Development of the antenna adjustable power coupler for 325 MHz superconducting cavities

In this paper, we present the multi-physics design and high-power tests of a fundamental power coupler for superconducting cavities. The fabricated power coupler was a coaxial capacitive-type coupler based on a conventional 31/8-inch Electronic Industries Alliance 50-Omega coaxial transmission line with a titanium nitride (TiN)-coated single ceramic window. The coupler had three diagnostic ports for vacuum, arc, and electron pickups to monitor radio frequency (RF) breakdowns. To reduce the thermal load required to reach the cryogenic temperature, the thermal intercepts were located at 4.5 K and 40 K for liquid helium use and at 4.5 K and 77 K for liquid helium and nitrogen use. A transition box for adjusting the antenna penetration depth and matching impedance was designed. A high-power test was performed with a 20 kW and 325 MHz solid-state power amplifier. The power coupler was connected to a rectangular test cavity with high vacuum and various measuring equipment, such as an arc detector, a power meter, and an electron pick-up probe. The interlock system under vacuum and arc instrumentations prevented the RF window from breaking the power coupler window during the high-power test. We conducted high-power tests for more than 12 h at 12 kW in a 325 MHz continuous wave mode to verify the performance of the designed power coupler.