Design and high-power testing of offline conditioning cavity for CiADS RFQ high-power coupler

To validate the design rationality of the power coupler for the RFQ cavity and minimize cavity contamination, we designed a low-loss offline conditioning cavity and conducted high-power testing. This offline cavity features two coupling ports and two tuners, operating at a frequency of 162.5 MHz with a tuning range of 3.2 MHz. Adjusting the installation angle of the coupling ring and the insertion depth of the tuner helps minimize cavity losses. We performed electromagnetic structural and multiphysics simulations, revealing a minimal theoretical power loss of 4.3%. However, when the cavity frequency varied by 110 kHz, theoretical power losses increased to 10%, necessitating constant tuner adjustments during conditioning.Multiphysics simulations indicated that increased cavity temperature did not affect frequency variation. Upon completion of the offline high-power conditioning platform, we measured the transmission performance, revealing a power loss of 6.3%,exceeding the theoretical calculation. Conditioning utilized efficient automatic range scanning and standing wave resonant methods. To fully condition the power coupler, a 15° phase difference between two standing wave points in the conditioning system was necessary. Notably, the maximum continuous wave power surpassed 20 kW, exceeding the expected target.