Space Charge Effects in Ultrafast Electron Diffraction and Imaging
AbstractUnderstanding space charge effects is central for the development of high-brightness
ultrafast electron diffraction and microscopy techniques for imaging material
transformation with atomic scale detail at the fs to ps timescales. We present methods
and results for direct ultrafast photoelectron beam characterization employing a shadow
projection imaging technique to investigate the generation of ultrafast, non-uniform,
intense photoelectron pulses in a dc photo-gun geometry. Combined with N-particle
simulations and an analytical Gaussian model, we elucidate three essential space-charge-led
features: the pulse lengthening following a power-law scaling, the broadening
of the initial energy distribution, and the virtual cathode threshold. The impacts of these
space charge effects on the performance of the next generation high-brightness ultrafast
electron diffraction and imaging systems are evaluated.
Z. Tao, H. Zhang, P. M. Duxbury, M. Berz, C.-Y. Ruan,
Journal of Applied Physics 111 (2012) 044316
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