Analytical Calculation and Optimization of Superconducting Gravimeter Temperature Effect

High-resolution superconducting gravimeters (SGs) require μK-level temperature control. Passive isolation can increase the risk of quenching the superconducting gravity sensing unit. Also, the use of a vacuum chamber for passive isolation increases complexity and complicates the operation of the instrument. Therefore, to investigate how to avoid using passive isolation, we developed an analytical computation model based on the Maxwell-London (ML) equations for calculating the magnetic levitation forces of the SG, taking into account the penetration depth characteristics of type II superconducting sphere. The model can be used to calculate the independent contributions of the upper and lower superconducting coils to the superconducting sphere levitation force, the magnetic gradient of the SG, and most importantly, the temperature coefficient of the SG temperature effect. Calculations show that temperature variations change the penetration depth and levitation force of the superconducting sphere and that the penetration depth determined at 4.2 K corresponds to a unique temperature coefficient, which means that the effect of the same temperature on the levitation force of the superconducting sphere is definite for a certain penetration depth. Further studies find that the temperature coefficient depends linearly on the effective penetration depth of the superconducting sphere, and the greater temperature coefficient than that of the smooth superconductor depends on the surface preparation and surface oxidation of the superconducting sphere. After discussion, it is clear that Nb coating on the surface of superconducting spheres is an effective solution to avoid passive isolation in the future.