Operation of the CEBAF linac with high beam loading

The superconducting RF, CW CEBAF accelerator will use a pair of antiparallel 400 MeV linacs connected by recirculation arcs for nominal 4 GeV in five passes. Single-pass, high power testing of the first linac has been conducted during the months preceding the conference. The RF control system has been designed to control cavity gradient and phase under a wide range of gradients and significant beam loading. At full beam current, the accelerating gradient is approximately equal to the accelerating voltage in the superconducting RF cavities. Even though the beam current during the high power testing is one-fifth of the full current, beam loading is substantial. Operational experience of the response of the RF system is presented. A tuning algorithm which compensates for beam loading effects has been developed and tested. Heavy beam loading, corresponding to five-pass operation, was studied by increasing the loaded Q of the cavities. A current modulation experiment addressed the issue of energy spread increase due to current fluctuations, and the effect of by-passed cavities on beam properties was investigated