We report simulation results showing the use of sacrificial bunch charge to achieve high brightness in photoinjector beamlines designed for Ultrafast Electron Diffraction (UED) and Inverse Compton Scattering (ICS). The beam undergoes nonlaminar focusing during which the tails dynamically linearize the core’s transverse phase space. An aperture then removes the resulting diffuse tails, leaving a beam with high brightness. We employ this scheme in C-band photoinjector guns, whose high gradients are attractive for both low (UED) and high charge (ICS) applications. In our simulations we use a 1.6 cell gun with a peak field at the cathode of 240 MV/m. We start with negligible intrinsic emittance and use a multi-objective genetic algorithm to obtain a Pareto front minimizing bunch length and emittance. For ICS applications, we obtain an extremely small minimum emittance of 80 nm at a final charge of 250 pC per bunch and 1.44 ps rms bunch length. For a final bunch charge of 1e+5 electrons, typical for UED experiments, we obtain an emittance of 1.2 nm at an rms bunch length of 50 fs. Both results far exceed the brightness state of the art for these applications.

The TopGun photoinjector is a 1.6-cell C-band gun developed by the University of California, Los Angeles team. Originally optimized for 100 pC operation, its low emittance design has been our starting point. However, the Los Alamos National Laboratory (LANL) needs to operate with a 250 pC bunch charge while maintaining an emittance below 100 nm. The initial optimization of the high charge TopGun-like photoinjector design, intended for construction at LANL, has been previously reported. That design had a single objective: achieving the lowest possible emittance, which was attained at significantly longer bunch length, thereby limiting improvements in beam brightness. Here, we present a multi-objective genetic optimization of the high charge TopGun-like photoinjector design to obtain a Pareto front minimizing bunch length and emittance. We have successfully reduced the bunch length from 8.18 ps to 5.67 ps while maintaining similar emittance values.

The Micro Electron Diffraction for Ultrafast Structural Analysis (MEDUSA) beamline is a 140 keV ultrafast electron diffraction (UED) beamline currently operational at Cornell. The MEDUSA beamline specializes in the study of small samples, with electron beam probe sizes down to the single micron scale. These samples can be pumped by lasers with wavelengths ranging from IR to UV. In this proceeding, we discuss the upgrades made to MEDUSA, with a focus on a pair of foil wound solenoids we built for post sample magnification of the resulting diffraction patterns, and a measurement of their aberrations. Additionally, we detail the cryogenic compatibility changes made to allow the study of samples down to liquid nitrogen temperatures.