Generating low emittance electron bunches from the photocathodes backing free-electron lasers (FEL) is a potential source of significant improvement in achievable X-ray peak powers. Temporally shaping the photoexcitation lasers with intensity profiles that are perfectly flattop or ellipsoidal has been demonstrated to improve the emittance of the emitted electron bunch. However, experimental techniques to achieve these profiles have not been demonstrated at the high-energy, high-repetition rate conditions required by next-generation XFELs, such as LCLS-II(HE). We present an experimental demonstration of the dispersion controlled nonlinear synthesis (DCNS) technique* which has been shown in theory to produce emittance-reducing laser profiles under these conditions. Our implementation generates 20 picosecond pulses in the ultraviolet with a flattop intensity profile. We compare the simulated emission of electron bunches to the currently implemented Gaussian temporal profiles and the performance of LCLS-II XFEL with electrons generated from both laser profiles. Finally, we suggest methods to adapt DCNS to non-uniform shaping and for lasers using other nonlinear conversion processes.

LCLS-II commissioning is well under way. As an indirect diagnostic, electron beam-induced higher order mode (HOM) signals from the RF cavities in the first LCLS-II cryomodule are routed outside the accelerator and filtered to select dipole modes. The signals are amplified and detected using a Schottky diode, following a design tested at Fermilab. The detected signal magnitude is proportional to the bunch charge and the transverse offset magnitude of the electron beam in the cavity. This hardware was initially tested at the Fermilab Accelerator Science and Technology (FAST) facility, and has been adapted to LCLS-II. In this paper, we describe commissioning tests of the system in LCLS-II at SLAC. This includes a description of the associated hardware, and the code under development for live monitoring and beam offset display, as well as the calibration of the signal magnitudes using magnet and beam position monitor data.