LCLS-II injector operational challenges and recent developments

LCLS-II has been in user operations since 2023 and has ramped the beam rate up to 33 kHz to date. The LCLS-II photoinjector has demonstrated a low-emittance (half-micron) beam operating at a high rate. During the injector operation, we have also encountered several challenges, such as e-beam splitting and large dark current from the gun. The split e-beam compromises FEL lasing, while the large gun dark current generates substantial amounts of ions, which backbombard and damage the photocathode, as well as cause beam loss downstream, frequently damaging components. Recently, these problems have been successfully addressed. On one hand, a relatively larger laser size was implemented on the photocathode, which helped suppress space charge effects, resulting in the disappearance of the split beam. On the other hand, an over-inserted photocathode, recently installed on the LCLS-II gun, has reduced the gun's dark current by a factor of 1000. The gun dark current reduction has significantly extended the photocathode's operational lifetime, eliminated the QE and FEL intensity dependence on beam rate, and substantially reduced beam loss downstream. This paper presents detailed analyses and solutions for LCLS-II photoinjector operational challenges.