The auto-tuning control system design of debuncher for CSNS-II linac

Purpose After the upgrade of the China Spallation Neutron Source (CSNS) linac, the new debuncher operates at 600 kW. During power ramp-up, cavity wall heating alters the equivalent inductance and capacitance, leading to a shift in resonant frequency. Movable tuners adjust tuning rod length to maintain resonance. It not only reduces the demand on the power source but also prevents power trip-offs caused by excessive reflection power due to cavity detuning.
Methods The tested debuncher has three movable tuners, with tuners 1 and 2 located on the cavity to adjust the cavity frequency and tuner 3 located on the coupler to adjust the coupling coefficient. The movable tuners use servo motors to drive the tuning rods and work in conjunction with limit switches, displacement sensors, and other hardware to complete the tuning of the debuncher. The underlying control logic is implemented through the distributed real-time control software Experimental Physics and Industrial Control System (EPICS), and the tuning control interface is developed using Control System Studio (CSS). An automatic tuning algorithm based on Python was also designed to automatically correct cavity detuning during the power ramp-up process.
Results The debuncher tuner implements two control modes: manual tuning and automatic tuning. The automatic tuning can control the detuning frequency within ± 2 kHz.
Conclusion The automatic tuning system can meet the tuning requirements of the debuncher. The tuning accuracy meets the closed-loop conditions of the low-level RF (LLRF). The flatness of the cavity field meets the requirements of the beam on the cavity field.