Numerical simulation of AC losses in superconducting gravimeter

Abstract

The annual drift of μGal level is an important indicator of superconducting gravimeters, which helps geophysicists to clarify the weak geophysical signals. In this paper, a finite element simulation model of the superconducting gravimeter sensor is developed based on the H formulation for evaluating the contribution of the excitation AC losses and the AC losses under operating conditions to the superconducting gravimeter’s drift. The model combines the H formulation and the heat transfer module of COMSOL Multiphysics software to calculate the AC losses of the superconducting gravimeter’s test mass and obtain the distribution images of the test mass’s temperature due to the AC losses-induced heating. The overall temperature rise of the test mass is obtained by assuming that it heats up uniformly and thus combines the temperature dependence factor (10 μGal/mK) of the superconducting gravimeter to derive the instrument drift induced by AC losses. Then, the long-term drift due to excitation losses can reach 0.847 μGal/year, while the operating losses can be 0.45 μGal/year or even less, according to the simulation. In addition, this paper discusses the effects of the parameters (index number, critical electric field and critical current density) in the E-J power law introduced by the H formulation on the AC loss evaluation. It is concluded that the AC losses are sensitive to the critical current density, and increasing the test mass’s critical current density helps enhance the stability of the superconducting gravimeter.

Properties of superconducting MgB2 spherical shells deposited on 2 mm and 1 mm diameter Si3N4 spheres

“…In the Gravity Probe B (GP-B) experiment aiming at testing the fundamental predictions of Einstein's theory of general relativity, there are four gyroscopes each comprising a fused silica sphere coated with 1.25 µm thick superconducting Nb [10], which are the heart of the instrument and critical to the success of the nearly 48 year GP-B mission [11,12]. Superconducting spherical shells, typically 2.5 cm diameter Nb, are also the basic elements of superconducting gravimeters [13], which use the levitation of the hollow superconducting sphere to measure the gravity change with high precision and have been deployed to study a wide variety of geophysical phenomena [14,15].…”