Ultrafast electron diffraction :source development, diffractometer design and pulse characterisation

[摘要] ENGLISH ABSTRACT: Ultrafast Electron Diffraction (UED) is a rapidly maturing field which allows investigation of theevolution of atomic arrangement in solids on timescales comparable to the vibrational period oftheir constituent atoms (~10-13 s). The technique is an amalgamation of conventional high energyelectron diffraction methods and pump-probe spectroscopy with femtosecond (1 fs = 10-15 s) laserpulses. Ultrafast pulsed electron sources generally suffer from limitations on the attainable electronnumber 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 electronpulses suitable for diffraction experiments and containing about 5000 electrons per shot isdescribed. The setup has been characterised by measurement of the transverse beam size andangular spread, and through recording and analyzing an electron diffraction pattern from a titaniumfoil. Measurement of the temporal duration of fs electron pulses is not trivial, and a specialisedcompact streak camera operating in accumulation mode has been developed as part of this study. Asub-200 fs temporal resolution has been achieved, and the dependence of temporal duration onelectron number per pulse was investigated for the current UED source. The observed trendscorrelate well with detailed electron bunch simulations. In order to investigate ultrafast processes onsamples that cannot be probed repeatedly, it becomes necessary to significantly increase thebrightness of current state of the art compact sources such as the one constructed in the presentstudy. UED sources employing electron pulse compression techniques offer this possibility.Traditional pulse compression schemes based on RF cavities, while simple in principle, posesignificant technical challenges in their realisation. The current thesis describes two novel UEDpulse compression methods developed by the author: achromatic reflectron compression and pulsedcavity compression. Both concepts are expected to be easier to realise than conventional RFcompression. Detailed simulations predict that such sources can attain a brightness improvement ofmore than one order of magnitude over compact sources that do not employ compressiontechniques. In addition, such sources show much promise for the attainment of pulse durations inthe sub-100 fs range.

[发布机构] Stellenbosch University