SIMULATION OF DARK CURRENTS IN X-BAND ACCELERATORSTRUCTURES

Abstract

In high gradient accelerator structures, such as are used in the main linac of the Next Linear Collider (NLC), electrons are emitted spontaneously from structure walls and then move under the influence of rf fields. A question of interest for the NLC is, what is the effect of such ''dark current'' electrons on the main linac bunch, e.g. can they significantly affect its orbit or emittance? To help answer this question we have taken a dark current simulation program and modified it to estimate such effects. In this report we use the dark current program to study properties of dark currents themselves. Dark currents have been studied by many authors, both experimentally and through numerical tracking. A difference from earlier numerical work is that, instead of choosing many random positions and times of emission, we consider all possible emissions (with some resolution) from the irises of a structure. We address questions such as: what fraction of dark current reaches the ends of a structure, what are the temporal and spectral distributions of outgoing dark current, and what is the gradient dependence. Our calculations do not include the generation of secondary electrons (we believe they are not important in their effectmore » on a bunch). This is thus a study of the behavior of primary dark current electrons in X-band accelerator structures. Note however that, in a real structure, primary electrons may be outnumbered by secondary electrons (as was found, for example, in a high gradient S-band study). More details of our results will be given in a future report.« less