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 $200\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$, for two final bunch charges: $1{0}^{5}$ electrons (16 fC) and $1{0}^{6}$ electrons (160 fC). Example optimal solutions are analyzed, and the effects of disordered induced heating estimated. In particular, a relative coherence length of ${L}_{c,x}/{\ensuremath{\sigma}}_{x}=0.27\text{ }\text{ }\mathrm{nm}/\ensuremath{\mu}\mathrm{m}$ was obtained for a final bunch charge of $1{0}^{5}$ electrons and final bunch length of ${\ensuremath{\sigma}}_{t}\ensuremath{\approx}100\text{ }\text{ }\mathrm{fs}$. For a final charge of $1{0}^{6}$ electrons the cryogun produces ${L}_{c,x}/{\ensuremath{\sigma}}_{x}\ensuremath{\approx}0.1\text{ }\mathrm{nm}/\ensuremath{\mu}\mathrm{m}$ for ${\ensuremath{\sigma}}_{t}\ensuremath{\approx}100--200\text{ }\text{ }\mathrm{fs}$ and ${\ensuremath{\sigma}}_{x}\ensuremath{\ge}50\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. These results demonstrate the viability of using genetic algorithms in the design and operation of ultrafast electron diffraction beam lines.