Design and validation of the 324 MHz double-spoke cavity tuner for CSNS-II

Abstract The CSNS-II linac will employ twenty 324 MHz double-spoke superconducting cavities, which work on pulsed operating mode. We report the design and experimental validation of a combined coarse-fine tuner that pairs a wedge-driven stepper motor with dual piezoelectric actuators. The tuner architecture is compensating Lorentz-force detuning, beam loading, helium pressure fluctuations, and microphonics, providing a fast-tuning allowance above 500 Hz. An axial stiffness model was built, and the stiffness of each component was simulated using COMSOL. The tuning efficiency of the tuner was calculated to be 61.15%, yielding effective coarse and fine tuning ranges of 342 kHz and 5.25 kHz at room temperature. Integrated cavity-string tests confirm linear slow-tuner response (0.246 Hz/pulse over 12.0 kHz at 300 K; 0.255 Hz/pulse over 63 kHz at 2 K) and quantify piezo hysteresis and stroke (3.67 kHz at 300 K; 0.66 kHz at 2 K). The requirements, design, assembly integration, and testing of the tuner will be presented, and the final tuner design will fully meet the requirements of the CSNS-II double spoke cavity.