Achieving phase and amplitude stability in pulsed superconducting cavities

Superconducting cavities exhibit a high susceptibility to mechanical perturbations due to the narrow bandwidth of the cavities. Significant phase and amplitude errors can be induced by the frequency variations driven by microphonics and Lorentz force detuning especially in the case of pulsed operation where mechanical resonances of the cavity can be excited. While the relativistic electron and positron beams in the TESLA linacs will permit the control of the vector-sum of many cavities driven by one klystron, the non-relativistic proton beam in the linacs for SNS and the JAERI/KEK Joint Project limits the number of cavities that can be controlled by one klystron. Considerable experience of RF control at high gradients (>15 MV/m) with pulsed RF and pulsed beam has been gained at the TESLA Test Facility in which presently 16 cavities are driven by one klystron. The RF control system employs a completely digital feedback system to provide flexibility in the control algorithms, precise calibration of the vector-sum, and extensive diagnostics and exception handling capabilities. Presently under study is a piezotranslator based active compensation scheme for the time varying Lorentz force detuning which if successful will reduce RF power requirements at gradients >25 MV/m considerably and provide improved field stability.