Instability Dynamics with Electron Cloud Buildup in Long Bunches

LONG BUNCHES M. Blaskiewicz , BNL 911B, Upton, NY 11973-5000, USA Abstract For long bunches, trailing edge multi-pacting causes the electron cloud density at the trailing edge of the bunch to be significantly larger than the cloud density during the early and central portions of the bunch. The effective wakefield is modeled and compared with hand estimates. Instabilty simulations, which include the effect of nonlinear space charge, are described. INTRODUCTION Fast electron cloud instabilities have been obvserved in the Los Alamos PSR [1, 2, 3, 4, 5, 6] and may be problematic for the Oak Ridge SNS [5, 6]. For both these machines the transverse electron oscillation period is much less than the bunch length. The electron motion is nearly adiabatic and the electron density can strongly increase as the tail of the bunch passes. This is illustrated in Fig. 1, which shows CSEC [6] simulation results for the PSR. The secondary emission parameters of the stainless steel wall agree well with the model in [7]. It is likely that significant wall conditioning would be needed before the PSR could reach a bunch intensity of , but the author knows of no detailed data under such conditions and it seemed more reasonable to adopt a well defined state. In any case, the purpose of the present paper is to describe computational techniques.