Design and development of long-pulse solid-state modulator for 324 MHz/648 MHz klystron power system of CSNS-II linac

The Phase-II linear accelerator (Linac) of the China Spallation Neutron Source (CSNS-II) is designed to accelerate H− ions to 300 MeV, comprising a Radio Frequency Quadrupole (RFQ), four drift tube linacs (DTLs), 20 superconducting spoke cavities, and 24 superconducting elliptical cavities. The RFQ and DTLs operate at 324 MHz, while the superconducting elliptical cavities are powered at 648 MHz, both utilizing klystron-based RF systems. To supply the required pulsed high voltage to these klystrons, Long-Pulse Solid-State Modulators (LPSSMs) have been developed. Each LPSSM consists of six series-connected modules, with each module incorporating low-voltage input and capacitive energy storage, high-frequency inversion through an IGBT H-bridge, voltage step-up via a high-frequency transformer, and passive rectification and filtering to generate pulsed high voltage. The stacked output provides a compact, efficient, and reliable high-voltage pulse with intra-pulse droop compensation and no requirement for a high-voltage crowbar protection system. A prototype LPSSM has been successfully developed and tested, achieving a maximum pulsed voltage of 122 kV, a current of 104 A, a pulse width of 1.5 ms, and a repetition rate of 25 Hz. Performance verification on both dummy load and a 648 MHz klystron demonstrated full compliance with design specifications. Batch-produced units will feature an increased repetition rate of 50 Hz with production scheduled for completion by June 2026.