Principles of STEP accelerometer design

Abstract

The superconducting differential accelerometers for the equivalence principle and geodesy experiments on the European STEP mission have been designed under common guiding principles. Both accelerometers rely on stable superconducting magnetic levitation. The suspension is stiff against all unwanted degrees of freedom while the axial differential mode is kept compliant to obtain a high intrinsic sensitivity of the differential accelerometer, . The sensitive axis of each component accelerometer is aligned to by adjusting persistent currents in the alignment coils and a restoring force is provided by persistent currents to prevent unwanted rotation about the sensitive axis. The dynamic alignment of the sensitive axes improves the common-mode rejection ratio to . The axial displacements of the two test masses in each differential accelerometer are coupled through two separate superconducting circuits to two DC SQUIDs. Persistent currents are stored in the two circuits such that the acceleration signals are differenced in one circuit and summed in the other before detection by the respective SQUIDs. The signal differencing before detection reduces the dynamic range requirement of the following amplifiers and analogue-to-digital (A/D) converters by orders of magnitude.