Development and testing of a 3.9 GHz power coupler for a superconducting particle accelerator cavity

Superconducting cavities offer the advantage of high field intensity for a given input power, making them an attractive contender for particle accelerator applications [1]. Power coupling into a superconducting cavity employed in a particle accelerator requires unique provisions to maintain high vacuum and cryogenic temperature on the cavity side, while operating with ambient conditions on the source side. Components introduced to fulfill mechanical requirements must show negligible obstruction of the propagation of the microwave with absence of critical locations that may give rise to electron multipaction and thermal breakdown, leading to a multiple section design, instead of an aperture, a probe, or a loop structure as found in conventional cavities. A coaxial power coupler for a superconducting multiple-cell cavity at 3.9 GHz has been developed at Fermilab. The cavity is intended to be employed as an accelerator to provide enhanced electron beam quality for a free-electron laser in the Hamburg user facility.