Development of an antenna adjustable fundamental power coupler for the 81.25 MHz superconducting cavity at the High Intensity Heavy-Ion Accelerator Facility

Abstract: The Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) has designed and prototyped fundamental power couplers for 81.25 MHz superconducting quarter-wave resonators (QWR) in the High-Intensity Heavy-Ion Accelerator Facility (HIAF). The QWR coupler operates in the pulsed or continuouswave mode with an average power of 6 kW. This coupler is designed with a 50 Ω coaxial structure and a dualdisk warm-window assembly. Focusing on the optimized heat leakage, a double-walled structure is applied to the outer conductor of the coupler for helium gas cooling at 5 K. To suppress multipacting (MP), a 10 nm titanium nitride (TiN) coating is applied to the vacuum side of the ceramic window, whereas a DC bias voltage is applied to the inner conductor. To reduce the production cost of the coupler and make it suitable for mass production, various coupler fabrication materials were tested, and the feasibility of the fabrication process was verified. The coupler includes two types of diagnostic ports for vacuum and ARC monitoring. Two prototype couplers were fabricated and tested for high-power conditioning. The QWR couplers proved their robustness with 10 h of 9 kW traveling wave conditioning and 12 h of 9 kW standing wave conditioning, achieving a stabilized 9 kW power level and a stable vacuum. As predicted, MP barriers were encountered at input powers ranging from 1 kW to 9 kW, and a DC bias voltage of 800 V effectively suppressed the occurrence of MP.