Precision measurement and modeling of superconducting magnetic bearings for the satellite test of the equivalence principle

The Satellite Test of the Equivalence Principle (STEP) is a modern version of Galileo's experiment of dropping two objects from the leaning tower of Pisa. The Equivalence Principle states that all objects fall with the same acceleration, independent of their composition. The primary scientific objective of STEP is to measure a possible violation of the Equivalence Principle one million times better than the best ground based tests. This extraordinary sensitivity is made possible by using cryogenic differential accelerometers in the space environment. Critical to the STEP experiment is a sound fundamental understanding of the behavior of the superconducting magnetic linear bearings used in the accelerometers. We have developed a theoretical bearing model and a precision measuring system with which to validate the model.; The accelerometers contain two concentric hollow cylindrical test masses, of different materials, each levitated and constrained to axial motion by a superconducting magnetic bearing. Ensuring that the bearings satisfy the stringent mission specifications requires developing new testing apparatus and methods. The bearing is tested using an actively-controlled table which tips it relative to gravity. This balances the magnetic forces from the bearing against a component of gravity. The magnetic force profile of the bearing can be mapped by measuring the tilt necessary to position the test mass at various locations. An operational bearing has been built and is being used to verify the theoretical levitation models.; The experimental results obtained from the bearing test apparatus were inconsistent with the previous models used for STEP bearings. This led to the development of a new bearing model that includes the influence of surface current variations in the bearing wires and the effect of the superconducting transformer. The new model, which has been experimentally verified, significantly improves the prediction of levitation current, accurately estimates the relationship between tilting and translational modes, and predicts the dependence of radial mode frequencies on the bearing current.; In addition, we developed a new model for the forces produced by trapped magnetic fluxons, a potential source of imperfections in the bearing. This model estimates the forces between magnetic fluxons trapped in separate superconducting objects.