A Study On The Cross-coupling Effect Of Magnetic Crosstalk In Superconducting Accelerometer

Because of the low basic noise and good stability,the instruments based on superconducting principles,including gravimeter and gravity gradiometer,show great potential for air-borne precision gravity measurements.For both,vertical superconducting accelerometer?VSA?is the fundamental unit for instrument construction.With the basic noise level lowered,the cross-talking noise becomes one of the most important factor which restricts the actual resolution of the VSA in dynamic environment.The cross-talking effects can be explained by a number of mechanisms,and the strategies to depress them are quite different for each mechanism.Among them,the one mediated by magnetic field contributes considerable cross-talking noise,especially for the VSA with test mass fully levitated.Therefor,it is necessary to precisely evaluate this effect for the design of superconducting gravity instruments.Due to the complex boundary conditions,it is difficult to do it by theoretical calculations.In this thesis,a special experimental set-up has been built,and the cross-talking coefficient has been separately measured for the magnetic-field mediated mechanism.The experimental set-up consists of a cryogenic unit and a set of room temperature electronics.The cryogenic unit copies the VSA structure so that the boundary conditions are the same for all coils.Different from the actual VSA,the test mass keeps fixed during measurement.A nano-positioners is used to adjust the position and altitude of the test mass.In the measurement,a sinusoidal current as modulation signal is applied to the coils belong to the non-sensitive degree of freedom.The flux coupled to the coil of sensitive degree of freedom is measured by a superconducting quantum device?SQUID?.Thus the magnetic-field mediated cross-talking coefficient is obtained after a simple calculation.The measurement results show that the geometrical relationship between the paired coils has decisive influence on the magnitude of the cross-talking coefficient.As for the coil pairs,which are both enclosed in the same cylinder of test mass,the cross-talking coefficient is at the order of magnitude of 10^-3?Wb/A.While for the coil pair which are separated by the cylinder wall,it is smaller by one order of magnitude in contrast.Also,it has been found for the former case,that the cross-talking effect can be reduced dramatically,if the current directions are set to be opposite to each other for a pair of side-wall coils.Based on measured cross-talking coefficients,the noise level resulted from magnetic cross-talking has been evaluated for laboratory environment and air-borne environment respectively.The results obtained in this work will provides important criteria for the design of superconducting gravity instruments.