Precise Adjustment Of The Persistent Current In Superconducting Gravity Instruments

Due to the extremely low intrinsic noise, superconducting gravity gradiometer?SGG? has been considered as one of the most promising candidate for next generation high-sensitivity tools in flight exploration survey. However, because of the notorious vibration of the aircraft, extremely high common-mode rejection ratio?CMRR? is also required for the SGG. Otherwise, the common-mode noise will dominate and there is no way to make use of the merits of SGG. To obtain a sensitivity of 1E / ?Hz?^1/2 in airborne environment, the CMRR of SGG must be larger than 1×109. For the best case, CMRR of 1×103 can be reached by elaborately matching the parameters of the two superconducting accelerometers?SA?. The left 6 order of magnitude must resort to precise adjustment of the persistent current in superconducting circuit, which compensates the unavoidable tiny difference in physical parameters between two SA. Obviously, the current setting and adjustment resolution must be better than 10-6, this is beyond the capability of commercial current source meters.In this thesis, the method to precisely adjust the superconducting loop current has been investigated. The strategy is to set the preminary current using a source meter, then reduce the current for the loop which need to do. To achieve this, an ultra-low resistor and a heat-switch that working in pulse mode are imbedded in superconducting circuit. While the heat-switch is turned on, the superconducting current is forced to pass through the ultra-low resistor and the current allow to decay for a given time. The resolution of current regulation is only determined by the resistance of the ultra low resistor and superconducting circuit resistive time when the coil inductance of superconducting circuit remains constant. To reach a relative resolution of 10-6, the resistance required is of the order of 10-8? and resistive time is of the order of 10-3s.In this thesis, the ultra-low resistor has been studied and the resistor that we have made is of the order of 10-8?, the lowest resistor that we have measured is?23±10?n?. The thermal structure of the heat-switch has also been optimized, and the shortest resistive time we have measured with the optimized structure is?5±1? ms. Combining the results above with the inductance coil parameters in actual superconducting circuit, the current could be adjusted to a relative resolution of about 3?2?×10-6, which satisfies our requirement.