Impedance of RF shield on ceramic chamber in the rapid cycling synchrotron of China Spallation Neutron Source

Abstract: In a rapid cycling synchrotron (RCS), the magnetic field is synchronized with the beam energy, creating a highly dynamic magnetic environment. A ceramic chamber with a shielding layer (RF shield), composed of a series of copper strips connected a capacitor at either end, is typically employed as the vacuum chamber to mitigate eddy current effects and beam coupling impedance. Consequently, the ceramic chamber exhibits a thin-walled, multilayered complex structure. Previous theoretical studies suggest that the impedance of such a structure has a negligible impact on the beam. However, recent impedance measurements of the ceramic chamber in the China Spallation Neutron Source (CSNS) RCS reveal a resonance in low-frequency range, which further theoretical analysis confirms as a source of beam instability in the RCS. Currently, the magnitude of this impedance cannot be accurately assessed through theoretical calculations. In this study, we utilize CST Microwave Studio to confirm the impedance of the ceramic chamber. Further simulations covering six different types of ceramic chambers are conducted to develop an impedance model in the RCS. Additionally, this paper investigates the resonant characteristics of the ceramic chamber impedance, finding that the resonant frequency is closely related to the capacitance of capacitors. This finding provides clear directions for further impedance optimization and is crucial for achieving the beam power of 500 kW for the CSNS Phase II project (CSNS-II). However, careful attention must be given to the voltage across the capacitors.