A lead-on-sapphire superconducting cavity of superior quality

A cavity consisting of a superconducting lead film on a sapphire substrate has been fabricated to obtain the enhanced frequency stability possible with this configuration. The cavity exhibits a quality value Q exceeding 2×109 in its TE011 mode with a resonant frequency of 2.689 GHz. Methods of fabrication and testing of the cavity are presented in this article. Since the interface between the film and substrate is exposed to the full value of the resonant magnetic field, our experiment is the most sensitive test to date for enhanced losses at the interface itself. We find no evidence of such losses. In fact, the measured values of the surface resistance match very well predictions for rf losses based on the BCS theory.