Superconducting resonant cavity R&D for dark matter axion experiment at CAPP

The IBS Center for Axion and Precision Physics Research (CAPP) in Korea is searching for axions using a tunable resonant cavity and conducting an R&D to enhance axion to photon conversion rates to a detectable level. The deposition of superconducting thin films on the inner surface of the cavity increases the Q factor and thereby enhances the conversion power significantly. However, in order to make superconducting cavity works in the experimental condition, especially in high DC magnetic field (> 8 T), we have to exploit the magnetic property of type II superconductors. Type II superconductors have a relatively high critical field (> 1 T) due to its magnetic vortex formation rather than type I superconductor. Still, we have to solve other problems due to vortex vibration and field penetration into the substrate by the resonant mode. They are the main factors to decrease Q factor. In this work, we will present various RF characteristics related to NbTi film and YBCO film on the inner wall of the sample cavities at the various temperature and the varied DC magnetic field, and describe the behavior of vortex loss and penetration length. From the result, we will discuss the way to make better performance.