MTCA.4-Based Upgrade of the CSNS RCS LLRF System for CSNS-II

The China Spallation Neutron Source (CSNS) is undergoing a second phase upgrade (CSNS-II) to increase the beam power of the Rapid Cycling Synchrotron (RCS) to 500 kW. The original CPCI based Low Level RF (LLRF) system has operated reliably for nearly nine years. However, the discontinuation of critical components, including legacy devices and chips, has significantly increased the difficulty of system maintenance and introduced potential risks to the long term safe and stable operation of the RF system. To address these challenges and provide a reliable control platform for future high power operation, a new MicroTCA.4 based LLRF system has been developed and deployed for the CSNS-II RCS RF system. The system provides independent LLRF controllers for eight ferrite loaded fundamental harmonic cavities and three Magnetic Alloy (MA) loaded second harmonic cavities. Accurate RF field regulation is achieved with feedback and iterative learning control algorithms under rapid frequency sweeping and pulsed operation conditions. The vector fitting and complex envelope analysis are employed for cavity modeling, boosting the accuracy of control system analysis and design. A multi harmonic feedback control architecture is adopted for the MA loaded cavity LLRF system, enabling independent regulation of the fundamental, second harmonic, and higher order harmonic components. The upgraded LLRF system was successfully installed during the 2025 summer maintenance period and has been operating stably in the CSNS-II RCS. Commissioning results demonstrate amplitude tracking errors below 0.5% and phase errors below 0.5 ∘ for both fundamental and second harmonic RF systems. The developed LLRF system provides a maintainable and scalable control platform to support future high power operation of the CSNS-II RCS.