Cavity clock experiments on space station

With the start of the Space Station era a new group of fundamental physics experiments is being developed to take advantage of the unique conditions available. Recently a microwave cavity clock experiment was selected to perform experiments in relativity. The project has four main goals: an improved test of Local Position Invariance, improved Kennedy–Thorndyke and Michelson–Morley type experiments, and an enhancement of the performance of atomic clocks being developed elsewhere for use in space. We describe the background and status of the project, which involves the development of superconducting microwave oscillators with high frequency stability. On the International Space Station unwanted cavity frequency variations are expected to be caused mainly by acceleration effects due to residual drag and vibration, temperature variations, and fluctuations in the energy stored in the cavity. At present, acceleration effects appear to be the predominant limit, and a new cavity support system has been fabricated to reduce the effect, and three assemblies are in test. Fractional frequency stabilities in the low 10⁻¹⁶ range are expected on time scales of minutes.