Thesis (PhD)--Stellenbosch University, 2012. / ENGLISH ABSTRACT: The objective of this research was to develop a neutron detection system that incorporates a mass spectrometer to measure high neutron flux in a nuclear reactor environment. This system consists of slow and fast neutron detector elements for measuring fluxes in those energy regions respectively. The detector should further be capable of withstanding the harsh conditions associated with a high temperature reactor. This novel detector which was initially intended for use in the PBMR reactor has possible applications as an in-core neutron and indirect temperature-monitoring device in any of the HTGR. Simulations of a generic HTGR core model were performed in order to obtain the neutron energy spectrum with emphasis on the behavior of three energy regions, slow, intermediate and fast neutrons within the core at different temperatures. The slow neutron flux which has the characteristic of a Maxwell- Boltzmann distribution were found to shift to larger values of neutron flux at higher energies as the fuel temperature increased, while fast neutron flux spectra remained relatively constant. In addition, the results of the fit of the slow neutron flux with a modified Maxwell-Boltzmann equation confirmed that in the presence of the neutron source, leakage and absorption, the effective neutron temperatures is above the medium temperatures. From these results, it was clear that the detection system will need to monitor both slow and fast neutron flux. Placing neutron detectors inside the reactor core, that are sensitive to a particular energy range of slow and fast neutrons, would thus provide information about the change of temperature in the fuel and hence act as an in-core temperature monitor. A detection mechanism was developed that employs the neutron-induced break-up reaction of 6Li and 12C into α-particles. These materials make excellent neutron converters without interference due to γ-rays, as the contributions from 6Li(γ,np)4He and 12C(γ,3α) reactions are negligible. The mass spectrometer measures the 4He partial pressure as a function of time under high vacuum with the help of pressure gradient provided by a high-vacuum turbomolecular pump and a positive-displacement fore-vacuum pump connected in series. A cryogenic trap, which contains a molecular sieve made of pellets 1.6 mm in diameter, was also designed and manufactured to remove impurities which cause a background in the lighter mass region of the spectrum. The development and testing of the high flux neutron detection system were performed at the iThemba Laboratory for Accelerator Based Sciences (LABS), South Africa. These tests were carried out with a high energy proton beam at the D-line neutron facility, and with a fast neutron beam at the neutron radiation therapy facility. To test the principle and capability of the detection system in measuring high fluxes, a high intensity 66 MeV proton beam was used to produce a large yield of α-particles. This was done because the proton inelastic scattering cross-section with 12C nuclei is similar to that of neutrons, with a threshold energy of about 8 MeV for both reactions. Secondly, the secondary fast neutrons produced from the 9Be(p,n)9B reaction were also measured with the fast neutron detector. The response of this detection system during irradiation was found to be relatively fast, with a rise time of a few seconds. This is seen as a sharp increase in the partial pressure of 4He gas as the proton or neutron beam bombards the 12C material. It was found that the production of 4He with the proton beam was directly proportional to the beam intensity. The number of 4He atoms produced per second was deduced from the partial pressure observed during the irradiation period. With a neutron beam of 1010 s−1 irradiating the detector, the deduced number of 4He atoms was 109 s−1. When irradiation stops, the partial pressure drops exponentially. This response is attributed to a small quantity of 4He trapped in the present design. Overall, the measurements of 4He partial pressure produced during the tests with proton and fast neutron beams were successful and demonstrated proof of principle of the new detection technique. It was also found that this system has no upper neutron flux detection limit; it can be even higher than 1014 n·cm−2·s−1. The lifetime of this detection system in nuclear reactor environment is practically unlimited, as determined by the known ability of stainless steel to keeps its integrity under the high radiation levels. Hence, it is concluded that this high flux neutron detection system is excellent for neutron detection in the presence of high γ-radiation level and provides real-time flux measurements. / AFRIKAANSE OPSOMMING: Die doel van hierdie navorsing was om ’n neutrondetektorstelsel te ontwikkel wat hoë neutronvloed binne in ’n kernreaktor kan meet. Die stelsel bevat twee aparte detektorelemente sodat die termiese sowel as snelneutronvloed gemeet kan word. Die detektor moet verder in staat wees om die strawwe toestande, kenmerkend aan ’n hoë temperatuur reaktor, te kan weerstaan. Die innoverende detektorstelsel, oorspronklik geoormerk vir gebruik in die PBMR reaktor, het toepassingsmoontlikhede as in-kern neutron- sowel as indirekte temperatuurmonitor. Simulasies van ’n generiese model van ’n HTGR reaktorkern is uitgevoer ten einde die neutronenergiespektrum in die kern by verskillende temperature te bekom met klem op die gedrag van neutrone in drie energiegroepe: stadig (termies), intermediêr en snel (vinnig). Daar is bevind dat die stadige neutrone, wat ’n Maxwell-Boltzman verdeling toon, in intensiteit toeneem en dat die piek na hoër energie verskuif met toename in temperatuur, terwyl die vinnige neutronspektrum relatief onveranderd bly. ’n Passing van die stadige spektrum op ’n gemodifiseerde Maxwell-Boltzmann verdeling het bevestig dat die effektiewe neutrontemperatuur weens die teenwoordigheid van bronterme, verliese en absorpsie, hoër as die temperatuur van die medium is. Hierdie resultate maak dit duidelik dat die detektorstelsel beide die stadige sowel as die vinnige neutronvloed moet kan waarneem. Deur detektorelemente wat sensitief is vir die onderskeie spekrale gebiede in die reaktorhart te plaas, kan informasie bekom word wat tot in-kern temperatuur herleibaar is sodat die stelsel inderdaad as indirekte temperatuurmonitor kan dien. Die feit dat alfa-deeltjies geproduseer word in neutron-geïnduseerde opbreekreaksies van 6Li en 12C is as die basis van die nuwe opsporingsmeganisme aangewend. Hierdie materiale funksioneer uitstekend as neutron-selektiewe omsetters in die teenwoordigheid van gamma-strale aangesien laasgenoemde se bydraes tot helium produksie via die 6Li(γ,np)4He en 12C(γ,3α) reaksies, weglaatbaar is. Die massaspektrometer meet die tydgedrag van die 4He parsiële druk binne ’n hoogvakuum wat met behulp van ’n seriegeskakelde kombinasie van ’n turbomolekulêre en positiewe-verplasingsvoorpomp verkry word. ’n Koueval met ’n molekulêre sif, bestaande uit 1.6 mm diameter korrels, is ontwerp en vervaardig om onsuiwerhede te verwyder wat andersins as agtergrond by die ligter gedeelte van die massaspektrum sou wys. Die ontwikkeling en toetsing van die hoëvloed detektorstelsel is te iThembaLABS (iThemba Laboratories for Accelerator Based Sciences) gedoen. Dit is uitgevoer deur gebruik te maak van die hoë energie protonbundel van die D-lyn neutronfasiliteit asook van die bundel vinnige neutrone by die neutronterapiefasiliteit. Om die beginsel en vermoë te toets om by ’n hoë neutronvloed te kan meet, is van die intense 66 MeV protonbudel gebruik gemaak om ’n hoë opbrengs alfa-deeltjies te verkry. Dit is gedoen omdat die reaksiedeursnit vir onelastiese verstrooiing van protone vanaf 12C kerne soortgelyk is aan die van neutrone, met ’n drumpelenergie van 8 MeV vir beide reaksies. Tweedens is die sekondêre vinnige neutrone afkomstig van die 9Be(p,n)9B reaksie ook met die neutrondetektor gemeet. Daar is bevind dat die reaksietyd van die deteksiestelsel tydens bestraling relatief vinnig is, soos gekenmerk deur ’n stygtyd van etlike sekondes. Laasgenoemde manifesteer as ’n toename in die parsiële druk van die 4He sodra die proton- of neutronbundel op die 12C teiken inval. Daar is verder bevind dat die 4He produksie direk eweredig aan die bundelintensiteit is. Vir ’n neutronbundel van nagenoeg 1010 s−1, invallend op die neutrondetektor, is vanaf die gemete parsiële druk afgelei dat die produksie van 4He atome sowat 109 s−1 beloop. In die geheel beoordeel, was die meting van die 4He parsiële druk tydens die toetse met vinnige protone en neutrone suksesvol en het dit die nuwe meetbeginsel bevestig. Dit is verder bevind dat die meetstelsel nie ’n beperking op die boonste neutronvloed plaas nie, maar dat dit vloede van selfs hoër as 1014 s−1 kan hanteer. Die leeftyd van die detektorstelsel in die reaktor is prakties onbeperk en onderhewig aan die bevestigde integriteit van vlekvrystaal onder hoë bestraling. Die gevolgtrekking is dus dat die nuwe detektorstelsel uitstekend geskik is vir die in-tyd meting van ’n baie hoë vloed van neutrone ook in die teenwoordigheid van intense gammabestraling.

Thesis (PhD (Physics))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: There are many applications where a Gaussian laser beam is not ideal, for example, in areas such as medicine, data storage, science, manufacturing and so on, and yet in the vast majority of laser systems this is the fundamental output mode. Clearly this is a limitation, and is often overcome by adapting the application in mind to the available beam. A more desirable approach would be to create a laser beam as the output that is tailored for the application in mind – so called intra-cavity laser beam shaping. The main goal of intra-cavity beam shaping is the designing of laser cavities so that one can produce beams directly as the output of the cavity with the required phase and intensity distribution. Shaping the beam inside the cavity is more desirable than reshaping outside the cavity due to the introduction of additional external losses and adjustment problems. More elements are required outside the cavity which leads to additional costs and larger physical systems. In this thesis we present new methods for phase and amplitude intra– cavity beam shaping. To illustrate the methods we give both an analytical and numerical analysis of different resonator systems which are able to produce customised phase and intensity distributions. In the introduction of this thesis, a detailed overview of the key concepts of optical resonators is presented. In Chapter 2 we consider the well–known integral iteration algorithm for intra–cavity field simulation, namely the Fox–Li algorithm and a new method (matrix method), which is based on the Fox–Li algorithm and can decrease the computation time of both the Fox–Li algorithm and any integral iteration algorithms. The method can be used for any class of integral iteration algorithms which has the same calculation integrals, with changing integrants. The given method appreciably decreases the computation time of these algorithms and approaches that of a single iteration. In Chapter 3 a new approach to modeling the spatial intensity profile from Porro prism resonators is proposed based on rotating loss screens to mimic the apex losses of the prisms. A numerical model based on this approach is presented which correctly predicts the output transverse field distribution found experimentally from such resonators. In Chapter 4 we present a combination of both amplitude and phase shaping inside a cavity, namely the deployment of a suitable amplitude filter at the Fourier plane of a conventional resonator configuration with only spherical curvature optical elements, for the generation of Bessel–Gauss beams as the output. In Chapter 5 we present the analytical and numerical analyses of two new resonator systems for generating flat–top–like beams. Both approaches lead to closed form expressions for the required cavity optics, but differ substantially in the design technique, with the first based on reverse propagation of a flattened Gaussian beam, and the second a metamorphosis of a Gaussian into a flat–top beam. We show that both have good convergence properties, and result in the desired stable mode. In Chapter 6 we outline a resonator design that allows for the selection of a Gaussian mode by diffractive optical elements. This is made possible by the metamorphosis of a Gaussian beam into a flat–top beam during propagation from one end of the resonator to the other. By placing the gain medium at the flat–top beam end, it is possible to extract high energy in a low–loss cavity. / AFRIKAANSE OPSOMMING: Daar is verskeie toepassings waar ʼn Gaussiese laser bundel nie ideaal is nie, in gebiede soos mediese veld, stoor van data, vervaardiging en so meer, en tog word die meeste laser sisteme in die fundamentele mode bedryf. Dit is duidelik ’n beperking, en word meestal oorkom deur aanpassing van die toepassing tot die beskikbare bundel. ’n Beter benadering sou wees om ʼn laser bundel te maak wat afgestem is op die toepassing - sogenaamde intra-resonator bundel vorming. Die hoofdoel van intra-resonator bundel vorming is om resonators te ontwerp wat direk as uitset kan lewer wat die gewenste fase en intensiteits-distribusie vertoon. Vorming van die bundel in die resonator is voordeliger omdat die vorming buite die resonator tot addisionele verliese asook verstellings probleme bydra. Meer elemente word benodig buite die resonator wat bydra tot hoër koste en groter sisteme. In hierdie tesis word nuwe fase en amplitude intra-resonator bundelvormings metodes voorgestel. Om hierdie metode te demonstreer word analitiese en numeriese analises vir verskillende resonator sisteme wat aangepaste fase en intensiteit distribusies produseer, bespreek. In die inleiding van die tesis word ʼn detailleer oorsig oor die sleutel konsepte van optiese resonators voorgelê. In hoofstuk 2 word die bekende integraal iterasie algoritme vir intraresonator veld simulasie, naamlik die Fox-Li algoritme, en ʼn nuwe metode (matriks metode), wat gebaseer is op die Fox-Li algoritme, en die berekeningstyd van beide die Fox-Li algoritme en enige ander integraal iterasie algoritme verminder. Die metode kan gebruik word om enige klas van integraal iterasie algoritmes wat dieselfde berekenings integrale het, met veranderde integrante (waar die integrand die veld van die lig golf is in die geval van die Fox-Li algoritme, IFTA, en die skerm metode. Die voorgestelde metode verminder die berekeningstyd aansienlik, en is benaderd die van ʼn enkel iterasie berekening. In hoofstuk 3 word ʼn nuwe benadering om die modellering van die ruimtelike intensiteitsprofiel van Porro prisma resonators, gebaseer op roterende verliese skerms om die apeks-verliese van die prismas te benader, voorgestel. ʼn Numeriese model gebaseer op hierdie benadering wat die uitset van die transversale veld distribusie in eksperimentele resonators korrek voorspel, word voorgestel. In hoofstuk 4 word ʼn tegniek vir die generering van Bessel-Gauss bundels deur die gebruik van ʼn kombinasie van amplitude en fase vorming in die resonator en ʼn geskikte amplitude filter in die Fourier vlak van ʼn konvensionele resonator konfigurasie met optiese elemente wat slegs sferiese krommings het, voorgestel. In hoofstuk 5 word die analitiese en numeriese analises van twee nuwe resonator sisteme vir die generering van sogenaamde “flat–top” bundels voorgestel. Beide benaderings lei na ʼn geslote vorm uitdrukking vir die resonator optika wat benodig word, maar verskil noemenswaardig in die ontwerptegniek. Die eerste is baseer op die terug voortplanting van plat Gaussiese bundel, en die tweede op metamorfose van Gaussiese “flat-top” bundel. Ons toon aan dat beide tegnieke goeie konvergensie het, en in die gevraagde stabiele modus lewer. In hoofstuk 6 skets ons die resonator ontwerp wat die selektering van ʼn Gaussiese modus deur diffraktiewe optiese element moontlik maak. Dit word moontlik deur die metamorfose van ’n Gaussiese bundel na ʼn “flat-top” gedurende die voortplanting van die een kant van die resonator na die ander. Deur die wins medium aan die “flat–top” kant van die bundel te plaas word dit moontlik om hoë energie te onttrek in ʼn lae verlies resonator.

Thesis (PhD (Physics))--University of Stelllenbosch, 2010. / ENGLISH ABSTRACT: In this dissertation a fully relativistic model for polarized inclusive quasielastic proton-nucleus scattering is developed. Using a standard relativistic impulse approximation (RIA) treatment of quasielastic scattering and a two-body SPVAT form of the current operator, it is shown how the behaviour of projectile and target can be decoupled. Subsequently, different models for projectile and target can be adopted and combined to examine a variety of relativistic effects. The most simplistic model of the target is provided by a mean-field nuclear matter approximation to the relativistic meson-nucleon model, quantum hadrodynamics (QHD). Here relativistic effects manifest as an effective mass, which is lower than the free mass, of the constituent nucleons. This model is improved upon by including many-body correlations through medium-modification of meson propagators in the relativistic random-phase approximation (RPA). Since it is generally accepted that the strong nuclear force and the extended range of the nuclear potential lead to distortion effects on the projectile and ejectile (seen as a modulation of the wave functions), our formalism is geared towards the use of relativistic distorted waves (RDWIA). The distorted waves are written as partial wave expansions and are solutions to the Dirac equation with potentials. The inclusion of distortions, however, greatly increases the computational burden and we show how a number of analytical and numerical techniques can be used to facilitate the process of calculation. It is also shown how the standard relativistic plane wave treatment (RPWIA) can, instead, be easily employed to obtain a baseline for determining the impact of distortions. A calculation is performed for the reaction 40Ca(!p, !p !) at a beam energy of 500 MeV. Here it is found that the effect of correlations on the RPWIA calculation can be seen as a quenching of the cross section that is expected to become more pronounced at lower energies or for higher density targets. A RDWIA calculation shows additional reduction and if target correlations are included this effect is enhanced. To our knowledge this is the first calculation that attempts to include both these effects (RPA and RDWIA) in the context of quasielastic proton-nucleus scattering. / AFRIKAANSE OPSOMMING: In hierdie proefskrif word ’n ten volle relatiwistiese model vir die berekening van inklusiewe kwasielastiese proton-kern verstrooiing daargestel. Deur gebruik te maak van ’n standaard relatiwistiese impulsbenadering (RIA) vir kwasi-elastiese verstrooiing asook ’n twee-deeltjie-SPVAT-vorm vir die stroom-operator, word daar gewys hoedat die gedrag van die projektiel en teiken ontkoppel kan word. Verskillende modelle kan dus vir die projektiel en teiken gebruik word om ’n verskeidenheid relatiwistiese effekte te bestudeer. Die mees simplistiese model vir die teiken word verskaf deur ’n gemiddelde-veld kernmateriaalbenadering tot die relatiwistiese meson-nukleon-model, kwantum-hadrodinamika (QHD). In hierdie model manifesteer relatiwistiese effekte as ’n effektiewe massa, wat kleiner is as die vrye massa, van nukleone in die kern. Hierdie model word verbeter deur die inagneming van veeldeeltjie korrelasies deur medium-gewysigde meson-propagators in die relatiwistiese ewekansige-fase-benadering (RPA). Aangesien dit algemeen aanvaar word dat die sterk-wisselwerking en die reikwydte van die kernpotensiaal aanleiding gee tot vervormingseffekte op die projektiel en ejektiel (gesien as die modulasie van golffunksies), is ons model optimaal geformuleer om gebruik te maak van relatiwistiese vervormde golwe (RDWIA). Die vervormde golwe word geskryf as parsi¨elegolf uitbreidings en dien as oplossings vir die Dirac-vergelyking met potensiale. Insluiting van vervormings vermeerder egter die berekeningslas geweldig en ons toon hoedat ’n aantal analitiese en numeriese tegnieke gebruik kan word om die proses te vergemaklik. Daar word ook aangetoon hoe die standaard- relatiwistiese-vlakgolf-benadering (RPWIA), in plaas van vevormde golwe, maklik gebruik kan word om ’n verwysingspunt vir die meting van die effek van vervormings te bepaal. ’n Berekening vir die reaksie 40Ca(!p, !p !) teen ’n projektiel-energie van 500 MeV word getoon. Hier word dit gevind dat die effek van korrelasies op die RPWIA-berekening gesien kan word as ’n verlaging van die kansvlak. Daar word verwag dat hierdie effek duideliker sal word by laer energie¨e en ho¨er kerndigthede. ’n RDWIA-berekening word getoon wat daarop dui dat addisionele verlaging in die kansvlak voorkom en indien korrelasies hier ingesluit word, word hierdie effek vergroot. Sover ons kennis strek, is hierdie die eerste berekening wat poog om beide hierdie effekte (RPA en RDWIA) in die konteks van kwasi-elastiese proton-kern verstrooiing in te sluit.

Thesis (PhD (Physics))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Ultrafast Electron Diffraction (UED) is a rapidly maturing field which allows investigation of the evolution of atomic arrangement in solids on timescales comparable to the vibrational period of their constituent atoms (~10-13 s). The technique is an amalgamation of conventional high energy electron diffraction methods and pump-probe spectroscopy with femtosecond (1 fs = 10-15 s) laser pulses. Ultrafast pulsed electron sources generally suffer from limitations on the attainable electron number per pulse (brightness) due to Coulomb repulsion among the electrons. In this dissertation, the design and construction of a compact UED source capable of delivering sub-300 fs electron pulses suitable for diffraction experiments and containing about 5000 electrons per shot is described. The setup has been characterised by measurement of the transverse beam size and angular spread, and through recording and analyzing an electron diffraction pattern from a titanium foil. Measurement of the temporal duration of fs electron pulses is not trivial, and a specialised compact streak camera operating in accumulation mode has been developed as part of this study. A sub-200 fs temporal resolution has been achieved, and the dependence of temporal duration on electron number per pulse was investigated for the current UED source. The observed trends correlate well with detailed electron bunch simulations. In order to investigate ultrafast processes on samples that cannot be probed repeatedly, it becomes necessary to significantly increase the brightness of current state of the art compact sources such as the one constructed in the present study. UED sources employing electron pulse compression techniques offer this possibility. Traditional pulse compression schemes based on RF cavities, while simple in principle, pose significant technical challenges in their realisation. The current thesis describes two novel UED pulse compression methods developed by the author: achromatic reflectron compression and pulsed cavity compression. Both concepts are expected to be easier to realise than conventional RF compression. Detailed simulations predict that such sources can attain a brightness improvement of more than one order of magnitude over compact sources that do not employ compression techniques. In addition, such sources show much promise for the attainment of pulse durations in the sub-100 fs range. / AFRIKAANSE OPSOMMING: Ultra vinnige elektron diffraksie is ‘n meettegniek wat tans in die proses is om vinnige ontwikkeling te ondergaan. Die tegniek het ten doel om strukturele omsettingsprosesse op ‘n lengteskaal van atoombindings en ‘n tydskaal van die vibrasie periode van atome in materie, ongeveer 10-13 s, te ondersoek. Dit word bewerkstellig deur die spasieresolusievermoë van gewone hoë energie elektron diffraksie met die tydresolusievermoë van femtosekonde (1 fs = 10-15 s) laserspektroskopie te kombineer. Die aantal elektrone per puls (intensiteit) van ultravinnige gepulsde elektronbronne word beperk deur die Coulomb afstootingskragte tussen die elektrone. Hierdie dissertasie beskryf die ontwerp en konstruksie van ‘n kompakte ultravinnige elektron bron. Die elektronpulse wat geproduseer word bevat tot 5000 elektrone per puls met ‘n tyd durasie van minder as 300 fs, en is geskik vir diffraksie eksperimente. Die aparaat is gekarakteriseer deur die volgende metings: elektronpulsdiameter, straaldivergensie, en ‘n titaan foelie se statiese diffraksie patroon. Dit is nie triviaal om die durasie van femtosekonde elektronpulse te meet nie, en n spesiale kompakte akkumulerende “streak camera” is vir die doeleindes van hierdie projek onwikkel. Die tydresolusie van hierdie “streak camera” is beter as 200 fs, en die afhanklikheid van die pulsdurasie wat deur die ultravinnige elektron bron geproduseer word as n funksie van die elektrongetal per puls is met behulp van hierdie toestel bepaal. Die resultate klop redelik goed met gedetaileerde simulasies van die elektron puls dinamika. Die karakterisasie van monsters wat nie herhaaldelik gemeet kan word nie vereis verkieslik ‘n nog hoër pulsintensiteit as wat met huidige bronne bereik kan word. ‘N verdere doelstelling is dus om ultravinnige elektron bronne te ontwikkel wat pulse met meer elektrone per puls kan genereer. Dit kan bewerkstellig word deur bronne wat van elektron puls kompressie tegnieke gebruik maak. Die tradisionele manier waarop dít gedoen word is deur middel van n kontinu gedrewe radio frekwensie holte. Hierdie metode gaan egter gepaard met aanmerklik hoë tegniese uitdagings. Om hierdie rede het die outeur twee alternatiewe puls kompressie konsepte ontwikkel: akromatiese reflektron kompressie and gepulsde holte kompressie. Albei konsepte sal waarskeinlik makliker wees om te realiseer as die tradisionele radio frekwensie kompressie, en is deur middel van gedetaileerde simulasies geverifiseer. Hierdie simulasies voorspel dat die intensiteit van genoemde bronne met ten minste n grooteorde meer kan wees as wat tans met kompakte ultravinnige elektron bronne moontlik is. Verder blyk dit dat sulke bronne n pulsdurasie van minder as 100 fs kan bereik.

Thesis (PhD)--Stellenbosch University, 2012. / Please refer to full text for abstract.

The effect of the refraction of a laser beam propagating through three different phase objects, i.e. a laser produced plasma and two different gas media, is investigated in this thesis. It is shown that these effects have useful applications. As an introduction to the work performed, a basic discussion of the theory of light is given. In the first experimental study, the accuracy of using the Refractive Fringe Diagnostic, as a tool to determine the electron density profiles of laser produced plasmas, is investigated [Buccellato et al. (1992)]. A comparative study is performed between an established method of determining the electron density profiles of laser produced plasmas, i.e. Nomarski interferometry, and the Refractive Fringe Diagnostic, by comparing experimental data obtained from the same laser shot. For the electron density profiles investigated, it is shown that the Refractive Fringe Diagnostic over-estimates the electron density by an order of magnitude. It is suggested that the electron density errors are due to the inherent assumptions of the Refractive Fringe Diagnostic. To verify this, a numerical simulation into the accuracy of the RFD is performed on a mathematically modelled plasma. The discrepancy in the numerical results are consistent with those of the experimental results and these can be attributed to the assumptions made by the Refractive Fringe Diagnostic. Laser light refracted by a gas medium, with a specific density profile, may produce a near diffraction limited focal spot. The remaining two experimental investigations deal with two novel gas lenses: the Pulsed Gas Lens and the Colliding Shock Lens. A radially expanding cylinder of gas produces a suitable density structure to focus laser light. A design of a gas lens, the Pulsed Gas Lens, using this principle is proposed as a final focusing lens for a laser fusion power station [Buccellato et al. (1993a)]. To establish the feasibility of such a lens a proof-of- principle design for the lens is given. A numerical simulation of this lens is performed by modelling the gas flow from the lens and raytracing through the determined density profiles inside the lens. It is found that this lens can be used as a focusing element. To establish certain practical aspects of the proof-of- principle design, a beam deflection device was constructed and tested. This beam deflection device models the lensing principle of the proposed lens. The laser beam deflection observed did not match the computed deflection. The opening mechanism for the proof-of-principle design did not produce an instantaneous opening of the chamber as was assumed in the simulation. The opening mechanism must be modified to decrease the opening time. Diverging spherical shock waves, produced by pairs of opposing electrodes evenly spaced on a circumference, produce a converging cylindrically symmetric shock wave. After convergence a suitable density structure exists for near diffraction li.mited focusing to occur. It is found that the Colliding Shock Lens is a varifocal lens: the focal length and lens diameter increase with time [Buccellato et al. (1993b)]. A numerical simulation is performed to model the operation of the Colliding Shock Lens. The numerical results compare favourably with the experimental results. From the simulation it is established that the lens diameter can be scaled up by increasing the physical size of the lens and the input energy to the lens. Potential applications of the colliding shock lens are discussed. To conclude this thesis, the results of the separate investigations are summarised. / Thesis (Ph.D.)-University of Natal, 1994.

Measurements by the TOMS instrument aboard the Nimbus 7 satellite, of total column ozone over polar regions have been studied to determine the effects of solar induced natural ozone modulation. Two different analysis methods were employed to ascertain short term (days to months) and long term (months to years) solar influences on polar ozone. Bursts of intense solar activity can result in solar proton events (SPE's). The high energy protons, originating in solar flares, produce secondary electrons which can generate large concentrations of odd nitrogen in the middle atmosphere. These reactive species can catalytically destroy ozone. Three case studies are presented in an attempt to quantify the effect of SPE's on ozone mass over a latitude region 90 to 70°. In order to monitor the ozone response following a SPE over both hemispheres simultaneously, the SPE must occur during the equinox period when both poles are irradiated. Fortuitously, a SPE was recorded in March 1989, the analysis of which forms a case study in this thesis. Ozone depletions of 7.4 x 10 to the power of 9 kg for the south polar cap and 8.0 x 10 to the power of 9 kg for the north polar cap indicate the degree of symmetry for this event. Longer term effects of solar variability are investigated by Fourier techniques. A Fourier transform of eleven years of total ozone mass values, over the region 90 to 70° S, was performed. Inspection of the Fourier spectrum reveals peaks associated with solar cycle, annual and semi-annual oscillations, that may be attributed directly to solar variation. Other peaks, corresponding to QBO and ENSO periodicities, may be ascribed to indirect solar influences i.e. thermally driven dynamics. Finally, a comparison between the phase of the solar cycle peak in this spectrum with that in a spectrum of daily values of solar radio flux, reveals that the austral polar ozone solar cycle periodicity lags solar forcing by 2.8 years. Portions of chapters have been reported at the 1990 South African Institute of Physics Annual Conference, University of Port Elizabeth, South Africa and as a poster at the 1992 Quadrennial Ozone Symposium, Charlottesville, United States of America, 4-13 June 1992. In addition, various parts of this work has been submitted for publication, viz: Stephenson, J. A. E. and M. W. J. Scourfield, Importance of energetic solar protons in ozone depletion, Nature, 352, 137: 1991. / Thesis (Ph.D.)-University of Natal, Durban, 1994.

Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: High-dimensional quantum entanglement offers an increase in information capacity per photon; a highly desirable property for quantum information processes such as quantum communication, computation and teleportation. As the orbital angular momentum (OAM) modes of light span an infinite-dimensional Hilbert space, they have become frontrunners in achieving entanglement in higher dimensions. In light of this, we investigate the potential of OAM entanglement of photons by controlling the parameters in both the generation and measurement systems. We show the experimental procedures and apparatus involved in generating and measuring entangled photons in two-dimensions. We verify important quantum tests such as the Einstein, Podolsky and Rosen (EPR) paradox using OAM and angle correlations, as well as a violation of a Bell-type inequality. By performing a full state tomography, we characterise our quantum state and show we have a pure, highly entangled quantum state. We demonstrate that this method can be extended to higher dimensions. The experimental techniques used to generate and measure OAM entanglement place an upper bound on the number of accessible OAM modes. As such, we investigate new methods in which to increase the spiral bandwidth of our generated quantum state. We alter the shape of the pump beam in spontaneous parametric down-conversion and demonstrate an effect on both OAM and angle correlations. We also made changes to the measurement scheme by projecting the photon pairs into the Bessel-Gaussian (BG) basis and demonstrate entanglement in this basis. We show that this method allows the measured spiral bandwidth to be optimised by simply varying the continuous radial parameter of the BG modes. We demonstrate that BG modes can be entangled in higher dimensions compared with the commonly used helical modes by calculating and comparing the linear entropy and fidelity for both modes. We also show that quantum entanglement can be accurately simulated using classical light using back-projection, which allows the study of projective measurements and predicts the strength of the coincidence correlations in an entanglement experiment. Finally, we make use of each of the techniques to demonstrate the effect of a perturbation on OAM entanglement measured in the BG basis. We investigate the self-healing property of BG beams and show that the classical property is translated to the quantum regime. By calculating the concurrence, we see that measured entanglement recovers after encountering an obstruction. / AFRIKAANSE OPSOMMING: Hoë-dimensionele kwantumverstrengeldheid bied ’n toename in inligtingskapasiteit per foton. Hierdie is ’n hoogs wenslike eienskap vir kwantum inligting prosesse soos kwantum kommunikasie, berekening en teleportasie. Omdat die orbitale hoekmomentum (OAM) modusse van lig ’n oneindig dimensionele Hilbertruimte beslaan, het dit voorlopers geword in die verkryging van verstrengeling in hoër dimensies. In die lig hiervan, ondersoek ons die potensiaal van OAM verstrengeling van fotone deur die parameters in beide die generering en meting stelsels te beheer. Ons toon die eksperimentele prosedures en apparaat wat betrokke is by die generering en die meet van verstrengelde fotone in twee dimensies. Ons verifieer kwantumtoetse, soos die Einstein, Podolsky en Rosen (EPR) paradoks vir OAM en die hoekkorrelasies, sowel as ’n skending van ’n Bell-tipe ongelykheid. Deur middel van ’n volledige toestand tomografie, karakteriseer ons die kwantum toestand en wys ons dat dit ’n suiwer, hoogs verstrengel kwantum toestand is. Ons toon ook dat hierdie metode uitgebrei kan word na hoër dimensies. Die eksperimentele tegnieke wat tydens die generasie en meet van OAM verstrengeling gebruik is, plaas ’n bogrens op die aantal toeganklik OAM modusse. Dus ondersoek ons nuwe metodes om die spiraal bandwydte van ons gegenereerde kwantum toestand te verhoog. Ons verander die vorm van die pomp bundel in spontane parametriese af-omskakeling en demonstreer die uitwerking daarvan op beide OAM en die hoekkorrelasies. Ons het ook veranderinge aan die meting skema gemaak deur die foton pare op die Bessel-Gauss (BG) basis te projekteer. Ons wys dat hierdie metode die gemeetde spiraal bandwydte kan optimeer deur eenvoudig die kontinue radiale parameter van die BG modes te verander. Ons demonstreer dat BG modusse verstrengel kan word in hoër dimensies as die heliese modusse, wat algemeen gebruik word, deur berekeninge te maak en te vergelyk met lineêre entropie en vir beide modusse. Ons wys ook dat kwantumverstrengling akkuraat nageboots kan word, met behulp van die klassieke lig terug-projeksie, wat die studie van projeksie metings toelaat en voorspel die krag van die saamval korrelasies in ’n verstrengeling eksperiment. Ten slotte, gebruik ons elk van die tegnieke om die effek van ’n storing op OAM verstrengling wat in die BG basis gemeet is, te demonstreer. Ons ondersoek die self-genesingseienskap van BG bundels en wys dat die klassieke eienskap vertaal na die kwantum-gebied. Deur die berekening van die konkurrensie (concurrence), sien ons dat die gemeetde verstrengeling herstel word nadat ’n obstruksie ondervind is.

Thesis (MSc (Physics))--University of Stellenbosch, 2006. / External cavity diode lasers are used increasingly as sources of light in applications ranging from industrial photonic systems to basic laboratory research on the interaction of light and atoms. External cavity diode lasers offer more stable output frequency and narrow spectral bandwidth than the typical free-running diode lasers. These characteristics are achieved by exploiting the sensitivity of diode lasers to external optical feedback. In this study the design and development of an external cavity diode laser system for future applications in spectroscopy and laser cooling of rubidium atoms is presented. The external cavity diode laser including mechanical components and control electronics of the system is developed from basic components. The system uses frequency selective optical feedback from a diffraction grating in a Littrow configuration to provide collimated, narrow-band, frequency tunable light near 780 nm. The external cavity diode laser is designed to increase the mode-hop-free frequency tuning range, and allow accurate frequency tuning and stabilisation. A low-noise current source and a temperature controller for thermal stability were developed as part of the system since the output frequency changes with temperature and current. The temperature controller is optimised experimentally for the thermal characteristics of the external cavity. An electronic sidelock servo circuit for frequency locking of the external cavity diode laser to an external reference for long term frequency stabilisation is proposed and discussed. The servo circuit electrically controls the grating tilt and the current through the diode laser in order to lock the frequency of the diode laser. The external cavity diode laser is optimised and characterised near 780 nm. Results obtained in this study indicate that the external cavity diode laser is suitable for future applications in spectroscopy and laser cooling of neutral rubidium atoms.

Thesis (MSc)--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: The selective excitation of an arbitrary vibrational level of a polyatomic molecule, without passage through an intermediary electronic excited state is demonstrated. This was achieved by simulating the interaction of a shaped, femtosecond pulse with one vibrational mode of the molecule. The carrier frequency of the pulse is chosen near resonant to the ground-to- rst-excited vibrational transition of the mode, and the pulse shape is optimized via closed-loop feedback. The simulation concentrates on the rst few vibrationally excited states since the density of states is still low, thus ensuring that the inter-vibrational decoherence time is relatively long compared to the pulse length. While various molecules were investigated this study focuses onUF6 for which detailed spectroscopic data for the v3 vibrational mode is available in literature. A multilevel model was developed and can be adapted for any number of levels. The model reported here was limited to a vibrational quantum number of four. The spectroscopic data included anharmonic splitting as well as forbidden transitions. The effect of rotational levels was not included. A density matrix approach was followed because this will allow for the introduction of dephasing of the coherent excitation via thermalizing collisions with the reservoir, as well as inter-vibrational relaxation. The time evolution of the density matrix is given by the Von Neumann equations. / AFRIKAANSE OPSOMMING: Die selektiewe opwekking van 'n arbitrêre vibrasionele vlak van 'n poliatomies molekule sonder oorgang na 'n intermediëre elektroniese opgewekte toetstand word gedemonstreer. Dit was bereik deur die interaksie te simuleer van 'n gevormde, femtosekonde pulse met een vibrasionele mode van 'n molekule. Die draer frekwensie van die pulse is so gekies dat dit naby resonansie van die grond-tot-eerste-opgewekte vibrasionele oorgang van die mode is, die puls vorm word geoptimeer deur 'n geslote-lus terugvoer. Die simulasie konsentreer op die eerste paar vibrasionele opgewekte toestande, omdat die digtheid van toestande nog steeds laag is, dus verseker dit dat inter-vibrasionele de-koherensie tyd relatief lank is in vergelyking met die puls se lengte. Verskillende molekules was ondersoek vir die studie. Die fokus is op UF6 waarvoor gedetaileerde spektroskopiese data vir die v3 vibrasionele beskikbaar is in die literatuur. 'n Multivlak model was ontwikkel en kan aangepas word vir enige aantal van vlakke. Die model wat hier aangemeld is, is beperk tot die vibrasionele kwantum getal van vier. Die spektroskopiese data het anharmonies splitting so wel as nie toegelaatbare oorgange bevat. Die effek van rotasionele vlakke was nie in berekening geneem nie. 'n Digtheids matriks benadering was gevolg, omdat dit toelaat vir die dekoherensie. Die tyd evolusie van die digtheids matriks word gegee deur die Von Neumann vergelykings.