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

Abstract

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 318-inch Electronic Industries Alliance 50-Ω coaxial transmission line with a titanium nitride (TiN)-coated single ceramic window. The coupler had three diagnostic ports for vacuum, arc, and electron pick-ups 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.