Experience with Multibunch Beam Stability at PEP-II

In this report we discuss high-current, multibunch issues at the PEP-II B factory. To achieve the required beam currents, new techniques are used to help stabilize multibunch beams. In the longitudinal planes of both the low energy (LER) positron ring and the high energy (HER) electron ring, residual phase oscillations are damped with higherorder mode (HOM) absorbers on the accelerating cavities, direct and comb rf loops, a low-frequency ' woofer ' link to correct multibunch modes supported by the cavity, and longitudinal bunch-by-bunch feedback. In the LER residual transverse motion has been successfully damped using transverse multibunch feedback up to the maximum current attained so far of 1.7 A. In the HER however a transverse instability has been observed at unexpectedly low beam currents. In this report we describe diagnostics used and summarize current ' thresholds ' and compare these with expectation. Next we present measurements made in the HER to better understand the apparently low threshold. We also show selected data using short bunch trains. Practical issues associated with very high beam currents are discussed including gap transients and the stabilizing influence of the beam-beam interaction on multibunch beam stability.