Cryogenic Instrumentation using Planar Inductor based Eddy Current Sensors

Abstract

Cryogenic sensors have become vital in the measurement of crucial parameters in modern scientific research. This work addresses the design, development and testing of planar inductor eddy current sensors and associated cold electronics for a variety of cryogenic applications. The first sensor designed is a multilayer planar inductor based eddy current proximity/displacement transducer. The initial part of the work focuses on the behaviour of PCB (FR4) based multilayer inductors at 4.2 K. The structural changes (warping) that were in the simulation studies were observed through the variation of capacitance between the layers of the inductors when the sensor was cooled. The second part of the work incorporates the designed multilayer inductor to develop a proximity sensor capable of measuring displacement in the range of (0-5mm) down to 4.2 K. Since the effective realization of the inductor based sensors require signal conditioning elements to be close to the sensing element, the electronic circuits which are capable of working at cryogenic temperatures without any drastic changes in parameters or at least predictable changes in parameters were developed. A detailed study of performance analysis of unbuffered inverter-based LC oscillator development is also discussed. The developed sensor has good thermal stability, sensitivity and repeatability at the cryogenic operating temperatures. The second sensor is a multilayer planar inductor array based eddy current angular position/rotation transducer working at 4.2 K using cold electronics signal conditioning circuits. A study on the rotor segments that would provide the most effective sensing (zero dead zone) is also done. The developed sensor is characterized for the entire temperature range (4.2 K – 300 K ) and shown to work satisfactorily. The final set of sensors are designed to measure the Residual Resistivity Ratio (RRR) of Nb samples. RRR is an important parameter that dictates the purity and in turn, the performance of the Superconducting Radio Frequency (SRF) cavities at low temperatures (<4.2 K). Here, three different non - contact RRR measurement techniques are presented which utilize the eddy current principles. The initial approach uses the ratio of the slope of lift - off lines generated by the impedance variation when the conductivity of the Nb sample changes to obtain the RRR value. The second approach utilizes the inflection point, which relates eddy current penetration depth to the conductivity of the metal. The third approach correlates the inductance variation of the sensing coil with the RRR of the sample through a cold electronics based multiplexed inductor LC oscillator circuit