Experimental studies of multipass beam breakup and energy recovery using the CEBAF injector linac

Beam breakup (BBU) instabilities in superconducting linacs are a significant issue due to the potentially high Q of the cavity higher-order modes (HOMs). The CEBAF accelerator, which employs five pass recirculation through two superconducting linacs to accelerate high CW current (up to 200 {dollar}mu{dollar}A), poses unique instability problems. An experimental investigation of multipass BBU at CEBAF has been completed using a single recirculation through the CEBAF injector linac. This recirculator is calculated to be more sensitive to the instability than the full CEBAF accelerator. Successful recirculation through the injector of a beam with greater than 200 {dollar}mu{dollar}A average current from an injection energy of 5.6 MeV to a final energy of 80.1 MeV indicates that BBU should not be a problem in the full CEBAF accelerator. In addition, we were able recover all the energy from a 30 {dollar}mu{dollar}A 42.8 MeV first pass beam by recirculating it out of phase and decelerating it to a final energy equal to the injection energy (5.6 MeV).