On-line measurement of beam phase and energy gain based on beam-cavity interaction

On-line monitoring of beam phase and energy gain in the cavity is of great importance for the stable operation of accelerators, especially in high-power scenarios. As cavities are typically calibrated with a low-current, short-pulse beam for safety purposes, the beam behavior may deviate from the calibrated case when running high-current beams, due to the strong space charge effect and other possible factors. Motivated by the high-power operation demand at European Spallation Source and the further understanding of beam-cavity interaction, methods for beam phase and energy gain on-line measurement have been developed. The beam phase was measured based on the steady-state cavity model, as demonstrated in the half wave resonator cavity at CAFE2. In this paper, this method was not only applied to a single-cell buncher cavity but also, for the first time, extended to the more complex low-energy multicell RFQ cavity for verification. Additionally, an energy-conservation-based method for measuring beam energy gain in a cavity was proposed and validated.