Multiobjective optimizations of a novel cryocooled dc gun based ultrafast electron diffraction beam line

Abstract

We present the results of multiobjective genetic algorithm optimizations of a single-shot ultrafast electron diffraction beam line utilizing a 225 kV dc gun with a novel cryocooled photocathode system and buncher cavity. Optimizations of the transverse projected emittance as a function of bunch charge are presented and discussed in terms of the scaling laws derived in the charge saturation limit. Additionally, optimization of the transverse coherence length as a function of final rms bunch length at the sample location have been performed for three different sample radii: 50, 100, and 200 μm, for two final bunch charges: 105 electrons (16 fC) and 106 electrons (160 fC). Example optimal solutions are analyzed, and the effects of disordered induced heating estimated. In particular, a relative coherence length of Lc,x / σx = 0.27 nm/μm was obtained for a final bunch charge of 105 electrons and final bunch length of σt ≈ 100 fs. For a final charge of 106 electrons the cryogun produces Lc,x / σx ≈ 0.1 nm/μm for σt ≈ 100-200 fs and σx ≥ 50 μm. These results demonstrate the viability of using genetic algorithms in the design and operation of ultrafast electron diffraction beam lines.