Multiobjective genetic algorithm optimizations of a single-shot ultrafast electron diffraction beam line comprised of a $100\mathrm{MV}/\mathrm{m}$ 1.6-cell normal conducting rf (NCRF) gun, as well as a nine-cell $2\pi /3$ bunching cavity placed between two solenoids, have been performed. These include optimization of the normalized transverse emittance as a function of bunch charge, as well as optimization of the transverse coherence length as a function of the rms bunch length of the beam at the sample location for a fixed charge of $10^6$ electrons. Analysis of the resulting solutions is discussed in terms of the relevant scaling laws, and a detailed description of one of the resulting solutions from the coherence length optimizations is given. For a charge of $10^6$ electrons and final beam sizes of ${\sigma }_x\ge 25\mu \mathrm{m}$ and ${\sigma }_t\approx 5\mathrm{fs}$, we found a relative coherence length of $L_{c,x}/{\sigma }_x\approx 0.07$ using direct optimization of the coherence length. Additionally, based on optimizations of the emittance as a function of final bunch length, we estimate the relative coherence length for bunch lengths of 30 and 100 fs to be roughly 0.1 and $0.2\mathrm{nm}/\mu \mathrm{m}$, respectively. Finally, using the scaling of the optimal emittance with bunch charge, for a charge of $10^5$ electrons, we estimate relative coherence lengths of 0.3, 0.5, and $0.92\mathrm{nm}/\mu \mathrm{m}$ for final bunch lengths of 5, 30 and 100 fs, respectively.

• Received 14 October 2016

DOI:https://doi.org/10.1103/PhysRevAccelBeams.20.033401

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Published by the American Physical Society