OBSERVATION AND MITIGATION OF MULTIPASS BBU IN CEBAF

The Continuous Electron Beam Accelerator Facility (CEBAF) recirculating accelerator at Jefferson Lab consists of two linacs carrying beam for up to five passes of acceleration. The design of this accelerator anticipated the onset of multipass beam break-up (BBU) at a beam current of approximately 20 mA, far above the operational peak current of 200 μA. For more than a decade of operation, no sign of BBU was ever observed. However, a specially designed acceleration cavity in a cryomodule installed in the summer of 2007 has been observed to cause BBU instability with as low as 40 μA of injected beam current. This presented an opportunity to study BBU in a five-pass accelerator. In this paper we will discuss multipass BBU, present observational data, and discuss the ways we have developed to maintain the instability threshold current to values above those required for operation.