Dark currents and their effect on the primary beam in an X-band linac

Abstract

We numerically study properties of primary dark currents in an X-band accelerating structure. For the H60VG3 structure considered for the Next Linear Collider (NLC) we first perform a fairly complete (with some approximations) calculation of dark-current trajectories. These results are used to study properties of the dark current leaving the structure. For example, at accelerating gradient of 65 MV/m, considering two very different assumptions about dark-current emission around the irises, we find that the fraction of emitted current leaving the structure to be a consistent ∼1%. Considering that ∼1 mA outgoing dark current is seen in measurement, this implies that ∼100 mA (or 10 pC per period) is emitted within the structure itself. Using the formalism of the Liénard-Wiechert potentials, we then perform a systematic calculation of the transverse kick of dark currents on a primary linac bunch. The result is ∼1 V kick per mA (or per 0.1 pC per period) dark current emitted from an iris. For an entire structure we estimate the total kick on a primary bunch to be ∼15 V. For the NLC linac this translates to a ratio of (final) vertical beam offset to beam size of about 0.2. However, with the assumptions that needed to be made—particularly the number of emitters and their distribution within a structure—the accuracy of this result may be limited to the order of magnitude.