In addition to the desired electron beam, RF photoinjectors such as the one in LCLS-II produce dark current via field emission. Left unchecked, the dark current can cause various operational issues in the accelerator, such as increased radiation, damage to accelerator components and diagnostics, and desorption of gases from vacuum chamber surfaces. In this contribution, we present measurements of the dark current in the LCLS-II injector, including imaging, current, and energy distributions of the observed dark current emitters. These measurements allow us to characterize each emitter in terms of the Fowler-Nordheim model of field emission, which in turn enables us to more accurately model the behavior of the dark current in the accelerator. Taking these results into account, we also present potential active and passive mitigation strategies.

LCLS-II 1-MeV CW electron source commissioning was successfully completed 2018-2020. Full 100 MeV injector system has been commissioned since summer 2022. CW RF gun and buncher operations are routinely established and e-beam is being ramped to 33 kHz now and eventually up to 1 MHz. About 0.5 μm of emittance has been achieved for 50 pC at desired bunch length. Dark current from the CW gun is systematically characterized and has been effectively mitigated with circular collimators after the gun at <1MeV of beam energy. In this contribution, we will present commissioning and operational experience for CW RF gun/buncher, dark current and high rate e-beam performance. Challenges for operating such CW NC gun based photoinjector and plans for future improvement of emittance and further mitigations of dark current are discussed.

In addition to the desired electron beam, RF photoinjectors such as the one in LCLS-II produce dark current via field emission. Left unchecked, the dark current can cause various operational issues in the accelerator, such as increased radiation, damage to accelerator components and diagnostics, and desorption of gases from vacuum chamber surfaces. In this contribution, we present measurements of the dark current in the LCLS-II injector, including imaging, current, and energy distributions of the observed dark current emitters. These measurements allow us to characterize each emitter in terms of the Fowler-Nordheim model of field emission, which in turn enables us to more accurately model the behavior of the dark current in the accelerator. Taking these results into account, we also present potential active and passive mitigation strategies.