Application of equivalent circuit algorithm in pre-tuning of CSNS-II ellipsoidal superconducting cavities

A rapid pre-tuning method based on an equivalent circuit model is proposed for multi-cell elliptical superconducting cavities in the CSNS-II project. Through electromagnetic analysis of elliptical superconducting cavities, an equivalent circuit model was established by decomposing the electromagnetic problem into circuit problem including inductance, intrinsic capacitance, and inter-cell coupling capacitance. A computational method was derived for quantifying frequency tuning requirements during electric field flatness optimization. Simulation results indicate that the cavity’s electric field flatness has been improved from 77% to over 98% after tuning. Based on this method, a pre-tuning system integrating high-frequency parameter measurement and mechanical tuning was designed. The experiment was conducted using a copper elliptical prototype cavity. After multiple iterations of tuning, the flatness of the electric field reached 96.5%, meeting the pre-tuning specifications of the CSNS-II superconducting cavity. This study provides a theoretical and engineering solution for high-precision pre-tuning of multi-cell superconducting cavities, showing significant application value for enhancing the performance of particle accelerators.