A supercritical superfluid helium cryostat for GEOSTEP: Preliminary study

The test of the equivalence principle to one part in 10(17) requires the use of a drag-free spacecraft to shield free-falling test masses from any environmental disturbances. Potential violations of the equivalence principle are detected by very small relative displacements of these test masses measured by high-sensitivity differential accelerometers. The accelerometer assembly is contained in a superfluid helium cryostat. The cryogenic temperature (about 2 K) is required to provide a high degree of mechanical (i.e. thermal) stability, to provide superconducting shielding against electromagnetic radiation and to ensure an ultra-high vacuum environment reducing gas-molecule impacts. For GEOSTEP, electrostatic suspension and capacitive sensing of the test masses are baselined. Helium cooling may also allow for the use of superconducting magnetic suspension and sensing. The cryogen boil-off also provides a source of propellant for the proportional gas thrusters used for the drag-free and attitude control.