Cryo-cooled sapphire oscillator operating above 35 K

We present preliminary design features for a second generation thermomechanically compensated sapphire resonator. Developed a few years ago, the “77 K CSO” resonator showed a quality factor Q=2×10⁶ at an operating temperature of 85 K, enabling a frequency stability of δf/f<1×10^-13. The new design promises a frequency stability of parts in 10¹⁵ with cooling provided by a cryocooler consuming several hundred watts or less. Optimization of the resonator design for a temperature of 40 K results in a mechanical tuning rate requirement (MHz/micron) reduced by a factor of 8, allowing for reduced EM fields at the surface of the sapphire and reduced sensitivity to mechanical deformation. This optimized EM design is implemented in a self-assembling mechanical design that allows easy disassembly for cleaning, together with a first-order cancellation for expected mechanisms of physical creep. The new design is expected to share the very short thermal time constants characteristic of the 10 K and 77 K CSO resonators, thus allowing effective compensation of rapid temperature fluctuations