Effect of Dark Currents on the Accelerated Beam in an X-band Linac

The metal surface of accelerating structures emits electron currents when operating at rf electric fields higher than 100 MV/m. These “dark currents” may have deleterious effects such as creating backgrounds for beam diagnostics, generating X-rays, loading rf power. They will also interact with the primary electron (or positron) bunch being accelerated in the structure, to kick the beam centroid (assuming that they are not axially symmetric) and they may dilute the beam emittance. For example authors of [1] suggest that such an interaction might have been a cause of beam jitter in SLAC Linear Collider (SLC). In this report we study the effect of dark currents on the primary beam in 11.4 GHz travelling wave structures that were developed for the NLC [2]. The structures are diskloaded waveguides with a near-constant on-axis gradient profile, designed to operate at 65 MV/m average gradient.