Development and Integration Testing of a Power Coupler for a 3.9-GHz Superconducting Multicell Cavity Resonator

A coaxial power coupler for a superconducting multicell cavity resonator at 3.9 GHz has been developed. The cavity is intended to be employed as an accelerator to provide enhanced electron beam quality in a free-electron laser. Due to cryogenic high-vacuum and high-power requirements, special provisions for two windows and two bellows were implemented. A simulation tool was employed to optimize the coupler structure for low reflection of incident power and dissipation while restraining the field at critical locations to prevent material breakdown. The procedures for testing the coupler on its own and integrated with the superconducting cavity are described, and the measurement results are presented. The coupler-cavity assembly was tested to exceed the requirement of 9.3-kW input power and axial field intensity of 14.5 MV/m in the cavity. Coupler return and insertion losses were estimated to be 21 and 0.2 dB, respectively.