Physics

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1-D particle-in-cell simulations of plasmas with kappa velocity distributions.
(2013) Abdul, Reginald Francis.; Mace, Richard Lester.
The main aim of this project was the development of a particle-in-cell (PIC) plasma simulation code. While particle-in-cell simulations are not new, they have largely focused on using an initial Maxwellian particle loading. The new feature the code implemented for this project is the use of kappa distributions as an initial loading. This specialises the code for the investigation of waves and instabilities in space plasmas having kappa-type velocity distributions. The kappa distribution has been found to provide a better fit to space plasma particle velocity distributions than the Maxwellian in a wide variety of situations. In particular, it possesses a power law tail which is a frequent feature of charged particle velocity distributions in space plasmas. Traditionally, the treatment of such out-of-equilibrium velocity distributions has been via a summation over several Maxwellians with different temperatures and average number densities. Instead, the approach used in this work is guided by recent advances in non-extensive statistical mechanics, which provide a rigorous underpinning for the existence of kappa distributions. As case studies, the simulation code was used to investigate the ion-acoustic instability as well as electrostatic Bernstein waves in both Maxwellian and kappa plasmas. Results were compared to kinetic theory and the differences in the Maxwellian and kappa plasma behaviours are discussed. To analyse the instabilities various diagnostics were used, including Fourier analysis of the wave fields to determine the dispersion relation, and particle binning to determine the particle velocity distributions. Both the Maxwellian and kappa particle loading algorithms were found to agree well with the theoretical velocity distributions and the dispersion relations were found to agree with kinetic theory for both kappa and Maxwellian plasmas. The code was developed in the C programming language using an incremental approach that enabled careful testing after each new level of sophistication was added. A version of the code was parallelised using Message Passing Interface (MPI) to take advantage of the distributed supercomputing environment provided by the CHPC.
57 Fe Mössbauer studies of 57 Mn* implanted III-V semiconductors InP and InAs.
(2011) Dlamini, Wendy Bonakele.; Naidoo, D.; Bharuth-Ram, Krishanlal.
III-V compound semiconductors such as Gallium Arsenide, Indium Phosphide as well as Indium Arsenide have recently demonstrated the capability of applications in high speed semiconductor devices compared to those made from Silicon. As a result, III-V compound semiconductors have drawn attention of material researchers, in particular in understanding the effects that may occur during manufacturing of these devices. Optical and electrical properties of a device may alter when a foreign atom is introduced during the manufacturing of the device. However, the foreign atom may also lead to the formation of lattice disorder (defects). A convenient way of introducing impurity atoms into a substrate and tailoring their functionality for particular applications is by ion implantation. Mössbauer spectroscopy is a useful technique usually utilized for understanding site location of the impurity atoms in a lattice and the formation of defect complexes. The focus of this dissertation is the study of lattice location of ion implanted ⁵⁷Mn/⁵⁷Fe ions in the III-V semiconductors InP, n-type InAs and p type InAs, and the annealing of implantation induced lattice damage in these samples. ⁵⁷Fe Mössbauer spectroscopy studies have been conducted on III-V semiconductors InP, n-type InAs and p-type InAs with the 57Fe Mössbauer state being populated following the implantation of radioactive ⁵⁷Mn⁺ ions which has the advantage that extremely low fluence implantations are sufficient to give data with good statistics. The ⁵⁷Mn⁺ ions were accelerated to 60 keV at the ISOLDE/CERN facility and implanted with fluences of up to 2x10¹² ions/cm² into single crystal samples which were held at 300 –700 K in an implantation chamber. βdecay of the Mn⁺ imparts an average recoil energy of 40 keV to the daughter ⁵⁷*Fe which are then re-distributed onto interstitial and/or substitutional sites, or trapped in defect complexes and damage sites. The Mössbauer spectra were collected with a light-weight parallel plate avalanche counter, with ⁵⁷Fe enriched stainless steel electrodes, mounted on a conventional drive unit outside the implantation chamber. The spectra were analyzed with the Mössbauer fitting code VINDA which allowed for simultaneous fits of the set of spectra for each sample collected at different temperatures. Acceptable fits to the Mössbauer spectra of the InP, n-type InAs and p-type InAs samples required three components: an asymmetric doublet attributed to Fe atoms in implantation induced damaged environments, a single line assigned to Fe on substitutional In sites, and a weak symmetric doublet assigned to impurity-vacancy complexes. In InP there is already an appreciable substitutional Fe (Feѕ) fraction on implantation at room temperature; while in the InAs samples FeS only becomes significant above 400 K. In all samples, the asymmetric doublet dominates the spectra below 400 K. Implantation damage, however, anneals quite rapidly and at high temperatures (above 400 K), the single line due to Feѕ dominates the spectra while the Fe-defect complex dissociates at 500 K. The implantation induced damage is observed to anneal fast in the arsenide samples compared to the phosphide sample. The slow annealing of the damage in InP was supported by the higher Debye temperature (290 K) extracted from the temperature dependence of the site population for the damage site in InP compared with InAs (194 K and 200 K for n-type and p-type, respectively). Variations in the isomer shift and quadrupole splitting for the damage site in InP at high temperatures (above 400 K) suggest structural changes in the neighborhood of the ⁵⁷Fe probe. Furthermore, the isomer shifts of the spectral components were consistent with near trivalent state and fully trivalent state i.e., Fe³⁺ with d⁵ electron configuration for Fe ions in the damage site and at the substitutional (In) site, respectively. The impurity Fe atoms associated with vacancies are identified to be in the Fe²⁺ state with a d⁶ electron configuration.
A study on the atmospheric and environmental impacts of aerosol, cloud and precipitation interaction.
(2022) Yakubu, Abdulaziz Tunde.; Chetty, Naven.
Understanding the mechanisms and processes of aerosol-cloud-precipitation interactions (ACPI) is essential in the determination of the specific role of aerosols in modulating extreme weather events and climate change in the long run. Atmospheric aerosols are mainly of various types and are emitted from differing sources. Considering they commonly exist in the heterogeneous forms in most environments, they significantly influence the incoming solar energy and the general perturbation of the clouds depending on their constituents. Thus, a systemic identification and characterisation of these particles are essential for proper representation in climate models. To better understand the process of climate change, this research explores the climate diversity of South Africa to examine aerosol sources and types concerning the atmospheric aerosol suspension over the region and their role in clouds and precipitation formation. The study further provided answers to the cause of extreme precipitation events, including drought and occasional flooding experienced over the region. Also, an insightful explanation of the process of ACPI is provided in the context of climate change. Furthermore, the research found that the effective radiative forcing (RF) over South Africa as monitored in Cape Town and Pretoria is negative (i.e., cooling effect) and provided an analysis of the cause. Similarly, the validation of some satellite datasets from MISR (Multiangle Imaging Spectroradiometer) and MODIS (Moderate Resolution Imaging Spectroradiometer) instruments against AERONET (Aerosol Robotic Network) is conducted over the region. Although a significant level of agreement is observed for the two instruments, intense improvements are needed, especially regarding measurements over water surfaces. Finally, the study demonstrated the proficiency of effective rainfall prediction from satellite instrument cloud datasets using machine learning algorithms.
Aerosol characteristics over different regions of southern Africa : using sunphotometer and satellite measurements.
(2015) Adesina, Joseph Ayodele.; Venkataraman, Sivakumar.
Aerosols and cloud play a major role in understanding and interpreting the varying earth’s energy budget. It is necessary to characterize these atmospheric particles by their sizes, chemical composition, water content etc. Aerosols can both cause heating and cooling depending on what they are made of; dust will generally tend to scatter leading to cooling effect while some species of black carbon will absorb sunlight thereby causing a heating effect. In order to assess their impact on global climate, a multiple measurement approach is necessary and specifically, we need long and short term ground-based measurements in clean and polluted environment and long term satellite measurements. In this thesis, we have used aerosol measurements from CIMEL Sunphotometer (part of the world-wide network; Aerosol Robotic Network: AERONET) over, Pretoria (25.75º S, 28.28º E) and Skukuza (24.9º S, 31.5º E) in South Africa, and satellite data from Moderate Resolution Imaging Spectroradiometer (MODIS) and Multiangle Imaging Spectroradiometer (MISR). Pretoria is situated in industrial region with adequate influence of urban/industrial aerosols while Skukuza is an agricultural based region with frequent burning of agricultural waste to clear the harvest during the late winter, spring and summer seasons. Thus, the study over industrial and agricultural regions explores more understanding about the regional radiative forcing in relation to aerosol loading and meteorology. MODIS satellite data was utilized for addressing long term trend in aerosol loading and cloud interaction studies over different locations of South Africa where no ground based sunphotometer data are available. Using six months sunphotometer data (July–December 2012), aerosol characteristics over Gorongosa were studied with particular attention to how aerosol loading evolves during the biomass burning season (spring) including pre- and post-months. The results revealed that the monthly mean aerosol optical depth (AOD₅₀₀) was at maximum in September and minimum in November. The study also investigated biomass burning and forest fire occurrences in Mozambique using MODIS active fire data. Using a year sunphotometer data (January – December 2012) obtained from Pretoria’s (CSIR_DPPS) AERONET site, aerosol was characterized by its optical, microphysical and radiative properties. The study explored meteorological effects on aerosol loading and aerosol direct radiative forcing over Pretoria. Maximum value of aerosol optical depth (AOD₅₀₀) was found during February (summer) and August (winter) while the atmospheric forcing was found to be independent of seasonal variation in AOD. Besides, AOD, Angstrom exponent (AE; α440-870), columnar water vapor (CWV), volume size distribution (VSD), single scattering albedo (SSA) and aerosol radiative forcing (ARF) were computed and their variations with their climatic implications were studied. Using the ground-based instrument of AERONET at Skukuza, we performed validation of MISR and MODIS (Terra and Aqua) level 3 AOD products using the data retrieved for the year 2010. We also carried out regression analysis on these satellite products using 10 years of dataset (2004-2013) to evaluate their performance at a hinterland and coastland stations with two distinct environments in SA. The validation showed that MISR was better correlated with sunphotometer having a coefficient of determination (R²=0.94), Aqua MODIS (R²=0.77) and Terra MODIS (R²=0.68). The long term regression analysis at the two selected locations showed MODIS products underestimating MISR. At the hinterland, MISR showed an increasing trend while MODIS products showed a decreasing trend over the study period but at the coastland MISR and Terra MODIS showed a negative trend while Aqua MODIS showed a positive trend. When the two MODIS products were compared, they were better correlated at the coastland (R²=0.66) than hinterland (R²=0.59) and when compared based on seasonal variation, they were better correlated in the winter season in both locations than any other season. The Ozone Monitoring Instrument (OMI) Ultra-Violet Aerosol Index (UVAI) which was used to monitor the absorption aerosol index showed an increasing trend over the two locations with 0.0089/yr hinterland and 0.0022/yr at coastland. In the present thesis, we also used data obtained from the Terra satellite onboard of the MODIS to investigate the spatial and temporal relationship between AOD and cloud parameters namely, water vapor (WV), cloud optical depth (COD), cloud fraction (CF), cloud top pressure (CTP) and cloud top temperature (CTT) based on 5 years (January 2008 -December 2012) of dataset over six locations in South Africa. AOD has high values during spring (September to November) but low values in winter (June to August) in all locations. In terms of temporal variation AOD was lowest at Bloemfontein 0.06±0.04 followed by Cape Town 0.08±0.02, then Potchefstroom 0.09±0.05, Pretoria and Skukuza had 0.11±0.05 each and Durban 0.13±0.05. The mean AE values for each location show a general prevalence of fine particles for most parts of the year. Our analysis of AOD and WV showed both quantities only co-vary at the beginning of the year but later in the year they tend to have opposite trend over all the locations. AOD and CF showed negative correlation for most of the locations while AOD and COD were positive over three of the locations. AOD and CTT, CTP showed similar variations in almost all the locations. The co-variation of CTT and CTP may be due to large scale meteorological variation.
Analysing transient effects in the ionosphere using narrowband VLF data.
(2009) Bremner, Sherry.; Collier, Andrew Blaine.
Very Low Frequency (VLF) radio waves propagate within the Earth-ionosphere waveguide with very little attenuation. Modifications of the waveguide geometry affect the propagation conditions, and hence, the amplitude and phase of VLF signals. Changes in the ionosphere, such as the presence of the D-region during the day, or the precipitation of energetic particles, are the main causes of this modification. Using narrowband receivers monitoring remote VLF transmitters, the amplitude and phase of these signals are recorded. A multivariate data analysis technique, Principal Component Analysis (PCA), is applied to the data in order to determine parameters such as seasonal and diurnal changes which affect the variation of these signals. Data was then analysed for effects from extragalactic gamma ray bursts, terrestrial gamma ray flashes and solar flares. Only X-rays from solar flares were shown to have an appreciable affect on ionospheric propagation.
The analysis and optimization of electrostatic electron optical lenses with rotational symmetry, through use of orthogonal functions.
(1978) Van der Merwe, Johannes Petrus.; Walker, Anthony David Mortimer.; Spalding, Dennis Raymond.
No abstract available.
An analysis of Pc5 pulsations observed in the SuperDARN radar data.
(2009) Magnus, Lindsay Gerald.; Rash, Jonathan Paul Stuart.; Walker, Anthony David Mortimer.
This thesis deals with the development of automatic methods for nding pulsation events in time series produced by the radars in the SuperDARN network. These methods are then applied to the detection and analysis of pulsation events illustrating the relative usefulness of radar data for the study of global pulsation dynamics. Each of the SuperDARN radars produces 1200 Doppler velocity records every hour. If backscatter is present, and there is a pulsation occurring in the same region as the scatter, the pulsation can be measured as periodic changes in the Doppler velocity of that record. There are over 85 million Doppler velocity data records for 2004. In order to identify pulsations in these data, an automated pulsation nder was developed. All records with signi cant peaks in the FFT spectra were tagged as having a pulsation present. If a record had less than 20% data missing it was termed a clean record as it was suitable for use with the automated pulsation nder. As pulsations can only be observed if there are scatter, an investigation into scatter characteristics are presented. It is shown that the occurrence of clean records is most strongly in uenced by IMF Bz, and the underlying spatial structure of the SuperDARN network. The results for the automated pulsation nder for 2004 are then presented. It was found that the average daily distribution of pulsation events, shown as a function of pulsation frequency, followed 1/f distribution with no distinct peaks. It was also found, however, that the standard deviation of the average showed peaks close to the "magic" frequencies indicating that on average there is more variation at these frequencies that any of the other pulsation frequencies measured. The occurrence of pulsations followed the clean scatter statistics both temporally and directionally telling us that the network is not suited for studying global pulsation dynamics because the variations in scatter dominated any variations in pulsation occurrence. Data from a few events identi ed by the pulsation nder are then presented to illustrate the advantages and disadvantages of using SuperDARN data for pulsation event studies. The events show a pulsation that occurs at its fundamental and third harmonics, an aliased pulsation, a pulsation, interrupted by sounding frequency changes, that shows how ionospheric scatter was tagged as ground scatter and how data from two overlapping pulsations in di erent radars can be merged to give the poloidal and toroidal characteristics of the event.
Application of machine learning in cosmology.
(2024) Nagarajappa, Chandan Ganjigere.; Ma, Yin-Zhe.
Abstract available in PDF.
Application of machine learning techniques to the description of open quantum systems.
(2023) Naicker, Kimara.; Petruccione, Francesco.; Sinayskiy, Ilya.
This work focuses on using classical machine learning (ML) models to study the quantum dynamics of excitation energy transfer (EET) within strongly coupled open quantum systems relevant to light harvesting complexes (LHCs). Direct evidence for long-lived quantum coherence has been found to play an important role in EET processes during the first step of photosynthesis in certain LHCs where excitation energy is transmitted from the antenna pigments to the reaction center in which photochemical reactions are initiated [1–3]. The numerically exact method used to simulate the dynamics in this work is the hierarchical equations of motion (HEOM) adapted by Ishizaki and Fleming to suit the quantum biological regime [4–6]. In the case of an open quantum system, such as a photosynthetic pigment-protein complex, evolving over time we can generate a set of time dependent observables that depict the coherent movement of electronic excitations through the system by solving a suitable set of quantum dynamic equations such as the HEOM. We have focused on solving two problems, the first being the inverse problem. That is, the objective is to determine whether a trained ML model can perform Hamiltonian tomography by using the time dependence of the observables as inputs. We demonstrate the capability of the convolutional neural network (CNN) to solve the inverse problem. That is, the trained CNN can accurately describe the system under study by predicting the parameters of the system Hamiltonian when given the aforementioned time dependent data. The models developed can predict Hamiltonian parameters such as excited state energies and inter-site couplings of a system up to 99.28% accuracy. The second use of the same data set of observables involves time-series analysis. Although various analytical solutions for the dynamics of open quantum systems such as the HEOM have been developed, these often require immense computational resources. We demonstrate that models such as SARIMA, CatBoost, Prophet, convolutional and recurrent neural networks can predict the long-time dynamics provided that the initial short-time dynamics is given. Our results suggest that SARIMA can serve as a computationally inexpensive yet accurate way to predict long-time dynamics.
Applications of digital holograms for the selection and detection of transverse laser modes.
(2014) Ngcobo, Sandile.; Forbes, Andrew.
The transverse mode of generally available commercial lasers in most instances is not suitable for desired applications. Shaping the laser beam either extra-cavity, that is outside the laser resonator, or intra-cavity, which is inside the laser resonator, is required to force the laser beam or cavity to oscillate on a selected desirable single laser mode. The shaped laser beam’s spatial intensity profile and propagation properties would then be suitable for the desired application. The crux of the work presented in this thesis involves intra-cavity beam shaping where we generate desirable transverse modes from inside the laser resonator and detecting such mode using digital holograms. In Chapter 1 we discuss a novel technique of modal decomposition of an arbitrary optical light field into underlying superposition of modes. We show that it can be used to extract physical properties associated with the initial light field such as the intensity, the phase and M2, etc. We show that this novel modal decomposition approach that requires no a priori knowledge of the spatial scale of the modes which lead to an optimised modal expansion. We tested the new technique by decomposing arbitrary modes of a diode-pumped solid-state laser to demonstrate its versatility. In Chapter 2 we experimentally demonstrate selective generation of Laguerre-Gaussian (LG) modes of variable radial order from 0 to 5, with zero azimuthal order. To generate these customised LG modes from within the laser resonator we show that a specialised optical element in a form of an amplitude mask is required to be inserted inside the laser resonator. The amplitude mask is designed and fabricated to contain absorbing rings which are immutably connected to the desired LG mode. The geometry of the absorbing ring radii are selected to match and coincide with the location of the selected LG mode zero intensity parts inside the cavity. We show for the first time that the generated LG modes using this method are of high mode purity and a gain mode volume similar to the desired LG mode. The results provide a possible alternative route to high brightness diode pumped solid state laser sources. In Chapter 3 we show that we can overcome the disadvantage of the specialised optical element being immutably connected to the selection of a particular mode by experimentally demonstrating a novel digital laser capable of generating arbitrary laser modes inside the laser resonator. The digital laser is realised by intra-cavity replacing an end-mirror of the resonator with a rewritable holographic mirror which is an electrically addressed reflective phase-only spatial light modulator (SLM). We show that by calculating a new computer-generated holographic gray-scale image on the SLM representing the desired customized laser mode digitally, the digital laser resonator is capable of generating the desired laser modes on demand. The results provide a new laser that can generate customized laser modes. In Chapter 4 we show that the digital laser can be used as a test bed for conceptualizing, testing, and proving ideas. We experimentally demonstrate this by using a simple laser cavity that contains an opaque ring which is digitally programmed on the SLM and an adjustable circular aperture on the output coupler mirror. We show that by manually varying the diameter of the aperture without realignment of the laser, the generated laser modes can be tuned from a Gaussian mode to a Flat-top mode. This opens up new digital methods that can be used to test laser beam shaping techniques. In Chapter 5 we outline a simple laser cavity comprising an opaque ring and a circular aperture that is capable of producing spatially tuneable laser modes, from a Gaussian beam to a Flat-top beam. The tuneability is achieved by varying the diameter of the aperture and thus requires no realignment of the cavity. We demonstrate this principle using a digital laser with an intracavity spatial light modulator, and confirm the predicted properties of the resonator experimentally. In Chapter 6 we discuss the techniques used to intra-cavity generate and detect LG beams with a non-zero azimuthal index since they are known to carry orbital angular momentum (OAM), and have been routinely created external to laser cavities. We show that the few reports of obtaining such beams from laser cavities suffer from inconclusive evidence of the real electromagnetic field. In this Chapter we revisit this question and show that an observed doughnut beam from a laser cavity may not be a pure Laguerre–Gaussian azimuthal mode but can be an incoherent sum of petal modes, which do not carry OAM. We point out the requirements for future analysis of such fields from laser resonators. In Chapter 7 we conclude and discuss future work.
Applications of Lie symmetry analysis to the quantum Brownian motion model.
(2008) Naicker, Viroshan.
Lie symmetry group methods provide a useful tool for the analysis of differential equations in a variety of areas in physics and applied mathematics. The nature of symmetry is that it provides information on properties which remain invariant under transformation. In differential equations this invariance provides a route toward complete integrations, reductions, linearisations and analytical solutions which can evade standard techniques of analysis. In this thesis we study two problems in quantum mechanics from a symmetry perspective: We consider for pedagogical purposes the linear time dependent Schrodinger equation in a potential and provide a symmetry analysis of the resulting equations. Thereafter, as an original contribution, we study the group theoretic properties of the density matrix equation for the quantum Brownian motion of a free particle interacting with a bath of harmonic oscillators. We provide a number of canonical reductions of the system to equations of reduced dimensionality as well as several complete integrations.
Applications of light scattering and refraction by atmospheric gases.
(2002) Moorgawa, Ashokabose.; Michaelis, Max M.
LIDAR, an acronym for LIght Detection And Ranging, is a system used for studying the scattering of laser light incident on a parcel of air. This thesis investigates the atmosphere above the Durban region using two atmospheric LIDARs, referred to, in this study, as the "old LIDAR" and the "new LIDAR". The old LIDAR was used in a campaign of observation from July to October 1997 in a study of aerosol concentrations over Durban. This thesis will focus on, among other things, the local aerosol profiles for low altitude (0 to 10 km) and high altitude (10 to 35 km). In particular, the focus will shift on any long persistence in this region (it was found that the aerosol layer observed by M. Kuppen (1996) on June 1994 at 25 km may have moved to the higher altitude of 28 km in October 1997. This may be explained by stratospheric upwelling, carrying the layer to higher altitude. These aerosols are known to influence the local climate). This investigation will give some useful insight into the local atmospheric dynamics. The new LIDAR system (Rayleigh-Mie LIDAR) has been used to measure atmospheric temperatures from 20 to 60 km as well as aerosol extinction coefficients from 15 to 40 km. Height profiles of temperature have been measured by assuming that the LIDAR returns are solely due to Rayleigh scattering by molecular species and that the atmosphere obeys the perfect gas law and is in hydrostatic equilibrium (Hauchecorne and Chanin 1980). Since its installation in April 1999, the new LIDAR has been used to monitor stratospheric temperatures and aerosol concentrations from 10 to 40 km. In this study, we discuss in chapter 7 the results of a validation campaign conducted during the period of April 1999 to December 2000. Average monthly LIDAR temperatures are computed from April 1999 to December 1999 and compared with radiosonde temperatures obtained from the South African Weather Service (SAWS) at Durban. The monthly LIDAR temperature profiles over two years (1999 and 2000) were also computed and compared with the climatological model Cospar International Reference Atmosphere (CIRA)-1986 and with the average monthly European Centre for Medium Range Weather Forecast (ECMWF) temperatures . The results show that there is good agreement between LIDAR and SAWS radiosonde temperatures in the 20 and 30 km altitude range. Between 20 and 40 km, the monthly LIDAR temperatures agree closely with the CIRA-86 and ECMWF profiles. However, during winter, in the altitude range 40 to 60 km, LIDAR temperatures are warmer than CIRA-1986 and ECMWF temperatures, and they show large variability. These variations could be due to relatively fast transient phenomena like gravity waves or planetary waves propagating vertically in the stratosphere. As part of the validation process, the aerosol extinction coefficients retrieved from the LIDAR data have also been compared with the extinction coefficients measured by Stratospheric Aerosol and Gas Experiment (SAGE) II close to the LIDAR location and on coincident days. Appendix E of this thesis also investigates the concept of refraction by atmospheric gases as applied to gas lenses. A simple spinning pipe gas lens (SPGL) has been used as the objective lens of a camera to take pictures of the moon and sun spots. The SPGL is a varifocal length lens which depends on the temperature of the pipe and the angular velocity at which it spins. For our purpose a focal length of 8 m has been used. The moon pictures are compared with a lunar map so as to identify the maria.
Assessment of the efficiency of solar radiation concentrating system.
(2009) Artur, Célia Domingas.; Pillay, Sadhasivan Rangan.; Løvseth, J.
A Solar Radiation Concentrating System for generation of high temperature heat for a solar oven was developed and evaluated at the University of KwaZulu-Natal, Westville Campus, Durban - South Africa. The system concentrates direct solar radiation on a small area receiver which absorbs the radiation and converts it into thermal energy that may be stored and used for several applications where food preparation and water pasteurization is the priority. The concentrator, area 2.2 m², is a half satellite communication dish covered with trapezoidal acrylic mirror tiles. The receiver/absorber is a spiral coil of blackened stainless steel pipe. Oil is circulated as heat transfer fluid. To determine the optimal dimension and position of the receiver, two experimental methods were used: (1) The tiles were scanned using a vertical, self adjusting laser beam to get a distribution of reflected radiation across the focal volume. (2) A thin, blackened stainless steel plate was placed at appropriate distances in the focal volume, and temperature distribution scanned using a temperature gun. The latter method proved to be the more useful. Results of the analysis of system performance showed that the system has the capability to produce high temperatures for domestic purposes. The efficiency of the system is about 35 %.
The association between VLF and ELF chorus emissions and electron precipitation.
(2009) Musumpuka, Remmy.; Hughes, Arthur R. W.; Collier, Andrew Blaine.
This is an investigation into the association between ionospheric absorption caused by electron precipitation and ELF:3 Hz-3 kHz and VLF:3 kHz-30 kHz, chorus. Ionospheric absorption was measured using the chain of riometers in Finland and related to chorus events recorded simultaneously at SANAE (L=4.2), Antarctica. The displacement in longitude of the Finnish riometers from SANAE’s conjugate point made it impossible to establish a clear relationship between chorus and riometer absorption. The diurnal variation of chorus has been established for the years 2002, 2004 and 2005 and it is shown that chorus can occur at any Local Time(LT) but has a well defined maximum probability of occurring between 0800 LT to 0900 LT. To study the occurrence of chorus automatically we have developed an Index of ELF/VLF activity which enables us to identify chorus and distinguish it from other emissions such as hiss and whistlers. This index of VLF Activity was established by computing the standard deviation of the VLF signal amplitude and it has been observed that the index is larger for the chorus signature as opposed to the hiss which is low and does not vary widely due to the hiss’ steady signal. This index is called ASD index of “VLF Activity”.
Aurora and associated VLF phenomena.
(1978) Duthie, Desmond D.; Scourfield, Malcolm W. J.
Observations have been made at Sanae (gm. lat. -63,71°) on occurring auroral forms, (diffuse and pulsating aurora), and simultaneous occurring VLF phenomena, (whistlers and auroral hiss) . Two studies are presented in this thesis. (a) A comparison of the positions of auroral forms and the positions of field lines, along which whistlers propagate, is made and it is found that: (i) Diffuse aurora occurs on closed field lines and indirect evidence shows that this is also the case for pulsating aurora. (ii) For two periods of data the separation of diffuse aurora from the plasmapause ranges from < 0,9 L to < 0,2 L but during a third period, the diffuse aurora lies, at least partially, within the plasmasphere. (b) An investigation into the association between pulsating aurora and pulsating auroral hiss is made and it is shown that: (i) A common identical pulsation period of 0,75 s and high coherency exists between the light intensity of an auroral patch and the intensity of the associated pulsating auroral hiss. This suggests a wave-particle interaction as a common modulation mechanism. (ii) Cyclotron instability (gyroresonance) or Cerenkov radiation mechanisms occurring in the equatorial plane do not account for the time delays, typically between 0,90 s and 0,157 s, found to be present between, the two phenomena, where the incident auroral electrons, responsible for the auroral patch light intensity, are observed to arrive before the auroral hiss emissions. The results of the analysis in (a) are reported in The Journal of Atmospheric and Terrestrial Physics, 39, 1429, 1977.
Avoidance of trappings in continual collapse of spherical stars.
(2019) Govender, Terricia.; Goswami, Rituparno.
In this dissertation we study the physical process of a spherically symmetric perfect uid experiencing a continuous gravitational collapse in concurrence with continuous radiating energy in an outward spacetime. Trapped surfaces are avoided and the nal fate of the collapse is a at spacetime. In addition, the collapsing matter conforms to the weak and dominant energy conditions at all epochs. Our investigation clearly unveils the purpose of the equation of state and reveals the bounds on the thermodynamic potentials the equation of state admits for such a model. We a rm that these models are generic without any of the issues and paradoxes attached to horizons and singularities, because the system of Einstein eld equations accepts such a theoretical account for an open set of initial data and the equation of state function in their respective functional spaces. High resolution radio telescopes of today, should ideally detect the existence of these compact bodies in the sky.
Baseline demand responsiveness framework for the conventional grid through appliance scheduling by evolutionary metaheuristics.
(2020) Doho, Goncalves Justino.; Matthews, Alan Peter.; Jarvis, Alan Lawrence Leigh.
A major problem of many energy environments nowadays, is an obsolete and highly inefficient electricity supply system, the Conventional Grid (CG), characterized by a high peak to average ratio, out of an uncontrollable demand, worsened by a native lack of communications infrastructures and resources for performing a proper automated demand side management, which has resulted in blackouts, harsh user discomfort, high electricity cost, huge economic losses and a high carbon footprint. Designed to tackle this problem is the emerging Smart Grid (SG). Most research works are devoted to providing automation and efficiency to the SG (or the intermediate SG-like) environments. There is a scarcity of research devoted to providing automated demand responsiveness to the information layer deprived CG environments, although as evident, an Automated Demand Response (ADR) is badly required, since there is still a long way until we get to the SG, all the more when developing world is concerned. Such context, set our focus towards the CG. So, this research work, developed a framework for providing a "blind" baseline Demand Responsiveness (bbDR) for CG environments, wherein, a pseudo real time electricity pricing function, built from a country load profile, is used as a guiding function for the autonomous scheduling of controllable appliances, which seeks to improve electricity consumption patterns, while also preserving user satisfaction by complying to their preferences. For performance evaluation, the optimized energy consumption patterns (peak load, peak to average ratio, load and cost profiles and mean energy rate) of the controlled use of appliances, are compared to those ones produced by their uncontrolled use. The controlled usage schedules are produced by an evolutionary metaheuristics, whilst the uncontrolled usage is stochastically generated from appliances’ rate-of-use probabilities sourced from the literature. The results proved that, such framework is capable of, without DR communications, delivering meaningful, ADR-like, performances to a communications deprived CG environment. As part of the work for simulating the above bbDR framework, we developed and demonstrated a Real Parameter Blackbox Optimization Approach to Appliance Scheduling (RPBBOAS) model, which describes the household, and provides the logical interface with the optimization algorithms. This real parameter model, vis-a-vis its discrete parameter counterpart, tackles combinatorial explosion by, in a novel way, reducing the problem dimension that is traded with the external blackbox optimization algorithms, in such a way that boosts performance and widens the window of applicable algorithms. While developing the above RPBBOAS model, readily available state-of-the-art metaheuristics showed a lackluster performance, which propelled us to design a novel hybrid evolutionary metaheuristics (Hy- PERGDx) that was eventually used in the bbDR simulations. It showed a better all-around performance and robustness vs the state-of-the-art, when benchmarked on a wide range of non-linear problems. Overall, such deliveries, demonstrated the potential of the proposed bbDR framework for improving demand patterns and quality of service figures, in a communication free way, which with an appropriate follow-up development, makes it suitable for application in severely affected, communications deprived (or communications limited), energy networks such as South Africa or worse energy ecosystems.
Beam generated instabilities in space plasma.
(1999) Misthry, Surversperi Suryakumari.; Bharuthram, Ramesh.
Electrostatic instabilities associated with a model applicable to the auroral acceleration region consisting of an ion beam, precipitating electrons and stationary background electrons are theoretically investigated. The kinetic dispersion relation is solved numerically without approximations. It is shown that two low-frequency plasma instabilities are present and these may generate the low-frequency electric field fluctuations (LEFs) that have been observed in the acceleration region. A parameter variation study is carried out in order to reveal the features of the instabilities. The model is adapted to suit two possible regions of study: (1) drifting cool ions and (2) drifting cool ions and counter-streaming hot electrons. The slow ion-acoustic instability which dominates at low ion beam drift velocities is studied by varying plasma parameters such as the propagation angle, ky/k, the wavenumber, k, the cold background electron density, nco, the hot electron temperature and the cool ion beam temperature. The second mode, the modified two-stream instability, which dominates at larger ion beam drift velocities and at oblique angles of propagation is investigated in a similar manner. To complete the study of these two instabilities, the effect of drifting hot electrons is examined briefly through a similar parameter variation study.
Binary and ternary molecules blend bulk heterojunction organic solar cell.
(2015) Mkhwanazi, Emmanuel Khulekani.; Mola, Genene Tessema.
Bulk heterojunction organic solar cells were prepared and characterized in terms of electrical and optical properties of the photosensitive medium. Two cases of organic solar cells were developed in this investigation with active layers composed of P3HT:PCBM molecules blend and P3HT:PTB7:PCBM ternary molecules blend, at stoichiometric ratio 1:1 and 2:1:1, respectively. The ternary molecules blend active layer attracted the attention of the re- searchers because of the possibility of broadening the absorption band. The performance of the devices was investigated and found better device per- formance in binary molecules blend while low charge mobility was observed in the ternary molecules blend active layer. The highest power conversion efficiency (n) obtained were 1.6 % for P3HT:PCBM blend and 0.56 % for P3HT:PTB7:PCBM active layer. The result indicated that the charge trans- fer mechanisms in the ternary molecules blend were not favorable on the extraction and transportation of charges to the electrodes. The morpholo- gies of the films and charge transfer properties in the two media are discussed based on SEM images and electrical measurements.
Biochemical-physical mechanisms of light-tissue interactions.
(2021) Buthelezi, Musawenkosi Doctor.; Chetty, Naven.; Adeleye, Oluwabamise.
Optical tissue phantom samples simulating the optical properties of the human prostates and brain tissues were fabricated. The experimental set-up was designed to be cost-effective but reliable, allowing for convenience in its usage and replication, making it ideal for biomedical optical measurements. Gel agar was the base material, and aluminum oxide (Al 2 O3 ) with black ink was employed as the scatter and absorber, respectively. The latter were mixed in various amounts into the gel agar to simulate the desired phantom tissues. Argon red laser and He-Ne green laser light, with wavelengths of 630 nm and 532 nm were incident on varying thicknesses of the phantom samples. The transmitted and incident light powers were measured to determine the scattering and absorption coefficients, from which the attenuation coefficients, penetration depth, and optical albedo were estimated. The optical penetration depths were found to be 0.30 for brain and 0.15 for prostate tissue phantoms. The fabricated tissues successfully mimicked the brain and prostate tissues, with µ a = 0.69 cm−1and µ a = 0.24 cm−1 absorption coefficients as well as 𝜇𝜇𝑠𝑠 = 1.73 cm−1 and µ s = 5.48 cm−1 scattering coefficients at 532 nm and 630 nm wavelengths, respectively. The optical albedo for brain phantom was found to be a = 0.71 and a = 0.96 for prostate phantom tissue. The results verify the reliability of the experimental technique and suitability of the fabricated tissues for use in biomedical, going forward, thus allowing for future work without the need for experimentally complex and expensive setups.

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