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 \({162.5}\,\textrm{MHz}\) with a tuning range of \({3.2}\,\textrm{MHz}\). 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 \({110}\,\textrm{kHz}\), 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 \({15}^\circ\) phase difference between two standing wave points in the conditioning system was necessary. Notably, the maximum continuous wave power surpassed \({20}\,\textrm{kW}\), exceeding the expected target.