Physics

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Field and model studies of the nearshore circulation.
(1967) Harris, Thomas Frank Wyndham.
Investigations into the characteristics and underlying mechanism of the circulation of water near the shore are reported. The two main types of circulation, one a cellular system resulting from Haves propagated nearly normally to the shore, and the other an essentially alongshore flow associated with oblique waves, are treated separately. The cellular circulation studies were made in the field at Virginia Beach and more extensively in wave tanks. From the field experiments data were collected about the dimensions of the cells, the way in which the Hater circulated, the rate of exchance of surf zone water and the extent of recycling. A method for measuring the changes in the mean sea level over intervals of time greater than those of the wave periods, was developed. The model experiments carried out in uniform wave tanks showed that the cellular circulations could be well simulated. Measurements were made of the cell dimensions, the velocity of the longshore and rip currents, and of the recycling regime. A finding from the wave tank studies Has the presence of standing waves formed by transverse edge waves. The interaction of these standing waves with the gene rated waves normal to the shore could be the initial cause of rip currents and the cellular circulation. Studies of the alongshore system were made in the field only. A method for measuring the volume of flow of longshore currents was developed, tested, and applied. Calculated volumes of flow using a theory based on continuity and the solitary wave theory (as proposed by Inman and Bagnold) compared tolerably well with the field observations. The calculations of volume of flow required a knowledge of the wave height spectra in the surf. This was established by making wave height recordings in the between-breaker zone . It was found that the characteristics of the spectra compared reasonably well with those pr e dict ed by the Longuet-Higgins theory, previously assumed to apply to deep water waves only. A mechanism for the transition from cellular to alongshore system is proposed.
Investigation of the fair weather electric field in the atmosphere.
(1975) Muir, Michael Stilwell.
No abstract available.
The morphology of radiation damage in copper irradiated with neutrons at elevated temperatures.
(1977) Kemm, Kelvin Richard.; Spalding, Dennis Raymond.
This thesis is an investigation of the radiation damage morphology of high purity copper crystals irradiated with fast neutrons at temperatures in the range of 250C to 4OO C. At these high temperatures neutron damage is found to accumulate into large 3-dimensional rafts up to 100 um in size, and the well known homogeneous distribution of black dot damage which is characteristic of irradiations at lower temperatures is not observed. The characteristics and composition of the rafts of damage 0 at different temperatures in the range 250 C to 400 C have been compared and found to differ to a large extent. It has also been shown that the background areas between rafts contain a rather low density of damage at all temperatures studied. It is therefore concluded that many of the interstitial atoms formed during irradiation migrate over large distances through the crystal lattice to precipitate at the sites of the dislocations forming the large rafts, and so denuded inter-raft areas are left behind. It is proposed that these large rafts originate from grown in dislocations present in the crystals before irradiation
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.
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.
The density dependence of the refractivity of gases.
(1978) Burns, Robert Charles.; Graham, Clive.
Towards an objective interpretation of quanum mechanics.
(1978) Wang, Derek Trygve.; Bedford, Donald.
Abstract on PDF file.
Theoretical studies of the crossfield current-driven ion acoustic instability.
(1979) Bharuthram, Ramashwar.; Hellberg, Manfred Armin.
Abstract available in PDF file.
X-ray crystallographic study of a trinuclear ruthenium organo-metallic complex.
(1984) Subramony, Loganathan.; Engel, D. W.; Moodley, K. G.
The crystal structure of Ru₃ (C0)₁₀(C₆H₅)₂PN(C₂H₅) P(C₆H₅)₂ (RUC)PNP) has been determined by single crystal X-ray analysis. The crystals are tri-- clinic with space group PĪ. The unit cell of dimensions a = 14,732; b = 12,386; c = 10,982 Å; α = 104,52; β = 100,64; γ = 94,89° contains two formula units. The unit cell and space group were determined by photographic deJong-Bouman and precession techniques using CuKα radiation. A procedure was developed for more accurate alignment of the crystal for rotating crystal techniques. Intensity data were collected on a Philips PW1100 four-circle diffractometer with MoKα radiation. The positions of the Ru atoms were determined from Patterson syntheses and the remaining atoms located using successive Fourier synthesis. The structure was refined by blocked full-matrix least-squares methods to a residual of R = 0,0537 for 3538 independent reflections with I > 5σ(I) with 300 parameters in the final refinement. The phenyl rings and the CH₂ and CH₃ moities were refined as rigid groups with H-atoms included at fixed positions. A difference Fourier synthesis was done and showed no significant peaks. RUCOPNP is derived from Ru₃(CO)₁₂ by substitution of two equatorial carbonyl ligands by the P atoms of a single edge-bridging (C₆H₅)₂PN(C₂H₅)P( C₆H₅)₂ ligand (PNP). In both complexes the Ru atoms are arranged in a triangle and have distorted octahedral geometry. The introduction of the PNP ligand to the symmetrical parent Ru₃(CO)₁₂ has the following effects: (a) It causes a contraction of the ligand-bridged Ru-Ru bond distance to 2,798 Å whereas the other Ru-Ru distances are 2,860 and 2,848 Å which are close to the distances in the parent compound; (b) It causes the adjacent equatorial carbonyl ligands to rotate towards the PNP ligand by 11,4°. (c) It causes considerable deviations of some of the axial carbonyl ligands from the normal to the plane containing the Ru atoms.
Sound transmission analysis by sound intensimetry.
(1985) Van Zyl, Barend Gideon.; Broadhurst, Anthony D.
This thesis represents the development and evaluation of a theory for sound transmission analysis by sound intensimetry. In the context of this study sound transmission analysis is understood to embrace the following: (1) The measurement of sound reduction indices. (2) Diagnostic analysis of sound transmission through panels and structures. The sound intensity method is examined against the theoretical background of the classic two-room method which forms the basis of currently used international standards. The flanking problem, which is one of the principle limiting factors in the use of the classic method, is analyzed. The standard formulation of the intensity method is expanded to account for leakage error, boundary interference effects and calibration mismatch. It is shown that the commonly observed low-frequency discrepancy between intensity and classic method results is resolved by application of the Waterhouse correction. Sound absorption by the test object on the receiving side is shown to cause an error which increases with the flanking factor and with the fraction of the receiving room absorption located on the surface of the test object. Guidelines are developed for the assessment and control of absorption error in practical situations. Using the common mode rejection index as a performance rating for sound intensity meters, the measurement of sound transmission in reactive fields is investigated. Derivation of a formula for the reactivity near the surface of a transmitting panel surrounded by a flanking structure in a reverberant field, leads to the development of a theoretical framework and criteria for the planning and evaluation of test arrangements for sound transmission analysis. Guidelines are given for the calculation of minimum receiving room absorption and the microphone spacing required in practical situations. A study of the characteristic properties of sound intensity fields in diffuse and non-diffuse environments is used as a basis in formulating a new method of measuring directional diffusivity. Based on the relationship between reactivity and the degree of directional balance in a sound intensity field, this method involves spatial averaging of the pressure level and determination of the magnitude of the total intensity vector at the point under consideration. A direct-reading diffusivity meter has been developed and employed in assessing diffusivity in practical situations. The effect of a lack of directional diffusivity on the accuracy of sound transmission analysis in reactive fields is examined. Criteria for calculating minimum diffusivity requirements in the source and receiving room are developed and evaluated experimentally.
A study of the 90Zr(n,d)89Y reaction.
(1986) Bawa, Ahmed Cassim.; Bharuth-Ram, Krishanlal.; McMurray, W. R.
A study has been made of the 90 Zr(n,d) 89 Y reactlion at an incident neutron energy of 22 MeV. The experimental aspect of the study was performed at the Van der Graaf facility at the National Accelerator Centre, using a particle spectrometer developed by K Bharuth-Ram and W R McMurray for the study of neutron-induced charged-particle emissions. The spectrometer, which consists of a telescope of three multiwire proportional counters and a curved plastic scintillator, permits the simultaneous accumulation of data over an angular range of 80°. Solid-angle- calculations have been performed to correct for the effect of the geometry of the system on the angular distribution of the cross-sections. A review has been made of the shell model of the nucleus, the optical potential model and the distorted waves method (or DWBA) for the analysis of direct nuclear reactions. A distorted-waves method analysis of the reaction is performed with the code DWUCK 4 and the resulting angular distribution of the various cross-sections are compared with the experimentally obtained data. ThIs comparison produces spectroscopic factors which are used to perform some analysis of the nuclear structure of the 90 Zr nucleus.
The crossfield current-driven ion acoustic instability in a two-ion plasma.
(1987) Govender, Jagathesan.; Bharuthram, Ramesh.
The behaviour of the crossfield current-driven ion acoustic instability in a plasma containing two ion species is theoretically examined. In our model the electrons are assumed to be hot and the ions cold, i.e. Tₑ »Tᵢ (~ 0), where both ion species are given the same temperature. The length and time scales are such that the electrons are magnetized and the ions unmagnetized. The linearised Vlasov equation is used to set up a dispersion relation for electrostatic waves for Maxwellian equilibrium velocity distributions of the electrons and ions. For the ion acoustic wave, a study is made of the dependence of the critical electron drift velocity (Vͨₒ) required to excite an instability on several parameters. The parameters include light ion fraction, heavy to light ion mass ratio, magnetic field strength and the propagation angle. In general the maximum value of Vͨₒ is found to be smaller than that for an unmagnetized plasma. Approximate analytic solutions of the dispersion relation are used to make comparisons with solutions from the full dispersion relation. The effect of drifts due to inhomogeneities in external magnetic field, perpendicular electron temperature and electron density on the growth rate of the ion acoustic instability are investigated in the ion rest frame. Finally, in a reference frame in which the electrons are stationary, both ion species are given external drifts. The effects of the ion drift velocities (both equal and unequal), electron to ion temperature ratio, light ion fraction, and heavy to light ion mass ratio on the growth rate of the ion acoustic instability are then studied.
Nuclear structure studies with (n,d) reactions.
(1988) Naidoo, Ravenderan Yagambaram.; Bharuth-Ram, Krishanlal.; McMurray, W. R.
The ²⁷AI(n,d)²⁶Mg and ⁵⁶Fe(n,d)⁵⁵Mn reactions have been studied at 22 MeV incident energy. The 6MV Van de Graaff facility at the National Accelerator Centre, Faure was used for the experimental aspects. An (n, charged particle) spectrometer was used to detect the energy and angle of the outgoing deuterons. The spectrometer allows for accumulation of particle discriminated data over an angular range of 80⁰. The intrinsic geometry of the spectrometer limits its' angular resolution to ~ 5⁰ (FWHM). The spectrometer has an energy resolution of ~ 0.7MeV(FWHM). A detailed study of the experimental system has been conducted and the proportional counters in particular were extensively investigated. A review of the relevant nuclear models for the target and residual nuclei is presented, together with a theoretical outline of the reaction mechanism for the (n,d) reaction. The distorted waves method approach is used in the analysis of the reaction cross sections. Optical potentials are used to simulate the incoming and outgoing distorted waves and thus generate the theoretical cross sections for the (n,d) reactions. The shapes of the angular distributions of the reaction cross sections for different orbital angular momentum transfers are compared to obtain a fit. Comparison of experimental and theoretical cross sections produce the spectroscopic factors which reveal the occupancy or vacany of level states and hence the single particle nature of these states. It was concluded from the study that the shell model of the nuclei under investigation gives a very good description of the results obtained for the (n,d) reactions.
Profile analysis of X-ray powder diffraction data.
(1988) Naicker, Vishnu Visvanathan.; Engel, D. W.
Various strategies have been tested for obtaining integrated intensities from x-ray powder diffractometer data. An asymmetric pseudo-Voigt profile function was used to fit the pattern in the region above 2θ = 35̊ (Cu-Kα radiation). At lower angles where the asymmetry was strongest and the profile function not suitable the peaks were integrated numerically. A smooth background function was estimated from the regions of minimum intensity of the pattern. The profile parameters were initially refined in small ranges of about 10̊ 2θ in order to determine their 2θ-dependence. Thereafter final refinements of peak intensities were undertaken using the profile parameters thus determined. Analysis of data from the mineral Fe-akermanite, Ca₂Mg₀. ₄Fe₀. δSi₂O₇, generated 173 integrated intensities with 1 > 2δ(1) out of 187 positions separated in 2θ by more than 0,02̊. Of the total of 213 reflections in the range, those overlapping exactly or separated by < 0,02̊ were treated as single peaks. The structure was refined using an overall isotropic temperature parameter and a parameter to compensate for preferred orientation, giving an unweighted residual of 10,4% for 14 parameters.
A study of the interaction of strong electromagnetic waves and anisotropic ion beams with a background plasma.
(1989) Singh, Pravin.; Bharuthram, Ramesh.
The interaction of an anisotropic (in velocity space) ion beam with an isotropic background hydrogen plasma is theoretically investigated. The length and time scales are such that both the ions and electrons are magnetized. Using linear theory, the electrostatic dispersion relation is derived, and solved fully, using no approximations. It is shown that the anisotropy can significantly enhance the instability growth rates as compared to the isotropic case. The importance of ion magnetization is illustrated. Comparisons are made with unmagnetized plasma results. The modulational instability of an arbitrarily-large-amplitude electron cyclotron wave along the external magnetic field is investigated, taking into account the relativistic electron quiver velocity and the relativistic ponderomotive force. Three types of plasma slow responses, the forced-Raman, quasistatic and forced-quasistatic, are considered and a parameter study of the instability growth rates is carried out.
High repetition rate continuously tunable CO2 laser system investigation.
(1990) Botha, Lourens Rasmus.; Michaelis, Max M.
The purpose of this thesis was to investigate certain factors important for the operation of a high repetition rate continuously tunable CO2 laser. The tuning range of specific importance for this research program was the R30 region within the ten micron band, since this frequency is of great importance for the laser isotope separation of uranium. This research program focused on: (i) Lowering the pressure at which viable continuous tunability could be achieved. {ii)Resonator design and analysis. {iii)Investigating the feasibility of using water capacitors in a high repetition rate laser system. A theoretical as well as an experimental investigation was done into the use of CO2 isotopes to lower the pressure at which continuous tunability in the R30, ten micron band could be achieved. A theoretical analysis was done into the use of a three mirror resonator with an etalon and a grating to ensure single longitudinal mode tuning in the R30 region. Such a resonator was designed and experimental results obtained were compared with those predicted by the theoretical analysis. A study was done into the use of water as a dielectric medium in a high repetition rate pulse power supply. A mathematical model, describing the electric breakdown of water,was developed. This was compared with published experimental results. Certain parameters important for the design of a water capacitor were experimentally measured. These include the intrinsic time constant,dielectric constant and resistivity of the water. A design proposal as well as a comparison between a water capacitor and other capacitor technologies are presented.
Linear properties of the cross-field ion acoustic instability in a double plasma device.
(1990) Dempers, Clemens Arnold.; Barrett, P. J.
This thesis deals with the dependence of the linear spatial growth rate of the cross-field ion acoustic instability on various plasma parameters. A kinetic theory model, with elastic and inelastic ion-neutral collisions included, is presented and used to conduct a numerical survey of the instability. The growth rate is computed as a function of distance into the plasma, taking into account the attenuation of the ion beam by charge exchange collisions. Further calculations show the variation in growth rate as a function of the following quantities: electron and ion beam temperature, electron density, beam velocity, background ion temperature, magnetic field, the angle between magnetic field direction and wave vector and the finite width of the plasma. The instability was observed in a double plasma device where an ion beam was passed through a background of stationary magnetized electrons. The magnetic field was sufficiently weak to allow approximately rectilinear ion motion. The growth rate of the wave was studied using interferometer techniques. It was identified by the dispersion relation as the cross-field ion acoustic wave propagating as the slow mode of the beam. It was found that the background ions play an important role in determining the phase velocity. Experimental data of the growth rate dependence on wave number, beam velocity and magnetic field strength were found to be well described by the theoretical model. The growth rate dependence of magnetic field direction on plasma width was furthermore found to be in qualitative agreement with the model.
A study of wave induced electron precipitation at low and middle latitudes.
(1991) Friedel, Reiner Hans-Walter.; Hughes, Arthur R. W.
Wave induced electron precipitation (WIEP) can modify the ionosphere above a sub-ionospherically propagating VLF signal in such a way as to perturb the amplitude and phase of the signal: The "Trimpi Event". In this thesis trimpi events are used in a study of WIEP events and in the responsible mechanism: The gyroresonant interaction. Trimpi activity at middle latitudes (SANAE, Antarctica, L = 4.02) and low latitudes (Durban, RSA , L = 1.69) together with the corresponding theory for the gyroresonant interaction is examined and compared. A newly developed computerised system for the detection and analysis of trimpi events has been developed in Durban. This system has been used to analyse tape data recorded at SANAE. Trimpi events were found on various transmitter paths to SANAE and a complete study of 1982 data has led to the establishment of trimpi characteristics as seen at SANAE: an absence of positive events and causative whistlers, a preference for short duration events (t < 25s), the occurrence of some very large events (up to 90% signal attenuation) , two minima in occurrence near 0015 and 0400 h Local Time, low occurrence and occurrence rate of events and evidence that interactions with non-ducted whistlers are of importance. The computerised sytem was then extended to collect data at Durban simultaneously from up to 20 transmitters worldwide. Examination of data from this survey showed very low occurrence rates of trimpis but yielded some daytime events for which the effectiveness of the gyroresonance interaction, which successfully explains the trimpi event at middle and low latitudes, had to be questioned. Thus a fully relativisic test particle simulation of the gyroresonant interaction was used to examine the effectiveness of gyroresonance at low L for producing trimpi events. This simulation was run for a wide range of interaction parameters and yielded the following constraints for effective pitch angle scattering (and hence precipitation) of electrons at low L: wave intensities in excess of 150 nT, wave frequencies in excess of 10 kHz and background electron densities at least one order of magnitude higher than normal. First data from the OMSKI project, a sophisticated VLF receiver operated at Durban as part of an international project, shows further evidence of low-latitude trirmpi activity. A survey of one month's continuous data is presented. In face of the evidence that trimpi events that occur at low L have the same signature as those at middle L but that the standard gyroresonance interaction is insufficient to cause them, alternate scenarios that could enhance the interaction were sought. In particular distortions in the ambient magnetic field (eg. PC-5 pulsations) were modelled using a new dipole-like background field model. This simulation showed that distortions which tend to reduce magnetic field curvature along field lines can significantly enhance the gyroresonant conditions and hence the interaction. A new set of conditions for effective gyroresonance at low L is thus established and contrasted with the more lenient conditions at middle L. A study of "frequency tracking" as a means to prolong resonance showed that natural whistlers do not posess the required frequency /time characteristics for this mechanism, and that artificial waves in a narrow range around the equatorial resonance frequency would ~ well suited for this purpose. An overview of the status of worldwide Trimpi detection networks together with the S.P.R.I. 's role in this regard is presented.
Linear and nonlinear electron-acoustic waves in plasmas with two electron components.
(1991) Mace, Richard Lester.; Hellberg, Manfred Armin.; Bharuthram, Ramesh.
Measurements of broadband electrostatic wave emIssons in conjunction with particle distributions in the earth's magnetosphere, have provided motivation for a number of studies of waves in plasmas with two electron components. One such wave-the electron-acoustic wave-arises when the two electron components have widely disparate temperatures (Watanabe & Taniuti 1977), and has a characteristic frequency that lies between the ion and electron plasma frequencies. Because of this broadband nature and because it is predominantly electrostatic, it provides a likely candidate for the explanation of the electrostatic component of "cusp auroral hiss" observed in the dayside polar cusp at between 2 and 4 earth radii as well as the broadband electrostatic noise (BEN) observed in the dayside polar regions and in the geomagnetic tail. The electron-acoustic wave and its properties provide the subjects for much of the investigation undertaken in this thesis. The thesis is divided into two parts. Part I is concerned with certain aspects of the linear theory of the electron-acoustic wave and is based on a kinetic description of the plasma. The dispersion relation for plane electrostatic waves obtained via linearisation of the Vlasov-Poisson system is studied in detail using analytical and numerical/geometrical techniques, and conditions under which the electron-acoustic wave arises are expounded. This work represents an extension of earlier works on Langmuir waves (Dell, Gledhill & Hellberg 1987) and the electron-acoustic wave (Gary & Tokar 1985). The effects of electron drifts and magnetization are investigated. These result, respectively, in a destabilization of the electron-acoustic wave and a modification of the dispersive properties. In this plasma configuration the model more closely replicates the conditions to be found in the terrestrial polar regions. We extend the parameter regimes considered in earlier works (Tokar &Gary 1984) and in addition, identify another electron sound branch related to the electron-cyclotron wave/instability. Effects of ion-beam destabilization of the electron-acoustic wave are also investigated briefly with a view to explaining BEN in the geomagnetic tail and also to provide a comparison with the electron-driven instability. In part II the nonlinear electron-acoustic wave is studied by employing a warm hydrodynamic model of the plasma components. We first consider weak nonlinearity and employ the asymptotic reductive perturbation technique of Washimi &Taniuti (1966) to render the hydrodynamical equations in the form of simpler evolutionary equations describing weakly-nonlinear electron-acoustic waves. These equations admit solitary-wave or soliton solutions which are studied in detail. Wherever possible we have justified our small amplitude results with full numerical integration of the original hydrodynamical equations. In so doing we extended the range of validity of our results to arbitrary wave amplitudes and also find some interesting features not directly predicted by the small amplitude wave equations. In this respect we were able to determine the important role played by the cool- to-hot electron temperature ratio for soliton existence. This important effect is in accordance with linear theory where the electron temperature ratio is found to be critical for electron-acoustic wave existence. The effects of magnetization on electron-acoustic soliton propagation is examined. We found that the magnetized electron-acoustic solitons are governed by a Korteweg-de Vries-Zakharov-Kusnetsov equation. In addition, it is shown that in very strong magnetic fields ion magnetization can become important yielding significant changes in the soliton characteristics. Multi-dimensional electron-acoustic solitons, which have greater stability than their plane counterparts, are also briefly discussed. Employing a weakly-relativistic hydrodynamic model of the plasma, the effect of a cool, relativistic electron beam on such soliton parameters as width, amplitude and speed is studied in detail. Both small- and large amplitude solitons are considered. The arbitrary-amplitude theory of Baboolal et al. (1988) is generalised to include relativistic streaming as well as relativistic thermal effects. The importance of the cool electron (beam)to- hot electron temperature in conjunction with the beam speed is pointed out. Finally, we derive a modified Korteweg-de Vries (mKdV) equation in an attempt to establish whether electron-acoustic double layers are admitted by our fluid model. Although double layers formally appear as stationary solutions to the mKdV equation, the parameter values required are prohibitive. This is borne out by the full fluid theory where no double layer solutions are found.
Plastic shear in a model amorphous solid.
(1992) Moji, Nthobane Cable.; Jackson, Paul J.
Abstract available in PDF.

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