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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/5359 |
| Date | 12 1900 |
| Creators | Kassier, Gunther Horst |
| Contributors | Schwoerer, Heinrich, Rohwer, Erich, University of Stellenbosch. Faculty of Science. Dept. of Physics. |
| Publisher | Stellenbosch : University of Stellenbosch |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 108 p. : ill. |
| Rights | University of Stellenbosch |
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