A Supercritical/Superfluid He-Cryostat for STEP

Publisher Summary The Satellite Test of the Equivalence Principle (STEP)-satellite is a candidate for ESA's next medium size scientific mission (M3). On STEP the fundamental physical law of the universality of free fall of anybody shall be checked up to an accuracy of 1 into 1017. Squids are foreseen to measure the relative motions of different test mass pairs in an absolute zero-g environment provided by a drag-free satellite control system. Any liquid movement in the required He-cryostat must therefore be avoided. The combination of a supercritical main cryostat with a superfluid shield under equal pressure offers the possibility to meet this requirement due to the avoidance of free liquid surfaces. With an actively cooled Cryostat Vacuum Vessel and equipped with a radiator a hybrid SCHeMe IIcryostat can be realized which provides the necessary quiet environment at < 2 K for the scientific payload of STEP while fulfilling stringent mass and volume requirements and providing a lifetime of over 6 months with 20 % margin.