Superconductor-sapphire cavity for an all-cryogenic SCSO

To develop a superconducting cavity stabilized oscillator (SCSO) as a frequency standard, we are studying the properties of cavities consisting of a single crystal of sapphire surrounded by a superconducting film. Measurements of quality factors of spherical and cylindrical samples of sapphire are reported. Loss values less than 2 × 10
^-9
have been measured at a temperature of 1.45K. A design for an all-cryogenic SCSO is described, with particular emphasis on the cavity requirements. We conclude that such a design would allow greatly enhanced stability of operation due substantially to the thermal and physical properties of the sapphire substrate. Cavity Q requirements are relatively modest, with better than 10
^-16
frequency stability predicted for a Q of 10
⁸
.