manifesting itself in a variety of ways, and include: channel-burning and solids elutriation. Primary thermal fragmentation occurring in the drying zone of a fixed-bed reactor is primarily a function of moisture content release with ensuing particle size reduction. Large particles tend to fragment more than finer particles, thus leading to hydrodynamic problems. From fragmentation studies it was elucidated that a thermal “stable size” is reached through the process of thermal fragmentation for optimum heat transfer and utilization during the drying and pyrolysis zone regions of the coarse coal utilization process. In this paper, the Sasol-Lurgi MK IV FBDB gasifier turn-out physical property profiles (bulk density and particle size distribution) results will be discussed. It was found that these profiles provided significant insight into the complex heterogeneous nature of the coal transformation processes occurring within the fixed-bed reactor. In the case of the bulk density profile, a shrinking core and flaking mechanism was proposed to explain the increase in density occurring in the bottom half of the gasifier. The +25 mm size fraction distribution profile was found to clearly show the fragmentation effects occurring within the reactor. Primary fragmentation was inferred as the mechanism responsible for causing breakage of this size fraction down to a remaining ca. 15% +25 mm fraction. The significant breakage of the coarse +25 mm fraction is expected to influence unstable gasifier conditions in the top part of the gasifier, due to pressure drop fluctuations caused by void packing. A good correlation was obtained for the relationship between bulk density versus the −25 mm + 6.3 mm size fraction content, indicating that the bed-packing density is highly dependent on the relative abundance of this intermediate size fraction. The −6.3 mm size fraction distribution profile was found to not be significantly different between the four reaction zones identified in the gasifier. Breakage of the coarser +6.3 mm sizes occurred continuously, and could possibly be related to breakage caused by the ash-grate when sampling. The Ergun Index was successfully used to profile the fragmentation zones identified and to show areas within the gasifier where pressure drop and resultant instability occurs. This is the first-ever identification of this phenomenon occurring within a fixed-bed gasifier and is expected to lead to significant optimization challenges to ensure better stability
An understanding of the behaviour of a number of element phases impacting on a commercial-scale Sasol-Lurgi FBDB gasifier
01 February 2010
North West University

Bunt, J.R.;Waanders, F.B.

Chemical properties of coal which impact on gasification performance relate to those processes which do effect a change in chemical constitution, these in turn may lead to changes in physical properties such as particle size distribution and surface area of the coal. Turn-out samples obtained from a commercial-scale Sasol-Lurgi fixed-bed dry bottom (FBDB) gasifier were characterized to understand and interpret the internal chemical property behaviour and are discussed in relation to the residual C, H, N, S and O distribution profiles obtained. Thermodynamic equilibrium simulation of the organic and inorganic speciation behaviour occurring within a fixed-bed gasifier was modelled using the Fact-Sage simulation package, and used to support the measured ultimate analysis profile data obtained. The measured gasifier ultimate analysis profiles provided good insight into understanding the development of aromaticity of the char, expressed by the carbon:hydrogen ratio calculated on a mass basis. Equilibrium compositional profiles calculated for C, H, N, S and O provided discernment regarding the speciation and partitioning behaviour occurring within the fixed-bed-reactor. Fact-Sage thermodynamic equilibrium modeling of the gasifier related to the ultimate analysis results, was found to be useful in identifying an oxygen scavenging effect created by the mineral transformation behaviour occurring during reduction. It was found that oxygen-containing species such as Mg2Al4Si5O18 (corderite) and Fe2Al4Si5O18 (ferro-corderite) form within the reduction zone. It would appear that mineral composition is a more fundamental property than merely ash content in the gasification process, when viewed on an oxygen consumption basis
Behaviour of calcium-containing minerals in the mechanism towards in situ CO2 capture during gasification
01 February 2010
North West University

Van Dyk, J.C.;Waanders, F.B.;Hack, K.

Mineral matter transformation and the behavior of mineral matter in the coal during gasification, provide more information on the suitability of a specific coal source for combustion or gasification purposes. Therefore, the chemistry and mineral interactions have to be understood in order to determine the suitability for fixed bed gasification purposes with regards to mineral matter transformations and slagging properties. Although a suite of minerals important for the gasification process were identified [Van Dyk JC, Melzer S, Sobiecki A. Mineral matter transformations during Sasol-Lurgi fixed bed dry bottom gasification – utilization of HT-XRD and FactSage modelling. Minerals Engineering 2006; 19: 1126–35], some of the minerals, i.e. anorthite and calcite, with a specific behavior at different concentrations in the mineral structure and the transformation thereof was not studied and highlighted in detail. A number of other researchers [Reifenstein AP, Kahraman H, Coin CDA, Calos NJ, Miller G, Uwins P. Behavior of selected minerals in an improved ash fusion test: quartz, potassium feldspar, sodium feldspar, kaolinite, illite, calcite, dolomite, siderite, pyrite and apatite. Fuel 1999; 78: 1449–61], [Kondratiev A, Jaks E. Predicting coal ash slag flow characteristics (viscosity model for the Al2O3–CaO–‘FeO’–SiO2 system). Fuel 2001; 80: 1989–2000] and [Kondratiev A, Jak E. Applications of the coal ash slag viscosity model for the slagging gasification technologies (viscosity model in the Al2O3–CaO–‘FeO’–SiO2 system), 18th Pittsburgh Coal Conference, Newcastle, Australia, December 2001]) also did not investigate these gasification changes and mineralogical deformation during specific gasification conditions in detail. The principle aim of this paper is to identify the role of Ca-containing mineral species towards the in situ capture of CO2 during gasification, as well as understanding the chemistry and interpret the mechanism of CO2 capture by means of high temperature X-ray diffraction (HT-XRD), in combination with FactSage modeling. The CaO content of a South African and another coal source investigated in the present study, were 6 mass% and 30 mass% respectively. The basic components present in the coal, or specifically CaO, only act as a fluxing component up to a specific percentage, where after the ash fusion temperature starts to increase again. At this turning point the (Si+Al):Ca molar mass ratio is 2.75, which implies that after the turning point, the formation of anorthite is maximized and can thereafter only remain at the same level. The anorthite formation, when the Ca content increases, follows the inverse trend of the ash flow temperature prediction curve with the coal containing 6% CaO. The decrease in anorthite formation, with increasing Ca content, after the turning point in the graph, can be explained by the fact that more of the crystalline phase becomes a liquid (slag), and thus also the increase in the amount CaO in the slag will be observed. At the turning point, it is also interesting to note the stabilisation of the amount of other Ca-containing species. These are the minerals that are responsible and available for the mechanism where CO2 can be captured on Ca to form CaCO3. The formation of CaCO3 can also be observed from the turning point where the (Si+Al):Ca molar mass ratio is 2.75. Anorthite (CaSi2Al2O8) forms within the gasification zone and all non-reacted Ca react with CO2 to form CaCO3 further down in the combustion zone
Chandra and HESS observations of the supernova remnant CTB 37B
09 February 2010
North West University

Aharonian, F.;Büsching, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

We discovered the >100 GeV γ-ray source, HESS J1713−381, apparently associated with the shell-type supernova remnant (SNR) CTB37B, using HESS in 2006. In 2007 we performed X-ray follow-up observations with Chandra with the aim of identifying a synchrotron counterpart to the TeV source and/or thermal emission from the SNR shell. These new Chandra data, together with additional TeV data, allow us to investigate the nature of this object in much greater detail than was previously possible. The new X-ray data reveal thermal emission from a ∼4 region in close proximity to the radio shell of CTB37B. The temperature of this emission implies an age for the remnant of ∼5000 years and an ambient gas density of ∼0.5 cm−3. Both these estimates are considerably uncertain due to the asymmetry of the SNR and possible modifications of the kinematics due to efficient cosmic ray (CR) acceleration. A bright (≈7× 10−13 erg cm−2 s−1) and unresolved (<1 ) source (CXOUJ171405.7−381031), with a soft (Γ ≈ 3.3) non-thermal spectrum is also detected in coincidence with the radio shell. Absorption indicates a column density consistent with the thermal emission from the shell, suggesting a genuine association rather than a chance alignment. The observed TeV morphology is consistent with an origin in the complete shell of CTB37B. The lack of diffuse non-thermal X-ray emission suggests an origin of the γ-ray emission via the decay of neutral pions produced in interactions of protons and nuclei, rather than inverse Compton (IC) emission from relativistic electrons
Discovery of very-high-energy gamma-ray emission from the vicinity of PSR J1913+1011 with HESS
12 February 2010
North West University

Aharonian, F.;Büsching, I.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

The HESS experiment, an array of four Imaging Atmospheric Cherenkov Telescopes with high sensitivity and large field-of-view, has been used to search for emitters of very-high-energy (VHE, >100 GeV) $\gamma$-rays along the Galactic plane, covering the region 30° $<\, l\, <$ 60°, 280° $<\, l\, <$ 330°, and -3° $<\, b\, <$ 3°. In this continuation of the HESS Galactic Plane Scan, a new extended VHE $\gamma$-ray source was discovered at $\alpha_{2000}$=19$^{\rm h}$12$^{\rm m}$49$^{\rm s}$, $\delta_{2000}$=+10°09´06´´(HESS J1912+101). Its integral flux between 1-10 TeV is ~10% of the Crab Nebula flux in the same energy range. The measured energy spectrum can be described by a power law d $N/{\rm d}E \, \sim \, E^{-\Gamma}$ with a photon index $\Gamma = 2.7 \pm 0.2_{\mbox{stat}}\pm 0.3_{\mbox{sys}}$. HESS J1912+101 is plausibly associated with the high spin-down luminosity pulsar PSR J1913+1011. We also discuss associations with an as yet unconfirmed SNR candidate proposed from low frequency radio observation and/or with molecular clouds found in 13CO data
Discovery of VHE gamma-rays from the high-frequency-peaked BL Lacertae object RGB J0152+017
17 February 2010
North West University

Aharonian, F.;Büsching, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Aims. The BL Lac object RGB J0152+017 (z=0.080) was predicted to be a very high-energy (VHE; >100 GeV) $\gamma$-ray source, due to its high X-ray and radio fluxes. Our aim is to understand the radiative processes by investigating the observed emission and its production mechanism using the High Energy Stereoscopic System (HESS) experiment. Methods. We report recent observations of the BL Lac source RGB J0152+017 made in late October and November 2007 with the HESS array consisting of four imaging atmospheric Cherenkov telescopes. Contemporaneous observations were made in X-rays by the Swift and RXTE satellites, in the optical band with the ATOM telescope, and in the radio band with the Nançay Radio Telescope. Results. A signal of 173 $\gamma$-ray photons corresponding to a statistical significance of 6.6$\sigma $ was found in the data. The energy spectrum of the source can be described by a powerlaw with a spectral index of $\Gamma =2.95\pm 0.36_{{\rm stat}}\pm 0.20_{{\rm syst}}$. The integral flux above 300 GeV corresponds to $\sim$2% of the flux of the Crab nebula. The source spectral energy distribution (SED) can be described using a two-component non-thermal synchrotron self-Compton (SSC) leptonic model, except in the optical band, which is dominated by a thermal host galaxy component. The parameters that are found are very close to those found in similar SSC studies in TeV blazars. Conclusions. RGB J0152+017 is discovered as a source of VHE $\gamma$-rays by HESS The location of its synchrotron peak, as derived from the SED in Swift data, allows clear classification as a high-frequency-peaked BL Lac (HBL)
Identification of the reaction zones occurring in a commercial-scale Sasol-Lurgie FBDB gasifier
14 April 2010
North West University

Bunt, J.R.;Waanders, F.B.

Gasification behaviour is particle dependent, whilst gasifier (reactor) behaviour is an averaging process of individual responses of each particle. It was hypothesized, that if it were possible to extract and analyze particles from different reaction zones within a gasifier, it may be likely to enhance the understanding of the contribution that these particles make towards gasification. This better understanding of the particle-type compositional responses could act as an enabler to further manipulate and improve gasifier performance. The primary focus of this study was to evaluate a sequential (axial) sampling “turn-out” methodology of a quenched fixed-bed commercial-scale Sasol–Lurgi gasifier, in order to present samples that accurately describe operational aspects occurring in the reaction zones within the reactor. Characterization of the chemical properties of the sample increments were expected to deliver distinct profiles of the drying, pyrolysis, reduction and combustion (ash-bed) zones, which could be used to advance the kinetic modeling capability of the process. In order to interpret the coal property transformational behaviour occurring within the commercial-scale gasifier, the proximate, Fischer tar, ultimate, and coal char CO2 reactivity analysis were conducted. The pyrolysis zone was found to be the largest reaction zone situated below the drying zone within the gasifier, followed by the reduction zone, and combustion (ash-bed) zones. Whilst the boundaries of the pyrolysis zone were very clearly defined by the residual volatile matter distribution profile, distinctive regional overlap with a “slow pyrolysis with gasification” region was observed in the bottom half of the pyrolysis zone, above which a “rapid de-volatilization” region existed. The reduction zone was found to also exhibit an overlap in zonal fronts, i.e. a gasification region occurred below the pyrolysis zone and co-existed in equal proportions, with an oxidation frontal region occurring above the combustion zone. The combustion zone was found to be very shallow, below which the ash-bed region existed. The findings clearly suggest that text book pictures showing axially-depicted reaction zones occurring within the fixed-bed gasifier, i.e. drying, pyrolysis, gasification and combustion, inadequately describe the “real” situation and in practice, overlap of reaction regions within zones indeed also transpire
Observations of the Sagitarrius dwarf galaxy by the HESS experiment and search for a dark matter signal
10 May 2010
North West University

Aharonian, F.;Büsching, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Observations of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy were carried out with the HESS array of four imaging air Cherenkov telescopes in June 2006. A total of 11 h of high quality data are available after data selection. There is no evidence for a very high energy γ-ray signal above the energy threshold at the target position. A 95% CL flux limit of View the MathML source3.6×10-12cm-2s-1 above 250 GeV has been derived. Constraints on the velocity-weighted cross-section 〈σv〉〈σv〉 are calculated in the framework of dark matter particle annihilation using realistic models for the dark matter halo profile of Sagittarius dwarf galaxy. Two different models have been investigated encompassing a large class of halo types. A 95% CL exclusion limit on 〈σv〉〈σv〉 of the order of View the MathML source2×10-25cm3s-1 is obtained for a core profile in the 100 GeV–1 TeV neutralino mass range
Upper limits from HESS active galactic nuclei observations in 2005-2007
10 May 2010
North West University

Aharonian, F.;Büsching, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Aims.Very high energy (VHE; GeV) γ-ray studies were performed for 18 active galactic nuclei (AGN) from a variety of AGN classes. Methods.VHE observations of a sample of 14 AGN, considered candidate VHE emitters, were made with the High Energy Stereoscopic System (HESS) between January 2005 and July 2007. Large-zenith-angle observations of three northern AGN (Mkn 421, Mkn 501, 1ES 1218+304), known to emit VHE γ-rays, were also performed in order to sample their spectral energy distributions (SEDs) above 1 TeV. In addition, the VHE flux from 1ES 1101-232, previously detected by HESS in 2004-2005, was monitored during 2006 and 2007. Results.As significant detections from the HESS observation program are reported elsewhere, the results reported here are primarily integral flux upper limits. The average exposure for each of the 14 VHE-candidate AGN is ~7 h live time, and the observations have an average energy threshold between 230 GeV and 590 GeV. Upper limits for these 14 AGN range from <0.9% to <4.9% of the Crab Nebula flux, and eight of these are the most constraining ever reported for the object. The brief (<2.2 h each) large-zenith-angle observations yield upper limits for Mkn 501 (<20% Crab above 2.5 TeV) and 1ES 1218+304 (<17% Crab above 1.0 TeV), and a marginal detection () of Mkn 421 (50% Crab above 2.1 TeV). 1ES 1101-232 was marginally detected (, 1.7% Crab above 260 GeV) during the 2006 (13.7 h live time) observations, but not in the 2007 (4.6 h live time) data. The upper limit in 2007 (<1.9% Crab above 260 GeV) is below the average flux measured by HESS from 2004-2006
A new approach to ensure successful implementation of sustainable demand side management (DSM) in South African mines
10 May 2010
North West University

Pelzer, R.;Mathews, E.H.;Le Roux, D.F.;Kleingeld, M.

Demand side management (DSM) is seen as a short-term solution to the imminent problem of electricity supply shortages in South Africa. DSM aims to reduce peak loads with immediate results in a short time. The mining sector in South Africa is a large energy user with pumping one of the largest consuming systems. Therefore, DSM potential (load shift) should be investigated on these pumping systems. For sustainable load shift, a system is required that simulates, optimises and controls the actual on-site situation. As no such equipment that performs all these processes could be found for deep South African mines, it was developed by HVAC International (Pty) Ltd. It is called the Real-time Energy Management System (REMS). With this system, maximum results can be obtained on a sustainable basis. In this study, four similar DSM projects were investigated. These are described as case studies at gold mines in the Free State Province. For each of these studies a different new innovation was implemented. The innovations described include the adaptation of REMS to handle multi-level intricate pump systems, mines without any instrumentation and control infrastructure, as well as Three Chamber Piped Feeder Systems (3CPFSs)
Electron anisotropies in the inner heliosphere
14 May 2010
North West University

Nkosi, G.S.;Potgieter, M.S.;Ferreira, S.E.S.

A three-dimensional numerical model is used to study the propagation and modulation of a few MeV Jovian and galactic electrons in the inner heliosphere. The main aim of the work is to compute the three-dimensional electron intensity anisotropy as described by a standard approach to the modulation of cosmic rays in the heliosphere, with enhanced perpendicular diffusion. In order to accomplish this, the spatial dependence of the full diffusion tensor has to be specified and the three-dimensional spatial intensity gradients have to be computed in detail. The emphasis is placed on the role that polar perpendicular diffusion plays in order to establish how large the anisotropy vector components may get in the inner heliosphere, for both galactic and Jovian electrons. The modelling results should be seen as upper limits of what standard modulation theory predicts and may serve as reference for the interpretation of electron observations and to establish what percentage of the observed electron anisotropy can be attributed to Jovian electrons and to galactic electrons, respectively. It is found that the total anisotropy for electrons is dominated by the contribution of the Jovian electron anisotropy in the equatorial regions, as expected, close to the Jovian electron source during solar minimum conditions, whereas at high heliolatitudes the galactic electron anisotropy contribution dominates the total anisotropy. An equatorial approach of the Jovian electron source produces a much sharper anisotropy–time profile than a latitudinal approach, the latter interpreted as indicative of the role of enhanced perpendicular diffusion
Introduction to the PBMR heat transfer test facility
26 May 2010
North West University

Rousseau, P.G.;Van Staden, M.

This paper provides an introduction to the Heat Transfer Test Facility (HTTF) that is currently being developed for PBMR (Pty.) Ltd. by M-Tech Industrial (Pty.) Ltd. in association with North-West University in South Africa. The paper provides an overview of the phenomena that will be studied and the envisaged test configurations for each of these phenomena. It also shows the layouts of the different test units namely the High Pressure Test Unit (HPTU) and the High Temperature Test Unit (HTTU) and provides an overview of the planned test schedule
Oil crops in biofuel applications: South Africa gearing up for a bio-based economy
18 January 2011
North West University

Marvey, B B

Large fluctuations in crude oil prices and the diminishing oil supply have left economies vulnerable to energy shortages thus placing an enormous pressure on nations around the world to seriously consider alternative renewable resources as feedstock in biofuel applications. Apart from energy security reasons, biofuels offer other advantages over their petroleum counterparts in that they contribute to the reduction in greenhouse gas emissions and to sustainable development. Just a few decades after discontinuing its large scale production of bioethanol for use as engine fuel, South Africa (SA) is again on its way to resuscitating its biofuel industry. Herein an overview is presented on South Africa’s oilseed and biofuel production, biofuels industrial strategy, industry readiness, challenges in switching to biofuels and the strategies to overcome potential obstacles.
Comparative analysis of energy impacts of traditional and partly electronic environmental auditing: Learning from the Japanese automotive industry case
26 January 2011
North West University

Mbohwa, C

This paper does a comparative analysis and assessment of energy impacts of traditional and partly electronic ISO 14001 environmental management system auditing that using videoconferencing systems. The focus is on the Japanese automotive industry. The idea is to quantify the environmental impacts of auditing and resolve the important question on whether or not the use of information and communication technology (ICT) in environmental auditing reduces environmental impacts in ISO 14001 environmental management systems. The main results show that there are no clear energy savings or environmental advantages associated with ICT use in environmental auditing for the set system boundary. However electronic auditing is shown to have clear advantages for stated variables within the model, for example when the distance travelled by the auditors by train and by car is less than 75 km and 17 km per day respectively. A comparison of modes of travel by auditors, shows that rail travel offers the most environmental and energy benefits. It is also noted that travel dominates in terms of energy consumption and environmental burdens compared to all other environmental aspects associated with auditing. The study contributes to knowledge in that among other things, it enables researchers to draw parallels from it applicable to other industrial sectors and other countries. Southern African countries can use this methodology in analysing auditing methods with little or no changes in the model used.
When the railway came to Grahamstown.
11 January 2012
North West University

Hunt, K S

• Opsomming: Die oopstelling van die spoorlyn tussen Alicedale en Grahamstad op 3 September 1879 was 'n gebeurtenis van besondere belang vir Grahamstad. Die gedagte om Grahamstad per spoor met die hawe in Port Elizabeth te verbind, het drie-en-twintig jaar vroeër by die sakemanne van Grahamstad ontstaan. Toe die spoorlyn uiteindelik gebou is, was die inwoners van Grahamstad eintlik meer ten gunste van ŉ spoorverbinding met die hawe van Port Alfred. Dit is egter eers vyf jaar later gebou. Toe die eerste sooi gespit is vir die bou van die spoorwegstasie en ook toe die eerste passasierstrein in Grahamstad aangekom het, is groot feesvierings en feestelikhede gereël.
Developing an Environmental Performance Index (EPI): a focus on impacts of information and communication technology use.
13 January 2012
North West University

Mbohwa, C;Agwa-Ejon, J

There is a growing need for environmental performance measures that can be used by all stakeholders like surrounding communities, customers, suppliers and shareholders to gauge the environmental performance of organizations. The environmental performance measures that are used worldwide are normally not suitable for benchmarking organizations. This paper develops an environmental performance index using indicator and weight matrices of the full life cycle phases of an organization’s energy use for environmental management system activities. This work is transdisciplinary in nature and applies mathematical matrices and environmental productivity approaches, and borrows from the development of quality indices to consider a variety of impacts that cut across various phases of a product life cycle and different functions within an organization. The focus is on information and communication technology use in these systems. The model is applied to the Japanese automotive industry and the findings show that it is feasible and effective for comparing the environmental performance of companies in the same sector using the same weight matrices and indicators agreed to. The work informs decision making on the development of environmental performance measures that have worldwide applications, across many disciplines, in situations where suitable data are recorded. It also contributes to efforts on economic and social sustainability. Research efforts in similar areas in Southern Africa can benefit from the development and improvement of the proposed methodology.
'The power of power': Power stations as industrial heritage and their place in history and heritage education.
14 February 2012
North West University

Krige, Sue

This article focuses on the Electrical Precinct in Newtown as an example of industrial heritage in the centre of Johannesburg. The author makes the case that industrial heritage has been neglected in terms of what is deemed to be culturally significant. Yet industrial landscapes provide a direct connection with a ‘cultural ‘experience common to all city dwellers. The author demonstrates how the heritage tourism company, Cultural Encounters, used field trips/visits to Newtown with learners from Grades 8-12, over the period 2003 - 2009, to grapple with the requirements of the Social Sciences and History curricula. She argues that this approach to industrial heritage could provide a radical new approach to the 2010 Curriculum Assessment Policy Statement (CAPS), including the Grade 10 heritage assignment.
Assessing the past and the present role of the National Nuclear Regulator as a public protector against potential health injuries: the West and Far West Rand as case study.
12 June 2012
North West University

Liefferink, Mariette

Recent national and international news media articles and television programmes, official public domain Government and peer reviewed academic reports drew the international and national public’s attention to the health risks and hazards and impacts pertaining to uraniferous waste from the gold mining industry within the Witwatersrand goldfields and the role of the National Nuclear Regulator (NNR) in this regard. The Dutch research organizations’ Wise and Somo’s recent report, entitled “Uranium from Africa – mitigation of uranium impacts on society and environment by industry and governments” found that that the lack of knowledge within institutions and lack of proper environmental management systems, both in industry and in the government, “render South Africa a poor example of environmental and human health protection….The government is failing.” It is especially the poor, the disempowered and the vulnerable members of mining communities that bear the highest risks and impacts. These communities have low adaptive capacity because of chronic and acute malnutrition and high HIV/Aids percentages. The World Health Organization (WHO) recently estimated that 34% of all childhood illness in the world (compared to 24% of all age illness) and 36% of deaths in children under age 14 are due to modifiable environmental factors. As with many illnesses, genes may create a loaded gun, but environment pulls the trigger. However, determining how impacts from gold mining waste within the West and Far West Rand as part of the Witwatersrand goldfields influence or determine human health is a challenge. This is due to the social, cultural and economic conditions (thus their broader well-being status) which influence the vulnerability of communities, and subsequently their resilience. The focus of the discussion will relate to this area and the NNR’s role – past and present – as public health protector in this area per se.
Time-dependent cosmic ray modulation
23 August 2012
North West University

Manuel, R.;Ferreira, S.E.S.;Potgieter, M.S.;Strauss, R.D.;Engelbrecht, N.E.

Time-dependent cosmic ray modulation is calculated over multiple solar cycles using our well established two-dimensional time-dependent modulation model. Results are compared to Voyager 1, Ulysses and IMP cosmic ray observations to establish compatibility. A time-dependence in the diffusion and drift coefficients, implicitly contained in recent expressions derived by Teufel and Schlickeiser (2002), Shalchi et al. (2004), Minnie et al. (2007), Engelbrecht (2008), is incorporated into the cosmic ray modulation model. This results in calculations which are compatible with spacecraft observations on a global scale over consecutive solar cycles. This approach compares well to the successful compound approach of Ferreira and Potgieter (2004). For both these approaches the magnetic field magnitude, variance of the field and current sheet tilt angle values observed at Earth are transported time-dependently into the outer heliosphere. However, when results are compared to observations for extreme solar maximum, the computed step-like modulation is not as pronounced as observed. This indicates that some additional merging of these structures into more pronounced modulation barriers along the way is needed
Modeling the acceleration and modulation of anomalous cosmic ray oxygen
23 August 2012
North West University

Strauss, R.D.;Potgieter, M.S.;Ferreira, S.E.S.

After the solar wind termination shock crossings of the Voyager spacecraft, the acceleration of anomalous cosmic rays has become a very contentious subject. In this paper we examine several topics pertinent to anomalous cosmic ray oxygen acceleration and transport using a numerical cosmic ray modulation model. These include the effects of drifts on a purely Fermi I accelerated spectra, the effects of introducing higher charge states of oxygen into the modulation model, examining the viability of momentum diffusion as a re-acceleration process in the heliosheath and examining energy spectra, and intensity gradients, in the inner heliosphere during consecutive drift cycles
Modelling of low-energy galactic electrons in the heliosheath
23 August 2012
North West University

Nkosi, G.S.;Potgieter, M.S.;Webber, W.R.

The modulation of cosmic ray electrons in the heliosphere plays an important role in improving our understanding and assessment of the processes applicable to low-energy galactic electrons. A full three-dimensional numerical model based on Parker’s transport equation is used to study the modulation of 10 MeV galactic electrons, in particular inside the heliosheath. The emphasis is placed on the role that perpendicular diffusion plays in causing the extraordinary large increase in the observed intensities of these electrons in the heliosheath. The modelling is compared with observations of 6–14 MeV electrons from the Voyager 1 mission. Results are shown for the radial intensity profiles of these electrons, as well as the modulation effects of varying the extent of the heliosheath by changing the location of the termination shock and the heliopause and the value of the local interstellar spectrum. We confirm that the heliosheath acts as a modulation ‘barrier’ for low-energy galactic electrons. The significance of this result depends on how wide the inner heliosheath is; on how high the very local interstellar spectrum is at these low energies (E < 100 MeV) and on how small perpendicular diffusion is inside the inner heliosheath
Analysis of South African Grade 10 learner's conceptual resources regarding the concept of energy in physics
29 August 2012
North West University

Lemmer, Miriam

More than a thousand Grade 10 Physical Science learners from four South African provinces participated in a study that probed their conceptions of energy. The purpose was to determine the learners’ conceptual resources, i.e. their initial conceptions, and identify the potentially productive resources from which they may construct physics concepts. The conceptions revealed by the learners in their responses to a questionnaire were categorised as core intuitive, implicit, conscious or verbal-symbolic. Notwithstanding learning the curriculum theme Energy and Change in their preceding school years, the results show that many of the learners have not yet mastered the concept of energy and related concepts ontologically well enough for progression to formal physics in the FET phase. However, their initial conceptions contain resources that can be refi ned to the science concepts. Implications and recommendations for learning and teaching in the FET phase are discussed in the framework of the conceptual refinement model.
On the methanol masers in G9.62+0.20E and G188.95+0.89
03 September 2012
North West University

Van der Walt, Diederick Johannes

A comparison between the observed light curves of periodic masers in G9.62+0.20E and G188.95+0.89 and the results of a simple colliding-wind binary model is made to establish whether the flaring and other time-dependent behavior of the masers in these two star-forming regions can be ascribed to changes in the environment of the masers or in the continuum emission from parts of the background Hii region. It is found that the light curves of widely different shape and amplitude in these two objects can be explained within the framework of a periodic pulse of ionizing radiation that raises the electron density in a volume of partially ionized gas against which the masers are projected. It is also shown that the decay of the 11.405 km s−1 maser in G188.95+0.89 can be explained very well in terms of the recombination of the ionized gas against which the maser is projected, while it would require very special conditions to explain it in terms of changes in the environment of the maser. We conclude that for G9.62+0.20E and G188.95+0.89 the observed changes in the masers are most likely due to changes in the background free-free emission which is amplified by the masers.
The role of international sustainable development law principles in enabling effective renewable energy policy - a South African perspective
03 September 2012
North West University

Barnard, M

It is universally accepted that renewable energy is an important contributing factor towards the promotion of sustainable development. The implementation of renewable energy needs to be regulated in an effective manner which in turn necessitates the formulation of law and policy geared towards sustainable development. Recent policy developments in South Africa propose to facilitate the promotion of sustainable development through the implementation of renewable energy, among others. In terms of existing energy policy in South-Africa, the interconnectivity of renewable energy and sustainable development is evident. Most notably, the White Paper on Renewable Energy of 2003 promotes increased access to affordable renewable energy in order to contribute to sustainable development. Moreover, the 2008 first review of the National Energy Efficiency Strategy of the Republic of South- Africa of 2005 states that in order for the country’s renewable energy policy to be considered sustainable, it needs to facilitate development in the social, economic and environmental spheres. Notwithstanding, attaining the goal of sustainable development depends on whether all its effecting principles are catered for in the policy developments. Accordingly, in order to ascertain whether South-African law and policy can successfully facilitate/enable sustainable development via the implementation of renewable energy, a specific methodology is proposed. In terms of the New Delhi Declaration of 2002 there are 7 principles of international law effecting sustainable development. These principles will be used as criteria in a principled assessment of South-African renewable energy law and policy in order to establish whether the goal of promoting sustainable development would be effected through the national policy developments.
Simultaneous multi-wavelength campaign on PKS 2005-489 in a high state
05 September 2012
North West University

Abramowski, A.;Büsching, I.;De Jager, O.C.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

TeV γ-ray (H.E.S.S.), GeV γ-ray (Fermi/LAT), X-ray (RXTE, Swift), UV (Swift) and optical (ATOM, Swift) bands. This campaign was carried out during a high flux state in the synchrotron regime. The flux in the optical and X-ray bands reached the level of the historical maxima. The hard GeV spectrum observed with Fermi/LAT connects well to the very high energy (VHE, E > 100 GeV) spectrum measured with H.E.S.S. with a peak energy between ∼5 and 500 GeV. Compared to observations with contemporaneous coverage in the VHE and X-ray bands in 2004, the X-ray flux was ∼50 times higher during the 2009 campaign while the TeV γ-ray flux shows marginal variation over the years. The spectral energy distribution during this multi-wavelength campaign was fit by a one zone synchrotron self-Compton model with a well determined cutoff in X-rays. The parameters of a one zone SSC model are inconsistent with variability time scales. The variability behaviour over years with the large changes in synchrotron emission and small changes in the inverse Compton emission does not warrant an interpretation within a one-zone SSC model despite an apparently satisfying fit to the broadband data in 2009.
Discovery and follow-up studies of the extended, off-plane, VHE gamma-ray source HESS J1507-622
05 September 2012
North West University

Acero, F.;Busching, I.;Davids, I.D.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Context. The detection of gamma-rays in the very-high-energy (VHE) range (100 GeV−100 TeV) offers the possibility of studying the parent population of ultrarelativistic particles found in astrophysical sources, so it is useful for understanding the underlying astrophysical processes in nonthermal sources. Aims. The discovery of the VHE gamma-ray source HESS J1507-622 is reported and possibilities regarding its nature are investigated. Methods. The H.E.S.S. array of imaging atmospheric Cherenkov telescopes (IACTs) has a high sensitivity compared with previous instruments (∼1% of the Crab flux in 25 h observation time for a 5σ point-source detection) and has a large field of view (∼5◦ in diameter). HESS J1507-622 was discovered within the ongoing H.E.S.S. survey of the inner Galaxy, and the source was also studied by means of dedicated multiwavelength observations. Results. A Galactic gamma-ray source, HESS J1507-622, located ∼3.5◦ from the Galactic plane was detected with a statistical significance >9σ. Its energy spectrum is well fitted by a power law with spectral index Γ = 2.24 ± 0.16stat ± 0.20sys and a flux above 1 TeVof (1.5± 0.4stat ± 0.3sys) × 10−12 cm−2 s−1. Possible interpretations (considering both hadronic and leptonic models) of the VHE gamma-ray emission are discussed in the absence of an obvious counterpart.
Detection of very-high-energy y-ray emission from the vicinity of PSR B1706-44 and G 343.1-2.3 with H.E.S.S.
05 September 2012
North West University

Abramowski, A.;Büsching, I.;Davids, I.D.;De Jager, O.C.;Holleran, M.;Venter, C.;H.E.S.S. Collaboration

The γ-ray pulsar PSRB1706−44 and the adjacent supernova remnant (SNR) candidate G343.1−2.3 were observed by H.E.S.S. during a dedicated observation campaign in 2007. As a result of this observation campaign, a new source of very-high-energy (VHE; E > 100 GeV) γ-ray emission, H.E.S.S. J1708−443, was detected with a statistical significance of 7σ, although no significant point-like emission was detected at the position of the energetic pulsar itself. In this paper, the morphological and spectral analyses of the newly-discovered TeV source are presented. The centroid of H.E.S.S. J1708−443 is considerably offset from the pulsar and located near the apparent center of the SNR, at αJ2000 = 17h08m11s ± 17s and δJ2000 = −44◦20ʹ ± 4ʹ. The source is found to be significantly more extended than the H.E.S.S. point spread function (∼0.1◦), with an intrinsic Gaussian width of 0.29◦ ± 0.04◦. Its integral flux between 1 and 10 TeV is ∼3.8 × 10−12 ph cm−2 s−1, equivalent to 17% of the Crab Nebula flux in the same energy range. The measured energy spectrum is well-fit by a power law with a relatively hard photon index Γ = 2.0 ± 0.1stat ± 0.2sys. Additional multi-wavelength data, including 330 MHz VLA observations, were used to investigate the VHE γ-ray source’s possible associations with the pulsar wind nebula of PSRB1706−44 and/or with the complex radio structure of the partial shell-type SNR G343.1−2.3.
Primary particle acceleration above 100 TeV in the shell-type supernova remnant RX J1713.7-3946 with deep H.E.S.S. observations (Corrigendum to article: Primary particle acceleration above 100 TeV in the shell-type supernova remnant RX J1713.7-3946 with deep HESS observations, 464(1):235-243.)
05 September 2012
North West University

Aharonian, F.;Büsching, I.;De Jager, O.C.;Holleran, M.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Primary particle acceleration above 100 TeV in the shell-type supernova remnant RX J1713.7-3946 with deep H.E.S.S. observations (Corrigendum)
Discovery of the source HESS J1356-645 associated with the young and energetic PSR J1357-6429
05 September 2012
North West University

Abramowski, A.;Büsching, I.;De Jager, O.C.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

Context. Several newly discovered very-high-energy (VHE; E > 100 GeV) γ-ray sources in the Galaxy are thought to be associated with energetic pulsars. Among them, middle-aged (≳104 yr) systems exhibit large centre-filled VHE nebulae, offset from the pulsar position, which result from the complex relationship between the pulsar wind and the surrounding medium, and reflect the past evolution of the pulsar. Aims. Imaging Atmospheric Cherenkov Telescopes (IACTs) have been successful in revealing extended emission from these sources in the VHE regime. Together with radio and X-ray observations, this observational window allows one to probe the energetics and magnetic field inside these large-scale nebulae. Methods. H.E.S.S., with its large field of view, angular resolution of ≲0.1◦ and unprecedented sensitivity, has been used to discover a large population of such VHE sources. In this paper, the H.E.S.S. data from the continuation of the Galactic Plane Survey (−80◦ < 𝓁 < 60◦, |b| < 3◦), together with the existing multi-wavelength observations, are used. Results. A new VHE γ-ray source was discovered at RA (J2000) = 13h56m00s, Dec (J2000) = −64◦30ʹ00__ with a 2_ statistical error in each coordinate, namely HESS J1356−645. The source is extended, with an intrinsic Gaussian width of (0.20 ± 0.02)◦. Its integrated energy flux between 1 and 10 TeV of 8 × 10−12 erg cm−2 s−1 represents ∼11% of the Crab Nebula flux in the same energy band. The energy spectrum between 1 and 20 TeV is well described by a power law dN/dE ∝ E−Γ with photon index Γ = 2.2 ± 0.2stat ± 0.2sys. The inspection of archival radio images at three frequencies and the analysis of X-ray data from ROSAT/PSPC and XMM-Newton/MOS reveal the presence of faint non-thermal diffuse emission coincident with HESS J1356−645. Conclusions. HESS J1356−645 is most likely associated with the young and energetic pulsar PSR J1357−6429 (d = 2.4 kpc, τc = 7.3 kyr and ˙E = 3.1 × 1036 erg s−1), located at a projected distance of ∼5 pc from the centroid of the VHE emission. HESS J1356−645 and its radio and X-ray counterparts would thus represent the nebula resulting from the past history of the PSR J1357−6429 wind. In a simple one-zone model, constraints on the magnetic field strength in the nebula are obtained from the flux of the faint and extended X-ray emission detected with ROSAT and XMM-Newton. Fermi-LAT upper limits in the high-energy (HE; 0.1−100 GeV) domain are also used to constrain the parent electron spectrum. From the low magnetic field value inferred from this approach (∼3−4 μG), HESS J1356−645 is thought to share many similarities with other known γ-ray emitting nebulae, such as Vela X, as it exhibits a large-scale nebula seen in radio, X-rays and VHE gamma-rays.
Search for Lorentz invariance breaking with a likelihood fit of the PKS 2155-304 flare data taken on MJD 53944
05 September 2012
North West University

Abramowski, A.;Büsching, I.;De Jager, O.C.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

Several models of Quantum Gravity predict Lorentz Symmetry breaking at energy scales approaching the Planck scale (∼1019 GeV). With present photon data from the observations of distant astrophysical sources, it is possible to constrain the Lorentz Symmetry breaking linear term in the standard photon dispersion relations. Gamma Ray Bursts (GRB) and flaring Active Galactic Nuclei (AGN) are complementary to each other for this purpose, since they are observed at different distances in different energy ranges and with different levels of variability. Following a previous publication of the High Energy Stereoscopic System (H.E.S.S.) collaboration [1], a more sensitive event-by-event method consisting of a likelihood fit is applied to PKS 2155-304 flare data of MJD 53944 (July 28, 2006) as used in the previous publication. The previous limit on the linear term is improved by a factor of ∼3 up to GeV and is currently the best result obtained with blazars. The sensitivity to the quadratic term is lower and provides a limit of 6.4 × 1010 GeV, which is the best value obtained so far with an AGN and similar to the best limits obtained with GRB.
Modeling the modulation of galactic and jovian electrons by stochastic processes
06 September 2012
North West University

Strauss, R.D.;Potgieter, M.S.;Büsching, I.;Kopp, A.

We present a newly developed numerical modulation model to study the transport of galactic and Jovian electrons in the heliosphere. The model employs stochastic differential equations (SDEs) to solve the corresponding transport equation in five dimensions (time, energy, and three spatial dimensions) which is difficult to accomplish with the numerical schemes used in finite difference models. Modeled energy spectra for galactic electrons are compared for the two drift cycles to observations at Earth. Energy spectra and radial intensity profiles of galactic and Jovian electrons are compared successfully to results from previous studies. In line with general drift considerations, it is found that most 100 MeV electrons observed at Earth enter the heliosphere near the equatorial regions in the A > 0 cycle, while they enter mainly over the polar regions in theA < 0 cycle. Our results indicate that 100 MeV electrons observed at Earth originate at the heliopause with ∼600 MeV undergoing adiabatic cooling during their transport to Earth. The mean propagation time of these particles varies between ∼180 and 300 days, depending on the drift cycle. For 10 MeV Jovian electrons observed at Earth, a mean propagation time of ∼40 days is obtained. During this time, the azimuthal position of the Jovian magnetosphere varies by ∼1◦. At a 50 AU observational point, the mean propagation time of these electrons increases to ∼370 days with an azimuthal position change of Jupiter of ∼20◦. The SDE approach is very effective in calculating these propagation times.
Identification of reaction zones in a commercial Sasol-Lurgi fixed bed dry bottom gasifier operating on North Dakota lignite
08 October 2012
North West University

Mangena, S.J.;Waanders, F.B.;Bunt, J.R.;Baker, G.

The Sasol-Lurgi fixed bed dry bottom gasification technology has the biggest market share in the world with 101 gasifiers in operation. To be able to further improve the technology and also to optimise the operating plants, it is important that the fundamentals of the process are understood. The main objective of this study was to determine the reaction zones occurring in the Sasol-Lurgi fixed bed dry bottom (S-L FBDB) gasifier operating on North Dakota lignite. A Turn-Out sampling method and subsequent chemical analyses of the gasifier fuel bed samples was used to determine the reaction zones occurring in the commercial MK IV, S-L FBDB gasifier operating on North Dakota lignite. The reaction zones were further compared with the same reactor operating on bituminous coal. Based on the results obtained from this study it was found that about two thirds of the gasifier volume was used for drying and de-volatilising the lignite thus leaving only about a third of the reactor volume for gasification and combustion. Nonetheless, due to the high reactivity of the lignite, the char was consumed within a third of the remaining gasifier volume. Clear overlaps between the reaction zones were observed in the gasifiers thus confirming the gradual transition from one reaction zone to another as reported in literature. Due to the high moisture content in the lignite, the pyrolysis zone in the gasifiers operating on North Dakota lignite occurred lower/deeper in the gasifier fuel bed as compared to the same gasifier operating on South African bituminous coal from the Highveld coalfield. All the other reaction zones in the gasifier operating on bituminous coal were also higher in the bed compared to the lignite operation. This can therefore explain the higher gas outlet temperatures for the S-L FBDB gasifiers operating on higher rank coals when compared to the gasifiers operating on lignite. The fact that the entire reactor volume was utilized for drying, de-volatilisation, gasification and combustion with carbon conversion of >98% makes the S-L FBDB gasifier very suitable for lignite gasification.
Pressure-drop predictions in a fixed-bed coal gasifier
08 October 2012
North West University

Bunt, John Reginald;Luckos, Adam

In the Sasol Synfuels plant in Secunda, Sasol-Lurgi fixed-bed dry-bottom gasifiers are used for the conversion of low-grade bituminous coals to synthesis gas (syngas). The gasifiers are fed with lump coal having a particle size in the range from 5 to 100 mm. Operating experience shows that the average particle size and particle-size distribution (PSD) of feed coal, char and ash influence the pressure drop across the bed and the gas-flow distribution within the bed. These hydrodynamic phenomena are responsible for stable gasifier operation and for the quality and production rate of the syngas. The counter-current operation produces four characteristic zones in the gasifier, namely, drying, de-volatilization, reduction and combustion. The physical properties of the solids (i.e. average particle size, PSD, sphericity and density) are different in each of these zones. Similarly, the chemical composition of the syngas, its properties (temperature, density and viscosity) and superficial velocity vary along the height of the bed. The most popular equation used to estimate the pressure drop in packed beds is that proposed by Ergun. The Ergun equation gives good predictions for non-reacting, isothermal packed beds made of uniformly sized, spherical or nearly spherical particles. In the case of fixed-bed gasifiers, predictions by the Ergun equation based on the average or inlet values of bed and gas flow parameters are unsatisfactory because the bed structure and gas flow vary significantly in the different reaction zones. In this study, the Ergun equation is applied to each reaction zone separately. The total pressure drop across the bed is then calculated as the sum of pressure drops in all zones. It is shown that the total pressure drop obtained this way agrees better with the measured result. (C) 2010 Elsevier Ltd. All rights reserved.
The random pore model with intraparticle diffusion for the description of combustion of char particles derived from mineral- and inertinite rich coal
08 October 2012
North West University

Everson, Raymond Cecil;Kaitano, Rufaro;Neomagus, Hendrik Willem Johannes P.

An investigation was undertaken to determine the applicability of the random pore model with intraparticle diffusion for the determination of the reaction kinetics for the combustion of chars particles derived from coals rich in minerals and inertinites. The char particles which were pyrolysed at 900 degrees C consisted of a dense carbon-containing fraction originating from the inert macerals (mainly inertinites), a high concentration of minerals and carbominerites, pores generated by the devolatilisation of the reactive macerals and cracks as a result of the presence of minerals. Combustion experimentation was carried out with a thermogravimetric analyser using 1 mm particles with 20% (mole) oxygen in nitrogen at 287.5 kPa and low temperatures (450-600 degrees C) and with high gas flow rates. The random pore model with intraparticle diffusion (pores and cracks) was solved numerically according to a method consisting of a step-wise regression procedure. This was achieved by using carbon conversion and reduced time relationships to calculate the structural parameter and the initial Thiele modulus followed by determination of an initial lumped reaction rate and validation with conversion versus real time results. The model is characterised by a decreasing Thiele modulus (increasing porosity) occurring during the reaction period which gives rise to a transition to a chemically controlled reaction system. It was found that the initial overall reaction rate was controlled by intraparticle diffusion with an increasing influence with increasing temperature. (c) 2011 Elsevier Ltd. All rights reserved.
Comparison of sintering and compressive strength tendencies of a model coal mineral mixture heat-treated in inert and oxidizing atmospheres
08 October 2012
North West University

Nel, Marika V.;Strydom, Christien A.;Schobert, Harold H.;Beukes, J. Paul;Bunt, John R.

The chemical interactions responsible for sintering in a coal mineral mixture were investigated in air and in N2. A mineral mixture was made up by mixing kaolin, pyrite, quartz, calcite, hydromagnesite, FeCO3 and anatase in a fixed ratio. The mineral mixture was pelletized and heat-treated up to 1100 °C in order to evaluate sintering by recording the compressive strength values and visual assessment with scanning electron microscopy (SEM). Chemical interactions responsible for the trends in the compressive strength results were investigated with simultaneous thermogravimetric and differential thermal analysis (TG/DTA), as well as X-ray diffraction. The results indicated that the formation of anhydrite (CaSO4) was responsible for increased mechanical strength in the mineral mixture pellets heated in air at temperatures higher that 400 °C. CaSO4 formed from the reaction of the decomposition products of pyrite and calcite (SOx and CaO). The TG/DTA results also indicated that the reaction with pyrite in air caused the decomposition of calcite in the mixture at a lower temperature than was observed for calcite only. The pellets heated in N2 did not increase in mechanical strength during heat-treatment due to the lack of CaSO4 formation in the inert atmosphere. However, SEM analysis indicated that sintering did occur at the higher temperatures in N2. A decrease was observed in the compressive strength values obtained in air at temperatures from 900 °C to 1100 °C. Reasons for the decreased compressive strengths may include increased porosity, decomposition of CaSO4, and changes in the characteristics of the aluminosilicate phases.
Structural analysis of chars generated from South African inertinite coals in a pipe-reactor combustion unit
08 October 2012
North West University

Bunt, John Reginald;Malumbazo, N.;Wagner, N.J.;Van Niekerk, D.;Assumption, H.

An inertinite-rich medium rank C bituminous South African coal was utilized to generate chars in a pipe-reactor combustion unit. This unit generates chars at atmospheric pressures and temperature was controlled with N2 to a maximum of 1250 °C. Chemical structural changes were investigated at different reaction zones identified in the pipe-reactor combustion unit. A combination of FTIR, XRD and Solid State NMR experiments were used to characterize the coal/char/ash fractions produced in the reactor. These techniques revealed that the coal structure becomes disordered in the drying zone as well as in the beginning of the pyrolysis zone in the reactor. As the temperature increases towards the base of the reactor the coal structure becomes more ordered and well aligned until char is formed and converted. Major structural changes were seen to occur in the drying to the pyrolysis zones. Structural changes within the molecular core were observed with FTIR and XRD results obtained from samples taken from the drying zone to the combustion zone. However, 13C CP/MAS and dipolar dephasing experiments were not able to corroborate these structural changes of the coal/char/ash fractions produced in the reactor occurring in the reduction and combustion zones.
Assessing the catalytic effect of coal ash constituents on the CO2 gasification rate of high ash, South African coal
08 October 2012
North West University

Hattingh, Burgert B.;Everson, Raymond C.;Neomagus, Hein W.J.P.;Bunt, John R.

The catalytic effect of inorganic species, within the ash, on the CO2 gasification of three South African coals containing similar carbon-structural properties (elemental, structural and petrographical properties) was assessed. The reactivity of the coals with a particle size between 150 and 250 μm was determined in a thermo gravimetric analyser. The reactivity was measured at temperatures between 900 and 1000 °C, pressures between 1 and 10 bar, and fractions of CO2 between 10 and 30%. For the selected coals, the reactivity decreased with ash content, and was found to be dependent on the composition of the ash. Specifically, the reactivity increased with calcium and magnesium content and alkali index.
Volatile trace element behaviour in the Sasol® fixed-bed dry-bottom (FBDB)™ gasifier treating coals of different rank
08 October 2012
North West University

Bunt, J.R.;Waanders, F.B.

Trace element simulation and validation of model predictions for the elements Hg, As, Se, Cd and Pb have recently been undertaken for the Sasol® FBDB™ gasification process operating on lump coal. The validation was conducted by interpolating the residual trace elements content remaining behind in the solid coal/char/ash fractions after sequential mining of a quenched commercial-scale gasifier operating on low rank grade C bituminous Highveld coal used for gasification in South Africa. This paper extends the research understanding by comparing the volatile trace element behaviour of these same elements, using the same gasification technology, but operating on North Dakota lignite. The focus will be on the behaviour of the volatile Class III trace elements: Hg, As, Se, Cd and Pb within the Sasol® FBDB™ gasifier as function of coal rank. This study excludes the downstream gas cleaning partitioning and speciation behaviour of these elements. Findings indicate that although the feed concentration and mode of occurrence of these elements differ quite substantially between the two coal types studied, that the volatilization profiles of the elements are indeed quite similar; being within 0.1%–15% lower in the case of the lignite when compared to the bituminous coal. In both cases, Hg was found to be the most volatile and As the least; with the volatility order varying slightly for the metals Se, Cd and Pb for the two coal types. The differences observed in the trace element volatilization rate are supported by the temperature profile which was inferred from the reflectance of vitrinite (RoV) measurements of the dissected fuel bed material. The highly reactive lignite, is successfully gasified at a lower temperature than is the case for bituminous coal using the Sasol® FBDB™ gasification process. Speciation predictions have earlier shown that: H2 Se, CdS, PbS/Pb/PbCl, and AsH3 species possibly exist in the gas phase. In reality, organically-associated trace elements will also be volatilized into the gas phase, but due to a lack of thermodynamic data for the lignite organo-metallic species at this stage only inorganic associations could be modelled.
Theory and design of low-noise multipath amplifiers
11 October 2012
North West University

Krüger, Petrus P.;Visser, Barend;De Jager, Ocker C.

The theoretical minimum noise measure of a multipath amplifier (an amplifier that has multiple parallel amplifiers) is achieved by using the optimum source impedance and optimum gain for each amplification path. This optimum source impedance and gain can be calculated by replacing each amplifier with its optimum source impedance. The resulting noise measure is the same as the minimum noise measure of the amplifiers used. The theory is demonstrated by applying it to distributed am plifiers. In an ideal distributed amplifier, the magnitude of the optimum gain of the amplification paths decreases and the phase delay increases the farther the stage is from the input, with the decrease in gain being faster for higher frequencies. The challenge in designing broadband low-noise distributed amplifiers is to achieve optimum gain matching over broad bandwidths. A numerical optimization procedure that separates source impedance and gain matching is proposed and demonstrated by optimizing a 0.5-2-GHz distributed amplifier. An average noise measure of 0.3 dB is achieved, which is only 0.1 dB higher than the minimum noise measure of the amplification stages used. This increase is due to transmission line loss and gain mismatch.
Pricing of single stock futures and dividend risk
22 October 2012
North West University

Venter, Emmerentia Hendrika Antoinetta;Swart, B.

In this paper we consider the fair pricing of single stock futures (SSFs) and the effect of dividend risk on the dividend compensation component in the pricing formulas. SSF valuation is subject to the pricing of discrete cash dividends (not percentages or dividend yields) in the underlying stock. Discrete cash dividends present modelling challenges which are not present when dividends are in the form of yields or percentages. Problems are created for market-makers and investors when the actual cash dividend is different from that predicted by analysts and used for pricing. We propose a new model for the fair price of a single stock futures contract which addresses dividend uncertainty.
On the propagation times and energy losses of cosmic rays in the heliosphere
25 October 2012
North West University

Strauss, R.D.;Potgieter, M.S.;Kopp, A.;Büsching, I.

We present calculations of the propagation times and energy losses of cosmic rays as they are transported through the heliosphere. By calculating these quantities for a spatially 1D scenario, we benchmark our numerical model, which uses stochastic differential equations to solve the relevant transport equation, with known analytical solutions. The comparison is successful and serves as a vindication of the modeling approach. A spatially 3D version of the modulation model is subsequently used to calculate the propagation times and energy losses of galactic electrons and protons in different drift cycles. We find that the propagation times of electrons are longer than those of the protons at the same energy. Furthermore, the propagation times are longer in the drift cycle when the particles reach the Earth by drifting inward along the heliospheric current sheet. The calculated energy losses follow this same general trend. The energy losses suffered by the electrons are comparable to those of the protons, which is in contrast to the generally held perception that electrons experience little energy losses during their propagation through the heliosphere.
Investigating leakage and bypass flows in an HTR using a CFD methodology
15 November 2012
North West University

Kleingeld, Marius;Janse van Rensburg, Jacobus Johannes

An area that has been identified as significantly important in the development of a high temperature reactor (HTR) is the prediction of leakage and bypass flows. It is therefore essential to understand the influence of leakage and bypass flows on the thermal performance of an HTR. A methodology was developed to conduct an integral thermal analysis of a reactor using a CFD approach. One of the main objectives was to include leakage and bypass flow paths in order to provide a capability for simulating these very detailed flows. This paper investigates leakage and bypass flows through the PBMR reactor unit. It was found that, although these flows are dependent on the pressure drop through the pebble bed, a change in pebble bed pressure drop does not result in a similar change in the predicted leakages flows. It is also shown that the ability to account for leakage and bypass flows in an integral manner can help designers to focus their efforts on the specific regions that need to be targeted for the improvement of the life expectancy of the graphite blocks. Furthermore, leakage and bypass flows were found to reduce the pressure drop across the reactor unit while increasing the peak fuel temperatures.
An integral CFD approach for the thermal simulation of the PBMR reactor unit
15 November 2012
North West University

Kleingeld, Marius;Janse van Rensburg, Jacobus Johannes

A CFD method was developed to conduct integral thermal reactor analysis for the complete Reactor Unit of the Pebble Bed Modular Reactor (Pty) Ltd (PBMR). The requirement was however also to include very detailed aspects such as leakage and bypass flow paths through the reflector blocks and sleeves. The aim was therefore to investigate the influence of leakage and bypass flow on the thermal performance of the Reactor Unit in an integral fashion. The focus of this paper is to discuss the methodology that was developed. The discussion will firstly highlight all the required inputs, elaborate briefly on the underlying theory and how this was implemented into the CFD modeling capability. Results will be discussed briefly, but the focus is on the methodology.
CFD applications in the Pebble Bed Modular Reactor Project: a decade of progress
15 November 2012
North West University

Kleingeld, Marius;Janse van Rensburg, Jacobus Johannes

Of all the systems and components that have to be designed for a nuclear plant, the Reactor Unit is the most significant since it is at the very heart of the plant. At Pebble Bed Modular Reactor (Pty) Ltd. (PBMR), the design of the Reactor Unit is conducted with the aid of extensive analysis work. Due to the rapid computational improvements, the analysis capabilities have had to evolve rather significantly over the last decade. This paper evaluates the evolution of RU Computational Fluid Dynamics (CFD) analysis in particular and presents a historical timeline of the analyses conducted at PBMR. The influence of advances in the hardware and software applications on the evolution of the analysis capabilities is also discussed. When evaluating the evolution of analysis, it is important to look not only at the advances in mesh generation and the representation of the geometry, but also at the improvements regarding the physics that were included in the models. The discussion evaluates the improvements from the pre-conceptual analyses, the concept design, the basic design and finally, the detail design. It is however important to note that the focus of this research was on establishing a methodology for the integrated CFD analysis of High Temperature Reactors. It is recognized however that results from this research can currently only be used to investigate and understand trends and behaviors rather than absolute values. It was therefore required to also launch an extensive V&V program of which the focus was to verify the approach and validate the methodology that was established. The final aim was therefore to combine the research into the methodology with that of V&V in an effort to determine uncertainty bands which would enable the researcher to supply absolute results with an uncertainty value attached.
LTI modelling of active magnetic bearings by means of system identification
07 December 2012
North West University

Van Schoor, George;Van Vuuren, Pieter Andries;Venter, Willem Christiaan

A relatively unknown phenomenon in active magnetic bearings (AMBs) is that the frequency content of their rotor position signal can induce nonlinear behaviour in the bearings. The existence of such frequency-induced nonlinear behaviour is experimentally and theoretically confirmed. Frequency-induced nonlinearity is characterised by means of a novel graphical representation. The resultant graph is quite useful in the specification of suitable excitation signals when AMBs are to be modelled by means of system identification.
Self-sensing for electromagnetic actuators. Part 1. A coupled reluctance network model approach
11 January 2013
North West University

Ranft, E.O.;Van Schoor, G.;Du Rand, C.P.

A self-sensing arrangement in active magnetic bearings (AMBs) comprises a single electromagnetic transducer to realize the actuation and sensing functions concurrently. Minimizing the number of sensing devices and associated interfacing directly reduces possible failure points, system costs, and system complexity. Currently, self-sensing performance is degraded due to problems such as magnetic cross-coupling, eddy currents, saturation, and high losses. This first paper in a two part series presents an integrated model for self-sensing of an 8-pole heteropolar magnetic bearing. The proposed self-sensing approach addresses mechanisms that contribute to modelling error and uncertainty by using several techniques in an integrated structure. A coupled reluctance network model (RNM) is developed which models the coil impedance at the switching frequency. The accuracy of the model is improved by incorporating terms for air gap fringing, complex permeability, and magnetic material nonlinearity. The RNM is verified and refined through a process of iteration using finite element method (FEM) results and experimental AMB measurements. The results demonstrate that a RNM with only 40 nodes can achieve high levels of accuracy when compared to an 80 000 node FEM analysis. In Part II of the series, the refined RNM is incorporated into a multiple input multiple output (MIMO) parameter estimation self-sensing scheme.
Self-sensing for electromagnetic actuators. Part 2. Position estimation
11 January 2013
North West University

Ranft, E.O.;Van Schoor, G.;Du Rand, C.P.

The second paper in a two part series presents the position estimation scheme for an 8-pole heteropolar active magnetic bearing (AMB). The integrated self-sensing assembly comprises a nonlinear MIMO parameter estimator together with a coupled reluctance network model (Part I), by which problems associated with magnetic cross-coupling and saturation are collectively addressed. The parameter estimator utilizes the first harmonic components of the current and voltage waveforms to determine estimates for the x and y rotor positions. Magnetic saturation is accounted for using a saturation factor, which scales the demodulated coil currents to ensure that the actuator with the lowest flux density contributes the most to the position estimate. Basic functionality and feasibility of the proposed self-sensing scheme are demonstrated via an experimentally validated transient simulation model (TSM). The TSM incorporates magnetic effects such as eddy currents, cross-coupling, and hysteresis. In the second part of this work, the static and dynamic performance of the self-sensing sensor is evaluated. The influence of magnetic cross-coupling, saturation, and duty cycle variation on the position estimate is documented. The results demonstrate the importance of including a mutual coupling term in the position estimation model in order to stably suspend the rotor. Furthermore, stability margin analyses indicate that the robustness of the magnetic bearing control is satisfactory for unrestricted long-term operation.
An integrated self-sensing approach for active magnetic bearings
11 January 2013
North West University

Ranft, E.O.;Van Schoor, G.;Du Rand, C.P.

Self-sensing permits active magnetic bearings (AMBs) to consolidate the actuation and sensing functions into a single electromagnetic transducer. Eliminating the position sensing device as well as interfacing reduce potential system failure points, costs, and complexity. Self-sensing performance at present faces technical challenges such as magnetic cross-coupling, saturation, eddy currents, and system robustness. This work proposes an integrated self-sensing approach to collectively address mechanisms that contribute to modelling errors and position estimation inaccuracy. The self-sensing approach is based on the amplitude modulation technique and comprises a coupled reluctance network model (RNM) that is embedded in a nonlinear multiple input multiple output parameter estimator. The estimator employs a frequency-shifted model that is solved at a lower frequency to increase system performance. Furthermore, the RNM incorporates air gap fringing, complex permeability, and magnetic material nonlinearity terms. Magnetic saturation is accounted for using current scaling weights in the position estimation scheme. Basic functionality of the integrated self-sensing approach is demonstrated using an experimentally verified transient simulation model of the magnetic bearing. Verification and refinement of the RNM is accomplished through an iteration process using finite element method (FEM) results and experimental measurements. The simulation results show that the 40 node RNM can be accurate compared to an 80 000 node FEM analysis. Evaluation of the system stability margin indicates that the robustness of the magnetic bearing control is suitable for unrestricted long-term operation.
Galactic cosmic rays in the outer heliosphere: theory and models
25 January 2013
North West University

Florinski, V.;Ferreira, S.E.S.;Pogorelov, N.V.

We review recent advances in the field of galactic cosmic ray transport in the distant heliosphere. The advent of globalMHD models brought about a better understanding of the three-dimensional structure of the interface between the solar system and the surrounding interstellar space, and of the magnetic field topology in the outer heliosphere. These results stimulated a development of galactic cosmic ray transport models taking the advantage of the available detailed plasma backgrounds and of the new Voyager results from the heliosheath. It emerges that the heliosheath plays a prominent role in the process of modulation and filtration of low-energy galactic ions and electrons. The heliosheath stores particles for a duration of several years thus acting as a large reservoir of galactic cosmic rays. Cosmic-ray trajectories, transit times, and entry locations across the heliopause are discussed. When compared to observations model calculations of low energy electrons show almost no radial gradient up to the termination shock, irrespective of solar activity, but a large gradient in the inner heliosheath. Intensities are however sensitive to heliospheric conditions such as the location of the heliopause and shock. In contrast, high energy proton observations by both the Voyager spacecraft show a clear solar cycle dependence with intensities also increasing with increasing distance. By comparing these observations to model calculations we can establish whether our current understanding of long-term modulation result in computed intensities compatible to observations.
The time structure of ground level enhancements in solar cycle 23
25 January 2013
North West University

Moraal, H.;McCracken, K.G.

In a recent paper McCracken et al. (J. Geophys. Res. 113:A12101, 2008) proposed that the Ground Level Enhancement (GLE) of 20 January 2005 may have been produced by more than one acceleration mechanism, with the first acceleration due to the solar flare and the second one due to the CME associated with that event. They also noted several other GLEs with similar multiple pulse structures. This paper systematically investigates all the GLEs of solar cycle 23, from GLE 55 on 6 November 1997 to GLE 70 on 13 December 2006, to study their morphology and pulse structure, and to determine whether the multiple structures that may be found in these events are qualitatively similar to that of the GLE of 20 January 2005. We use all the data of all NMs that saw each event, to have as much directional and spectral information as possible. It is shown that three of these 16 events do contain such double-pulse structures, and the properties of these three are discussed in some detail.
Cosmic rays in the inner heliosphere: insights from observations, theory and models
25 January 2013
North West University

Potgieter, M.S.

The global modulation of galactic cosmic rays in the inner heliosphere is determined by four major mechanisms: convection, diffusion, particle drifts (gradient, curvature and current sheet drifts), and adiabatic energy losses. When these processes combine to produce modulation, the complexity increases significantly especially when one wants to describe how they evolve spatially in all three dimensions throughout the heliosphere, and with time, as a function of solar activity over at least 22 years. In this context also the global structure and features of the solar wind, the heliospheric magnetic field, the wavy current sheet, and of the heliosphere and its interface with the interstellar medium, play important roles. Space missions have contributed significantly to our knowledge during the past decade. In the inner heliosphere, Ulysses and several other missions have contributed to establish the relative importance of these major mechanisms, leading to renewed interest in developing more sophisticated theories and numerical models to explain these observations, and to understand the underlying physics that determines galactic cosmic ray modulation at Earth. An overview is given of some of the observational and modeling highlights over the past decade.
Law, the laws of nature and ecosystem energy services: a case of wilful blindness
14 August 2013
North West University

Hodas, David R

Ecosystems services include the collection, concentration, and storage of solar energy as fossil fuels (e.g., coal, petroleum, and natural gas). These concentrated forms of energy were produced by ancient ecosystem services. However, our legal and economic systems fail to recognise the value of the ecosystem service subsidies embedded in fossil fuels. This ecosystem services price subsidy causes overuse and waste of fossil fuels in the free market: fossil fuels are consumed more quickly than they can be replaced by ecosystem services and in far larger quantities than they would be if the price of fossil fuels included the cost of solar energy collection, concentration and manufacturing of raw fossil fuels. Moreover, burning fossil fuels produces enormous environmental, human health and welfare costs and damage. Virtually no legal literature on ecosystem services, sustainable development, or sustainable energy, considers fossil fuels in this context. Without understanding stored energy as an ecosystem service, we cannot reasonably expect to manage our fossil fuel energy resources sustainably. International and domestic energy law and policy systems generally ignore this feature of fossil fuel energy, a blind spot that explains why reducing greenhouse gas emissions from fossil fuels is fundamentally a political challenge. This paper will use new understandings emerging from the field of complex systems to critique existing legal decision-making models that do not adequately account for energy ecosystem services in policy design, resource allocation and project approvals. The paper proposes a new "least-social-cost" decision-making legal structure that includes ecosystem energy services.
Modelling of galactic carbon in an asymmetrical heliosphere: effects of asymmetrical modulation conditions
23 September 2013
North West University

Ngobeni, M.D.;Potgieter, M.S.

Observations of galactic cosmic rays (GCRs) from the two Voyager spacecraft inside the heliosheath indicate significant differences between them, suggesting that in addition to a possible global asymmetry in the north–south dimensions (meridional plane) of the heliosphere, it is also possible that different modulation (turbulence) conditions could exist between the two hemispheres of the heliosphere. We focus on illustrating the effects on GCR Carbon of asymmetrical modulation conditions combined with a heliosheath thickness that has a significant dependence on heliolatitude. To reflect different modulation conditions between the two heliospheric hemispheres in our numerical model, the enhancement of both polar and radial perpendicular diffusion off the ecliptic plane is assumed to differ from heliographic pole to pole. The computed radial GCR intensities at polar angles of 55° (approximating the Voyager 1 direction) and 125° (approximating the Voyager 2 direction) are compared at different energies and for both particle drift cycles. This is done in the context of illustrating how different values of the enhancement of both polar and radial perpendicular diffusion between the two hemispheres contribute to causing differences in radial intensities during solar minimum and moderate maximum conditions. We find that in the A > 0 cycle these differences between 55° and 125° change both quantitatively and qualitatively for the assumed asymmetrical modulation condition as reflected by polar diffusion, while in the A < 0 cycle, minute quantitative differences are obtained. However, when both polar and radial perpendicular diffusion have significant latitude dependences, major differences in radial intensities between the two polar angles are obtained in both polarity cycles. Furthermore, significant differences in radial intensity gradients obtained in the heliosheath at lower energies may suggest that the solar wind turbulence at and beyond the solar wind termination shock must have a larger latitudinal dependence.
Adapting plant measurement data to improve hardware fault detection performance in pressurised water reactors
24 October 2013
North West University

Cilliers, Anthonie Christoffel;Mulder, Eben Johan

With the fairly recent adoption of digital control and instrumentation systems in the nuclear industry a lot of research now focus on the development expert fault identification systems. The fault identification systems enable detecting early onset faults of fault causes which allows maintenance planning on the equipment showing signs of deterioration or failure. This includes valve and leaks and small cracks in steam generator tubes usually detected by means of ultrasonic inspection. Detecting faults early during transient operation in NPPs is problematic due to the absence of a reliable reference to compare plant measurements with during transients. The distributed application of control systems operating independently to keep the plant operating within the safe operating boundaries complicates the problem since the control systems would not only operate to reduce the effect of transient disturbances but fault disturbances as well. This paper provides a method to adapt the plant measurements that isolates the control actions on the fault and re-introduces it into the measurement data, thereby improving plant diagnostic performance.
Fault diagnosis of generation IV nuclear HTGR components. Part 2. The area error enthalpy-entropy graph approach
24 October 2013
North West University

Du Rand, C.P.;Van Schoor, G.

The second paper in a two part series presents the area error method for generation of representative enthalpy–entropy (h–s) fault signatures to classify malfunctions in generation IV nuclear high temperature gas-cooled reactor (HTGR) components. The second classifier is devised to ultimately address the fault diagnosis (FD) problem via the proposed methods in a multiple classifier (MC) ensemble. FD is realized by way of different input feature sets to the classification algorithm based on the area and trajectory of the residual shift between the fault-free and the actual operating h–s graph models. The application of the proposed technique is specifically demonstrated for 24 single fault transients considered in the main power system (MPS) of the Pebble Bed Modular Reactor (PBMR). The results show that the area error technique produces different fault signatures with low correlation for all the examined component faults. A brief evaluation of the two fault signature generation techniques is presented and the performance of the area error method is documented using the fault classification index (FCI) presented in Part I of the series. The final part of this work reports the application of the proposed approach for classification of an emulated fault transient in data from the prototype Pebble Bed Micro Model (PBMM) plant. Reference data values are calculated for the plant via a thermo-hydraulic simulation model of the MPS. The results show that the correspondence between the fault signatures, generated via experimental plant data and simulated reference values, are generally good. The work presented in the two part series, related to the classification of component faults in the MPS of different HTGRs, suggest that the proposed h–s graph approach provides a suitable FD framework for generation IV nuclear energy systems.
Fault diagnosis of generation IV nuclear HTGR components. Part 1. The error enthalpy-entropy graph approach
24 October 2013
North West University

Du Rand, C.P.;Van Schoor, G.

Fault diagnosis (FD) is an important element in modern nuclear power plant (NPP) diagnostic systems. In this respect, FD of generation II and III water-cooled nuclear energy systems has become an active research topic to continually improve levels of reliability, safety, and operation. However, evolutionary advances in reactor and component technology together with different energy conversion methodologies support the investigation of alternative approaches to FD. Within this context, the basic aim of this two part series is to adopt the application of the enthalpy–entropy (h–s) graph approach for FD of generation IV nuclear high temperature gas-cooled reactor (HTGR) components. In Part I, the error method is utilized to derive fault patterns from the h–s graph in order to classify malfunctions via the fault classification index (FCI) in the nuclear reactor, turbo-machinery (gas turbine and compressors), heat exchangers (pre-cooler, intercooler, and recuperator) and the primary transporting medium of the working fluid. The study is conducted on a 165 MW model of the main power system (MPS) of the Pebble Bed Modular Reactor (PBMR) that is based on a single-shaft, closed-loop, direct Brayton thermodynamic cycle. Illustrative signatures that correspond to 24 single fault transients, categorized in three fault classes by means of a sensitivity analysis of a simplified HTGR, are presented. FD is demonstrated for steady state operation as well as load following of the MPS during normal power operation of the plant. In Part II of the series, a second classifier named the area error method is devised for NPP supervision to ultimately address the FD problem using a multiple classifier system. The application of the proposed h–s graph approach (both methods) is specifically illustrated for classification of an emulated fault transient in data from the real prototype Pebble Bed Micro Model (PBMM) plant.
Pebble Bed Micro Model system identification
30 October 2013
North West University

Venter, W.C.;Lamprecht, E.C.

Linear system identiﬁcation techniques will be applied to obtain mathematical models of a simulation of the Pebble Bed Micro Model (PBMM). The PBMM is a conceptual model of the Pebble Bed Modular Reactor (PBMR), a new high temperature gas nuclear reactor. The basis of the PBMR and PBMM is a three-phase Brayton Power Generation Cycle. The Brayton cycle will in this paper be simulated by the use of Flownex, a general thermal-ﬂuid network analysis code solver. A description of the Brayton Power Generation Cycle, as well as description of the Flownex modelling program, will also be given.
A multiwavelength view of the flaring state of PKS 2155-304 in 2006
31 October 2013
North West University

Abramowski, A.;Büsching, I.;De Jager, O.C.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

Context: Multiwavelength (MWL) observations of the blazar PKS 2155-304 during two weeks in July and August 2006, the period when two exceptional flares at very high energies (VHE, E ≳ 100 GeV) occurred, provide a detailed picture of the evolution of its emission. The complete data set from this campaign is presented, including observations in VHE γ-rays (H.E.S.S.), X-rays (RXTE, Chandra, Swift XRT), optical (Swift UVOT, Bronberg, Watcher, ROTSE), and in the radio band (NRT, HartRAO, ATCA). Optical and radio light curves from 2004 to 2008 are compared to the available VHE data from this period, to put the 2006 campaign into the context of the long-term evolution of the source. Aims: The data set offers a close view of the evolution of the source on different time scales and yields new insights into the properties of the emission process. The predictions of synchrotron self-Compton (SSC) scenarios are compared to the MWL data, with the aim of describing the dominant features in the data down to the hour time scale. Methods: The spectral variability in the X-ray and VHE bands is explored and correlations between the integral fluxes at different wavelengths are evaluated. SSC modelling is used to interpret the general trends of the varying spectral energy distribution. Results: The X-ray and VHE γ-ray emission are correlated during the observed high state of the source, but show no direct connection with longer wavelengths. The long-term flux evolution in the optical and radio bands is found to be correlated and shows that the source reaches a high state at long wavelengths after the occurrence of the VHE flares. Spectral hardening is seen in the Swift XRT data. Conclusions: The nightly averaged high-energy spectra of the non-flaring nights can be reproduced by a stationary one-zone SSC model, with only small variations in the parameters. The spectral and flux evolution in the high-energy band during the night of the second VHE flare is modelled with multi-zone SSC models, which can provide relatively simple interpretations for the hour time-scale evolution of the high-energy emission, even for such a complex data set. For the first time in this type of source, a clear indication is found for a relation between high activity at high energies and a long-term increase in the low frequency fluxes.
Discovery of extended VHE Y-ray emission from the vicinity of the young massive stellar cluster Westerlund 1
07 November 2013
North West University

Abramowski, A.;Büsching, I.;De Jager, O.C.;Sheidaei, F.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

Aims. Results obtained in very-high-energy (VHE; E≥100GeV) γ-ray observations performed with the H.E.S.S. telescope array are used to investigate particle acceleration processes in the vicinity of the young massive stellar cluster Westerlund 1 (Wd 1). Methods. Imaging of Cherenkov light from γ-ray induced particle cascades in the Earth’s atmosphere is used to search for VHE γ rays from the region around Wd 1. Possible catalogued counterparts are searched for and discussed in terms of morphology and energetics of the H.E.S.S. source. Results. The detection of the degree-scale extended VHE γ-ray source HESS J1646–458 is reported based on 45 h of H.E.S.S. observations performed between 2004 and 2008. The VHE γ-ray source is centred on the nominal position of Wd 1 and detected with a total statistical significance of ~20σ. The emission region clearly extends beyond the H.E.S.S. point-spread function (PSF). The differential energy spectrum follows a power law in energy with an index of Γ=2.19±0.08stat±0.20sys and a flux normalisation at 1 TeV of Φ = (9.0±1.4stat±1.8sys)×10-12 TeV-1 cm-2s-1. The integral flux above 0.2 TeV amounts to (5.2±0.9)×10-11 cm-2 s-1. Conclusions. Four objects coincident with HESS J1646–458 are discussed in the search of a counterpart, namely the magnetar CXOU J164710.2−455216, the X-ray binary 4U 1642–45, the pulsar PSR J1648–4611 and the massive stellar cluster Wd 1. In a single-source scenario, Wd 1 is favoured as site of VHE particle acceleration. Here, a hadronic parent population would be accelerated within the stellar cluster. Beside this, there is evidence for a multi-source origin, where a scenario involving PSR J1648–4611 could be viable to explain parts of the VHE γ-ray emission of HESS J1646–458.
Discovery of gamma-ray emission from the extragalactic pulsar wind nebula N 157B with H.E.S.S
07 November 2013
North West University

Abramowski, A.;Casanova, S.;De Jager, O.C.;Pekeur, N.W.;Sheidaei, F.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

We present the significant detection of the first extragalactic pulsar wind nebula (PWN) detected in gamma rays, N 157B, located in the large Magellanic Cloud (LMC). Pulsars with high spin-down luminosity are found to power energised nebulae that emit gamma rays up to energies of several tens of TeV. N 157B is associated with PSR J0537−6910, which is the pulsar with the highest known spin-down luminosity. The High Energy Stereoscopic System telescope array observed this nebula on a yearly basis from 2004 to 2009 with a dead-time corrected exposure of 46 h. The gamma-ray spectrum between 600 GeV and 12 TeV is well-described by a pure power-law with a photon index of 2.8 ± 0.2stat ± 0.3syst and a normalisation at 1 TeV of (8.2 ± 0.8stat ± 2.5syst) × 10-13 cm-2 s-1 TeV-1. A leptonic multi-wavelength model shows that an energy of about 4 × 1049 erg is stored in electrons and positrons. The apparent efficiency, which is the ratio of the TeV gamma-ray luminosity to the pulsar’s spin-down luminosity, 0.08% ± 0.01%, is comparable to those of PWNe found in the Milky Way. The detection of a PWN at such a large distance is possible due to the pulsar’s favourable spin-down luminosity and a bright infrared photon-field serving as an inverse-Compton-scattering target for accelerated leptons. By applying a calorimetric technique to these observations, the pulsar’s birth period is estimated to be shorter than 10 ms.
Discovery of hard-spectrum y-ray emission from the BL Lacertae object 1ES 0414+009
07 November 2013
North West University

Abramowski, A.;Büsching, I.;De Jager, O.C.;Sheidaei, F.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

Context. 1ES 0414 + 009 (z = 0.287) is a distant high-frequency-peaked BL Lac object, and has long been considered a likely emitter of very-high-energy (VHE, E > 100 GeV) γ-rays due to its high X-ray and radio flux. Aims. Observations in the VHE γ-ray band and across the electromagnetic spectrum can provide insights into the origin of highly energetic particles present in the source and the radiation processes at work. Because of the distance of the source, the γ-ray spectrum might provide further limits on the level of the extragalactic background light (EBL). Methods. We report observations made between October 2005 and December 2009 with H.E.S.S., an array of four imaging atmospheric Cherenkov telescopes. Observations at high energies (HE, 100 MeV–100 GeV) with the Fermi-LAT instrument in the first 20 months of its operation are also reported. To complete the multi-wavelength picture, archival UV and X-ray observations with the Swift satellite and optical observations with the ATOM telescope are also used. Results. Based on the observations with H.E.S.S., 1ES 0414 + 009 is detected for the first time in the VHE band. An excess of 224 events is measured, corresponding to a significance of 7.8σ. The photon spectrum of the source is well described by a power law, with photon index of ΓVHE = 3.45 ± 0.25stat ± 0.20syst. The integral flux above 200 GeV is (1.88 ± 0.20stat ± 0.38syst) × 10-12 cm-2 s-1. Observations with the Fermi-LAT in the first 20 months of operation show a flux between 200 MeV and 100 GeV of (2.3 ± 0.2stat) × 10-9 erg cm-2 s-1, and a spectrum well described by a power-law function with a photon index ΓHE = 1.85 ± 0.18. Swift/XRT observations show an X-ray flux between 2 and 10 keV of (0.8−1) × 10-11 erg cm-2 s-1, and a steep spectrum ΓX = (2.2−2.3). Combining X-ray with optical-UV data, a fit with a log-parabolic function locates the synchrotron peak around 0.1 keV. Conclusions. Although the GeV–TeV observations do not provide better constraints on the EBL than previously obtained, they confirm a low density of the EBL, close to the lower limits from galaxy counts. The absorption-corrected HE and VHE γ-ray spectra are both hard and have similar spectral indices (≈1.86), indicating no significant change of slope between the HE and VHE γ-ray bands, and locating the γ-ray peak in the SED above 1–2 TeV. As for other TeV BL Lac objects with the γ-ray peak at such high energies and a large separation between the two SED humps, this average broad-band SED represents a challenge for simple one-zone synchrotron self-Compton models, requiring a high Doppler factor and very low B-field.
Discovery of VHE emission towards the Carina arm region with the H.E.S.S. telescope array: HESS J1018-589
07 November 2013
North West University

Abramowski, A.;Büsching, I.;Casanova, S.;De Jager, O.C.;Pekeur, N.W.;Sheidaei, F.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

The Carina arm region, containing the supernova remnant SNR G284.3–1.8, the high-energy (HE; E > 100 MeV) binary 1FGL J1018.6–5856 and the energetic pulsar PSR J1016–5857 and its nebula, has been observed with the H.E.S.S. telescope array. The observational coverage of the region in very-high-energy (VHE; E > 0.1 TeV) γ-rays benefits from deep exposure (40 h) of the neighboring open cluster Westerlund 2. The observations have revealed a new extended region of VHE γ-ray emission. The new VHE source HESS J1018–589 shows a bright, point-like emission region positionally coincident with SNR G284.3–1.8 and 1FGL J1018.6–5856 and a diffuse extension towards the direction of PSR J1016–5857. A soft (Γ = 2.7 ± 0.5stat) photon index, with a differential flux at 1 TeV of N0 = (4.2 ± 1.1) × 10-13 TeV-1 cm-2 s-1 is found for the point-like source, whereas the total emission region including the diffuse emission region is well fit by a power-law function with spectral index Γ = 2.9 ± 0.4stat and differential flux at 1 TeV of N0 = (6.8 ± 1.6) × 10-13 TeV-1 cm-2 s-1. This H.E.S.S. detection motivated follow-up X-ray observations with the XMM-Newton satellite to investigate the origin of the VHE emission. The analysis of the XMM-Newton data resulted in the discovery of a bright, non-thermal point-like source (XMMU J101855.4–58564) with a photon index of Γ = 1.65 ± 0.08 in the center of SNR G284.3–1.8, and a thermal, extended emission region coincident with its bright northern filament. The characteristics of this thermal emission are used to estimate the plasma density in the region as n ≈ 0.5 cm-3 (2.9 kpc/d)2. The position of XMMU J101855.4–58564 is compatible with the position reported by the Fermi-LAT collaboration for the binary system 1FGL J1018.6–5856 and the variable Swift XRT source identified with it. The new X-ray data are used alongside archival multi-wavelength data to investigate the relationship between the VHE γ-ray emission from HESS J1018–589 and the various potential counterparts in the Carina arm region.
Discovery of VHE y-ray emission and multi-wavelength observations of the BL Lacertae object 1RXS J101015.9-311909
07 November 2013
North West University

Abramowski, A.;Büsching, I.;Casanova, S.;De Jager, O.C.;Pekeur, N.W.;Sheidaei, F.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

1RXS J101015.9 − 311909 is a galaxy located at a redshift of z = 0.14 hosting an active nucleus (called AGN) belonging to the class of bright BL Lac objects. Observations at high (HE, E > 100 MeV) and very high (VHE, E > 100 GeV) energies provide insights into the origin of very energetic particles present in such sources and the radiation processes at work. We report on results from VHE observations performed between 2006 and 2010 with the H.E.S.S. instrument, an array of four imaging atmospheric Cherenkov telescopes. H.E.S.S. data have been analysed with enhanced analysis methods, making the detection of faint sources more significant. VHE emission at a position coincident with 1RXS J101015.9 − 311909 is detected with H.E.S.S. for the first time. In a total good-quality livetime of about 49 h, we measure 263 excess counts, corresponding to a significance of 7.1 standard deviations. The photon spectrum above 0.2 TeV can be described by a power-law with a photon index of Γ = 3.08 ± 0.42stat ± 0.20sys. The integral flux above 0.2 TeV is about 0.8% of the flux of the Crab nebula and shows no significant variability over the time reported. In addition, public Fermi/LAT data are analysed to search for high energy emission from the source. The Fermi/LAT HE emission in the 100 MeV to 200 GeV energy range is significant at 8.3 standard deviations in the chosen 25-month dataset. UV and X-ray contemporaneous observations with the Swift satellite in May 2007 are also reported, together with optical observations performed with the atom telescope located at the H.E.S.S. site. Swift observations reveal an absorbed X-ray flux of F(0.3-7) keV = 1.04+0.04-0.05 × 10-11 erg cm-2 s-1 in the 0.3 − 7 keV range. Finally, all the available data are used to study the multi-wavelength properties of the source. The spectral energy distribution (SED) can be reproduced using a simple one-zone Synchrotron Self Compton (SSC) model with emission from a region with a Doppler factor of 30 and a magnetic field between 0.025 and 0.16 G. These parameters are similar to those obtained for other sources of this type.
Identification of HESS J1303-631 as a pulsar wind nebula through y-ray, x-ray, and radio observations
07 November 2013
North West University

Abramowski, A.;Büsching, I.;Casanova, S.;Pekeur, N.W.;Sheidaei, F.;Venter, C.;Vorster, M.

Aims. The previously unidentified very high-energy (VHE; E > 100 GeV) γ-ray source HESS J1303−631, discovered in 2004, is re-examined including new data from the H.E.S.S. Cherenkov telescope array in order to identify this object. Archival data from the XMM-Newton X-ray satellite and from the PMN radio survey are also examined. Methods. Detailed morphological and spectral studies of VHE γ-ray emission as well as of the XMM-Newton X-ray data are performed. Radio data from the PMN survey are used as well to construct a leptonic model of the source. The γ-ray and X-ray spectra and radio upper limit are used to construct a one zone leptonic model of the spectral energy distribution (SED). Results. Significant energy-dependent morphology of the γ-ray source is detected with high-energy emission (E > 10 TeV) positionally coincident with the pulsar PSR J1301−6305 and lower energy emission (E < 2 TeV) extending ~0.4° to the southeast of the pulsar. The spectrum of the VHE source can be described with a power-law with an exponential cut-off N0 = (5.6 ± 0.5) × 10-12 TeV-1 cm-2 s-1, Γ = 1.5 ± 0.2) and Ecut = (7.7 ± 2.2) TeV. The pulsar wind nebula (PWN) is also detected in X-rays, extending ~2−3′ from the pulsar position towards the center of the γ-ray emission region. A potential radio counterpart from the PMN survey is also discussed, showing a hint for a counterpart at the edge of the X-ray PWN trail and is taken as an upper limit in the SED. The extended X-ray PWN has an unabsorbed , Bò of 6.5σ. The SED is well described by a one zone leptonic scenario which, with its associated caveats, predicts a very low average magnetic field for this source. Conclusions. Significant energy-dependent morphology of this source, as well as the identification of an associated X-ray PWN from XMM-Newton observations enable identification of the VHE source as an evolved PWN associated to the pulsar PSR J1301−6305. This identification is supported by the one zone leptonic model, which suggests that the energetics of the γ-ray and X-ray radiation are such that they may have a similar origin in the pulsar nebula. However, the large discrepancy in emission region sizes and the low level of synchrotron radiation suggest a multi-population leptonic nature. The low implied magnetic field suggests that the PWN has undergone significant expansion. This would explain the low level of synchrotron radiation and the difficulty in detecting counterparts at lower energies, the reason this source was originally classified as a “dark” VHE γ-ray source.
Probing the extent of the non-thermal emission from the Vela X region at TeV energies with H.E.S.S.
07 November 2013
North West University

Abramowski, A.;Casanova, S.;Krüger, P.P.;Pekeur, N.W.;Sheidaei, F.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

Context. Vela X is a region of extended radio emission in the western part of the Vela constellation: one of the nearest pulsar wind nebulae, and associated with the energetic Vela pulsar (PSR B0833-45). Extended very-high-energy (VHE) γ-ray emission (HESS J0835−455) was discovered using the H.E.S.S. experiment in 2004. The VHE γ-ray emission was found to be coincident with a region of X-ray emission discovered with ROSAT above 1.5 keV (the so-called Vela X cocoon): a filamentary structure extending southwest from the pulsar to the centre of Vela X. Aims. A deeper observation of the entire Vela X nebula region, also including larger offsets from the cocoon, has been performed with H.E.S.S. This re-observation was carried out in order to probe the extent of the non-thermal emission from the Vela X region at TeV energies and to investigate its spectral properties. Methods. To increase the sensitivity to the faint γ-ray emission from the very extended Vela X region, a multivariate analysis method combining three complementary reconstruction techniques of Cherenkov-shower images is applied for the selection of γ-ray events. The analysis is performed with the On/Off background method, which estimates the background from separate observations pointing away from Vela X; towards regions free of γ-ray sources but with comparable observation conditions. Results. The γ-ray surface brightness over the large Vela X region reveals that the detection of non-thermal VHE γ-ray emission from the PWN HESS J0835−455 is statistically significant over a region of radius 1.2° around the position α = 08h35m00s, δ = −45°36′00′′ (J2000). The Vela X region exhibits almost uniform γ-ray spectra over its full extent: the differential energy spectrum can be described by a power-law function with a hard spectral index Γ = 1.32 ± 0.06stat ± 0.12sys and an exponential cutoff at an energy of (14.0 ± 1.6stat ± 2.6sys) TeV. Compared to the previous H.E.S.S. observations of Vela X the new analysis confirms the general spatial overlap of the bulk of the VHE γ-ray emission with the X-ray cocoon, while its extent and morphology appear more consistent with the (more extended) radio emission, contradicting the simple correspondence between VHE γ-ray and X-ray emissions. Morphological and spectral results challenge the interpretation of the origin of γ-ray emission in the GeV and TeV ranges in the framework of current models.
A generalized diffusion tensor for fully anisotropic diffusion of energetic particles in the heliospheric magnetic field
08 November 2013
North West University

Effenberger, F.;Strauss, R.D.;Fichtner, H.;Scherer, K.;Barra, S.;Kleimann, J.

The spatial diffusion of cosmic rays in turbulent magnetic fields can, in the most general case, be fully anisotropic, i.e., one has to distinguish three diffusion axes in a local, field-aligned frame. We reexamine the transformation for the diffusion tensor from this local to a global frame, in which the Parker transport equation for energetic particles is usually formulated and solved. Particularly, we generalize the transformation formulae to allow for an explicit choice of two principal local perpendicular diffusion axes. This generalization includes the “traditional” diffusion tensor in the special case of isotropic perpendicular diffusion. For the local frame, we describe the motivation for the choice of the Frenet–Serret trihedron, which is related to the intrinsic magnetic field geometry. We directly compare the old and the new tensor elements for two heliospheric magnetic field configurations, namely the hybrid Fisk and Parker fields. Subsequently, we examine the significance of the different formulations for the diffusion tensor in a standard three-dimensional model for the modulation of galactic protons. For this, we utilize a numerical code to evaluate a system of stochastic differential equations equivalent to the Parker transport equation and present the resulting modulated spectra. The computed differential fluxes based on the new tensor formulation deviate from those obtained with the “traditional” one (only valid for isotropic perpendicular diffusion) by up to 60% for energies below a few hundred MeV depending on heliocentric distance.
Modeling drift along the heliospheric wavy neutral sheet
11 November 2013
North West University

Burger, R.A.

Drift along the wavy heliospheric neutral sheet is believed to play an important role in cosmic-ray modulation and can explain the peaked versus flat intensity profiles during consecutive solar magnetic epochs. Modulation models are becoming more and more realistic and in order to determine the role of the wavy neutral sheet more accurately, we revisit a previous calculation for drift along it. While mathematically correct, we argue that the previous expression for neutral sheet drift, which follows naturally from the standard expression for gradient and curvature drift, must be adapted in order for the drift speed to be less than particle speed. We compare the effect of both the previous and the current more accurate version of neutral sheet drift on cosmic-ray modulation with results obtained by other methods.
Spectral analysis and interpretation of the Y-ray emission from the starburst galaxy NGC 253
13 November 2013
North West University

Abramowski, A.;Büsching, I.;Casanova, S.;Pekeur, N.W.;Sheidaei, F.;Venter, C.;Vorster, M.;H.E.S.S. Collaboration

Very high energy (VHE; E ≥ 100 GeV) and high-energy (HE; 100 MeV ≤ E ≤ 100 GeV) data from γ-ray observations performed with the H.E.S.S. telescope array and the Fermi-LAT instrument, respectively, are analyzed in order to investigate the non-thermal processes in the starburst galaxy NGC 253. The VHE γ-ray data can be described by a power law in energy with differential photon index Γ = 2.14 ± 0.18stat ± 0.30sys and differential flux normalization at 1 TeV of F 0 = (9.6 ± 1.5stat(+ 5.7, –2.9)sys) × 10–14 TeV–1 cm–2 s–1. A power-law fit to the differential HE γ-ray spectrum reveals a photon index of Γ = 2.24 ± 0.14stat ± 0.03sys and an integral flux between 200 MeV and 200 GeV of F(0.2-200 GeV) = (4.9 ± 1.0stat ± 0.3sys) × 10–9 cm–2 s–1. No evidence for a spectral break or turnover is found over the dynamic range of both the LAT instrument and the H.E.S.S. experiment: a combined fit of a power law to the HE and VHE γ-ray data results in a differential photon index Γ = 2.34 ± 0.03 with a p-value of 30%. The γ-ray observations indicate that at least about 20% of the energy of the cosmic rays (CRs) capable of producing hadronic interactions is channeled into pion production. The smooth alignment between the spectra in the HE and VHE γ-ray domain suggests that the same transport processes dominate in the entire energy range. Advection is most likely responsible for charged particle removal from the starburst nucleus from GeV to multiple TeV energies. In a hadronic scenario for the γ-ray production, the single overall power-law spectrum observed would therefore correspond to the mean energy spectrum produced by the ensemble of CR sources in the starburst region.
The high-energy impulsive ground-level enhancement
13 November 2013
North West University

McCracken, K.G.;Moraal, H.;Shea, M.A.

We have studied short-lived (21 minute average duration), highly anisotropic pulses of cosmic rays that constitute the first phase of 10 large ground-level enhancements (GLEs), and which extend to rigidities in the range 5-20 GV. We provide a set of constraints that must be met by any putative acceleration mechanism for this type of solar-energetic-particle (SEP) event. The pulses usually have very short rise-times (three to five minutes) at all rigidities, and exhibit the remarkable feature that the intensity drops precipitously by 50% to 70% from the maximum within another three to five minutes. Both the rising and falling phases exhibit velocity dispersion, which indicates that there are particles with rigidities in the range 1 90 MeV gamma-ray bursts, indicating that freshly accelerated SEPs had impinged on higher-density matter in the chromosphere prior to the departure of the SEP pulse for Earth. This study was based on an updated archive of the 71 GLEs in the historic record, which is now available for public use.
Three-dimensional wavy heliospheric current sheet drifts
13 November 2013
North West University

Pei, C.;Burger, R.A.;Bieber, J.W.;Clem, J.

We present an analytic method to determine the directions of the three-dimensional (3D) heliospheric current sheet (HCS) drift for any tilt angle based on Parker's heliospheric magnetic field and compare it with published two-dimensional and quasi-3D methods. We also present a new approach to determine the magnitude of the 3D HCS drift numerically. Implications of these new methods for the solar modulation of Galactic cosmic rays are considered and compared with results from prior methods reported in the literature. Our results support the concept that HCS drift plays an important role in the solar modulation of cosmic rays.
Modelling heliospheric current sheet drift in stochastic cosmic ray transport models
13 November 2013
North West University

Strauss, R.D.;Potgieter, M.S.;Büsching, I.;Kopp, A.

Drifts are one of the major cosmic ray modulation mechanisms in the heliosphere. Three types of drifts occur in the background heliospheric magnetic field, namely curvature, gradient and current sheet drifts. The last compo- nent occurs because of the switch in magnetic field polarity across the heliospheric current sheet and is the main topic of study. We discuss and implement a new approach to model drifts in a numerical modulation model. The model employs stochastic differential equations to solve the relevant trans- port equation in five (three spatial, energy and time) dimen- sions. What is of interest is the fact that the model can han- dle current sheet tilt angles up to the theoretical maximum of α = 90° and still remain numerically stable. We use the additional insights gained from the numerical model to in- vestigate the effectiveness of drifts along the current sheet by examining the relationship between the current sheet path length and the cosmic ray propagation time. It is found that diffusion can disrupt the drift process very effectively, lead- ing to diffusive short circuiting of the current sheet by the cosmic rays.
Cheap 0.36 dB broadband multipath LNA
17 January 2014
North West University

Krüger, Petrus Paulus

A two-path low-noise amplifier (LNA) designed for the planned MeerKAT radio telescope is presented. The LNA has a noise figure (NF) of less than 0.36 dB and input return loss of less than 12 dB from 0.7 to 1.8 GHz. It consumes 100 mW of power and costs about $10 to manufacture.
Environmental impact of an industrial compressed air system for solar power in South Africa
20 January 2014
North West University

Gouws, Rupert

Almost 14% of the electricity generated by the national energy supplier (Eskom) in South Africa is sold directly to the mining sector and almost 20% is utilized directly by the compressed air systems of the mining sector. The industrial compressed air systems in South Africa therefore have a substantial impact on the environment in terms of emissions output. In this paper a solar powered compressor is installed parallel to an existing industrial compressed air system. The environmental (emissions) impact and energy consumption of the industrial compressed air system with the solar powered compressor is calculated and the results are provided. It is shown from the results that the solar powered air compressor improved the overall system efficiency and lowered the carbon footprint of the industrial compressed air system. The impact of the energy improvement on the amount of trees required to offset the calculated amount of CO2 is also provided.
The second Fermi Large Area Telescope catalog of gamma-ray pulsars
11 April 2014
North West University

Abdo, A.A.;Venter, C.;Ajello, M.;Allafort, A.;Baldini, L.;Fermi LAT

This catalog summarizes 117 high-confidence ≥0.1 GeV gamma-ray pulsar detections using three years of data acquired by the Large Area Telescope (LAT) on the Fermi satellite. Half are neutron stars discovered using LAT data through periodicity searches in gamma-ray and radio data around LAT unassociated source positions. The 117 pulsars are evenly divided into three groups: millisecond pulsars, young radio-loud pulsars, and young radio-quiet pulsars. We characterize the pulse profiles and energy spectra and derive luminosities when distance information exists. Spectral analysis of the off-peak phase intervals indicates probable pulsar wind nebula emission for four pulsars, and off-peak magnetospheric emission for several young and millisecond pulsars. We compare the gamma-ray properties with those in the radio, optical, and X-ray bands. We provide flux limits for pulsars with no observed gamma-ray emission, highlighting a small number of gamma-faint, radio-loud pulsars. The large, varied gamma-ray pulsar sample constrains emission models. Fermi's selection biases complement those of radio surveys, enhancing comparisons with predicted population distributions.
The status of H.E.S.S. and CTA, and their role in a multiwavelength context
22 April 2014
North West University

Venter, C.

The High Energy Stereoscopic System (H.E.S.S.) is a world-class experiment located in Namibia and consists of an array of four 13-m telescopes which investigate the non-thermal universe in the 100 GeV to 100 TeV energy range via the Cherenkov technique. H.E.S.S. is sensitive to fluxes of a few thousandths of that of the Crab Nebula, has a wide field of view (FoV; 5°), sub-degree angular resolution (<0.1°), accurate pointing (< 1000), and good spectral resolution (10−20%). Its excellent location affords a clear view of the Galactic Centre as well as many Galactic sources. H.E.S.S. Phase II, entailing the addition of a 28-m central telescope to the existing four, is already underway. This will result in increased energy coverage (with an expected threshold of ~20 GeV), sensitivity, and angular resolution. Efforts towards the design and construction of a next-generation gamma-ray observatory called the Cherenkov Telescope Array (CTA) are gaining momentum. This will consist of a northern and southern component, unifying the global gamma-ray astronomy community, and will boast an order-of-magnitude increase in sensitivity. The status of the Southern African site bid for hosting CTA-South is discussed. Lastly, it is important to view our knowledge of the very-high-energy (VHE) sky within the greater multiwavelength context, it being complementary to observations at lower energies (e.g., high-energy gamma rays, X-rays, optical, and radio waves). The rich opportunities created by such a synergy will bolster the continued study of some of the most violent and energetic phenomena in the Universe.
The Vela pulsar: results from the first year of Fermi LAT observations
23 April 2014
North West University

Abdo, A.A.;Venter, C.;Ackermann, M.;Ajello, M.;Allafort, A.;Fermi LAT

We report on analysis of timing and spectroscopy of the Vela pulsar using 11 months of observations with the Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope. The intrinsic brightness of Vela at GeV energies combined with the angular resolution and sensitivity of the LAT allows us to make the most detailed study to date of the energy-dependent light curves and phase-resolved spectra, using a LAT-derived timing model. The light curve consists of two peaks (P1 and P2) connected by bridge emission containing a third peak (P3). We have confirmed the strong decrease of the P1/P2 ratio with increasing energy seen with EGRET and previous Fermi LAT data, and observe that P1 disappears above 20 GeV. The increase with energy of the mean phase of the P3 component can be followed with much greater detail, showing that P3 and P2 are present up to the highest energies of pulsation. We find significant pulsed emission at phases outside the main profile, indicating that magnetospheric emission exists over 80% of the pulsar period. With increased high-energy counts the phase-averaged spectrum is seen to depart from a power law with simple exponential cutoff, and is better fit with a more gradual cutoff. The spectra in fixed-count phase bins are well fit with power laws with exponential cutoffs, revealing a strong and complex phase dependence of the cutoff energy, especially in the peaks. By combining these results with predictions of the outer magnetosphere models that map emission characteristics to phase, it will be possible to probe the particle acceleration and the structure of the pulsar magnetosphere with unprecedented detail.
Discovery of pulsed γ-rays from PSR J0034-0534 with the Fermi Large Area Telescope: a case for co-located radio and γ-ray emission regions
24 April 2014
North West University

Abdo, A.A.;Venter, C.;Ackermann, M.;Ajello, M.;Allafort, A.;Fermi LAT

Millisecond pulsars (MSPs) have been firmly established as a class of γ-ray emitters via the detection of pulsations above 0.1 GeV from eight MSPs by the Fermi Large Area Telescope (LAT). Using 13 months of LAT data, significant γ-ray pulsations at the radio period have been detected from the MSP PSR J0034–0534, making it the ninth clear MSP detection by the LAT. The γ-ray light curve shows two peaks separated by 0.274 ± 0.015 in phase which are very nearly aligned with the radio peaks, a phenomenon seen only in the Crab pulsar until now. The ≥0.1 GeV spectrum of this pulsar is well fit by an exponentially cutoff power law with a cutoff energy of 1.8 ± 0.6 ± 0.1 GeV and a photon index of 1.5 ± 0.2 ± 0.1, first errors are statistical and second are systematic. The near-alignment of the radio and γ-ray peaks strongly suggests that the radio and γ-ray emission regions are co-located and both are the result of caustic formation.
HESS upper limit on the very high energy γ-ray emission from the globular cluster 47 Tucanae
29 April 2014
North West University

Aharonian, F.;Holleran, M.;Büsching, I.;Davids, I.D.;De Jager, O.C.;Raubenheimer, B.C.;Venter, C.;H.E.S.S. Collaboration

Observations of the globular cluster 47 Tucanae (NGC 104), which contains at least 23 ms pulsars, were performed with the HESS telescope system. The observations lead to an upper limit of F(E > 800 GeV) < 6.7 × 10−13 cm−2 s−1 on the integral -ray photon flux from 47 Tucanae. Considering millisecond pulsars as the unique potential source of -rays in the globular cluster, constraints based on emission models are derived: on the magnetic field in the average pulsar nebula and on the conversion efficiency of spin-down power to γ -ray photons or to relativistic leptons.
PSR J2021+4026 in the gamma cygni region: the first variable γ-ray pulsar seen by the Fermi LAT
30 April 2014
North West University

Allafort, A.;Venter, C.;Baldini, L.;Ballet, J.;Barbiellini, G.;Fermi LAT

Long-term monitoring of PSR J2021+4026 in the heart of the Cygnus region with the Fermi Large Area Telescope unveiled a sudden decrease in flux above 100 MeV over a timescale shorter than a week. The "jump" was near MJD 55850 (2011 October 16), with the flux decreasing from (8.33 ± 0.08) × 10−10 erg cm−2 s−1 to (6.86 ± 0.13) × 10−10 erg cm−2 s−1. Simultaneously, the frequency spindown rate increased from (7.8 ± 0.1) × 10−13 Hz s−1 to (8.1 ± 0.1) × 10−13 Hz s−1. Significant (>5σ) changes in the pulse profile and marginal (<3σ) changes in the emission spectrum occurred at the same time. There is also evidence for a small, steady flux increase over the 3 yr preceding MJD 55850. This is the first observation at γ-ray energies of mode changes and intermittent behavior, observed at radio wavelengths for other pulsars. We argue that the change in pulsed γ-ray emission is due to a change in emission beaming and we speculate that it is precipitated by a shift in the magnetic field structure, leading to a change of either effective magnetic inclination or effective current.

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