Methods for Measuring and Controlling Beam Breakup in High Current ERLS

It is well known that high current Energy Recovery Linacs (ERL) utilizing superconducting cavities are susceptible to a regenerative type of beam breakup (BBU). The BBU instability is caused by the high impedance transverse deflecting higher-order modes (HOMs) of the cavities. This multipass, multibunch instability has been observed at Jefferson Laboratory's FEL Upgrade driver. Some preliminary measurements are presented. To combat the harmful effects of a particularly dangerous mode, two methods of directly damping HOMs through the cavity HOM couplers were demonstrated. In an effort to suppress the BBU in the presence of multiple, dangerous HOMs, a conceptual design for an injector beam-based transverse feedback system has been developed. By implementing beam-based feedback, the threshold for instability can be increased substantially