Development of high power solid-state RF amplifier for RAON SSR superconducting cavity

Abstract The rare isotope accelerator complex for on-line experiments (RAON) for advanced research in various scientific fields has been developed based on a heavy ion accelerator. To reach 200 MeV/u of uranium, three types of superconducting radio-frequency (SRF) cavities are installed or under development: 81.25 MHz quarter-wave resonators (QWRs), 162.5 MHz half-wave resonators (HWRs), and 325 MHz single-spoke resonators (SSRs). For SSRs, a compact, narrowband solid-state power amplifier (SSPA) of 20 kW continuous-wave (CW) RF power was designed to operate reliably under full-reflection conditions at all times, accounting for the operational characteristics of RAON SRF cavities. The system consists of four 6 kW high power amplifier (HPA), four high power circulators, a 4-way high-power hybrid combiner, a power distribution unit, an EPICS controller, and cooling water manifolds. Each 6 kW amplifier is constructed from six 1.2 kW power amplifier modules utilizing LDMOS FETs, which are equipped with internal circulators and RF loads. The performance test showed that the SSPA system can be operated at 20 kW output power with a total gain of 73 dB and an efficiency of 61.1%. In a long-time operation test, the stability in the phase and the power is less than 2 degrees and 0.2 dB, respectively. The circulator exhibited a return loss of better than -15 dB under all phase conditions in full-reflection tests. In the paper, the design, implementation, and performance evaluation of the high-power SSPA are presented.