Cryo-cooled sapphire oscillator operating above 35K

We present preliminary design features for a second generation thermomechanically compensated sapphire resonator. Developed a few years ago, the "77K CSO" resonator showed a quality factor Q = 2x10(6) at an operating temperature of 85K enabling a frequency stability of deltaf/f < 1x10(-13). The new design promises a frequency stability of parts in 10(15) with cooling provided by a cryocooler consuming several hundred watts or less. Optimization of the resonator design for a temperature of 40K results in a mechanical tuning rate requirement (MHz/micron) reduced by a factor of 8, allowing for reduced EM fields at die 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 10K and 77K CSO resonators, thus allowing effective compensation of rapid temperature fluctuations.