Characterization of a hybrid superconducting RF cavity for use in axion searches
We report on the development of a hybrid superconducting radio-frequency (SRF) cavity for axion haloscopes. The Axion Dark Matter eXperiment (ADMX) Sidecar has been a program of ADMX for testing technologies with the potential for improving high frequency cavity haloscopes in the 4-6 GHz range. A replica of the Sidecar cavity, an aluminum clamshell microwave resonator loaded with a niobium tuning rod coated with Nb3Sn, was characterized in a dilution fridge through its critical temperature transitions down to 25 mK and through magnetic fields up to 6 T. The quality factor of five resonant modes of the cavity system were monitored throughout the process, including the TM010 mode used in axion searches. Distinct superconducting transitions were observed near the expected critical temperatures of Nb3Sn, pure niobium, and aluminum. The TM010 mode exhibited a factors of improvement in quality factor of 1.86 and 2.87 through the two niobium transitions, which implied the rod had a zero-field surface resistance of roughly 79% that of copper at cryogenic temperatures. Magnet tests showed a distinct superconducting transition for aluminum, and gradual degradation of quality factor expected for Nb3Sn in a mixed superconducting state. This hybrid resonator design has been integrated in the ADMX insert for performing an axion-like particle search.