Principle of the STEP accelerometer design

Abstract

The superconducting differential accelerometers both for the Equivalence Principle experiment and geodesy within the European STEP mission have common design principles. The test masses are suspended by stable superconducting magnetic levitation. The suspension design makes all the degrees of freedom of the masses stiff, except for the axial differential mode which is made compliant in order to obtain a high intrinsic sensitivity of the differential accelerometer, ⪅ 10 −14 m s −2 Hz −1 2 . Trimming of persistent currents circulating in the levitation system allows to achieve rejection ratios for the unwanted common and radial accelerations ⪆ 10 6. Two separate superconducting circuits couple the axial displacements of the test masses to two SQUIDs. Persistent currents are stored in the two circuits such that one SQUID is only coupled to the differential displacement while the other only senses the common one. By differencing the signal before its detection, one highly reduces the dynamic range needs of the SQUIDs, of the following amplifiers and of the final A/D converters.