Determination of field amplitude and synchronous phase using the beam-induced signal in an unpowered superconducting cavity

At the Spallation Neutron Source superconducting linear accelerator, RF fields excited by a pulsed, high-intensity drifting beam in an unpowered superconducting cavity are measured with the cavity control circuit, in order to determine the synchronous phase and to calibrate the pickup probe of the cavity. Measurement errors arising from noise and from the excitation of other pass-band modes in the cavity are analyzed. The phase change due to beam acceleration as well as due to cavity detuning is computed with a model which is described in this paper. Simulation results show excellent agreement with measurements performed in drifting beam experiments. The measured cavity synchronous phase and accelerating gradient determined from the drifting beam measurements are in good agreement with those obtained from a time-of-flight based technique.