Experimental Studies on Thermal Behavior of 6T Cryogen-Free Superconducting Magnet System

Abstract

The main aim of this thesis is to report very comprehensive studies and analysis that I have carried out on different thermal aspects of a 6T Cryogen-free Superconducting Magnet System (CFMS) which is designed and built at IUAC. One of important parameter for any superconducting magnet is to achieve the desired field homogeneity by choosing proper dimensional parameter. The role dimensional parameters for the 6T NbTi magnet have been studied to achieve 0.07% field homogeneity at 10mm diametrical spherical volume at the magnetic center. The stability of the 6T NbTi magnet is greatly dependent by its operational margin which is determined by the current sharing temperature of the NbTi. The role of the operational load line has been discussed in determining the stability of the 6T NbTi magnet. The current sharing temperature of the 6T magnet has also been estimated. Two stage GM cryocooler is one of the critical components of the CFMS. The refrigeration load curves for both stages of SRDK-415 GM cryocooler have been generated in a cryocooler based test rig. The refrigeration load curves for the 1st stage of the cryocooler have been generated between 25K and 65K with 0-1.5W of load at the 2nd stage of the cryocooler. Similarly, the refrigeration load curves for the 2nd stage have been generated between 2.4K and 4.4K with 0-47.5W load at 1st stage of the cryocooler. The magnetic field profile for 6T solenoid magnet has been generated with the use of finite element magnetostatic simulation software (OPERA/TOSCA). Hybrid current lead is one of the crucial components of the CFMS. Thermal performance of inter-lead joint between hybrid lead and cold heads of the cryocooler plays a significant role in determining the stability of the NbTi magnet in the CFMS. A prototype lead joint has been characterized in the temperature range of 4-40K in a test rig with different interfacial material (Aluminium nitride/ Kapton) to achieve thermally conducting and electrically isolated joint. The extensive FEM analysis has also done using ANSYS for different types of conduction-cooled leads, optimized for 102A. Quenching is one of the important phenomenon for a superconducting magnet. The protection system for a superconducting magnet needs to be designed ingeniously. In this report, extensive FEM analysis has been done, using OPERA-3D/QUENCH for the 6T NbTi magnet. The analysis has shown the hot spot temperature of the 6T NbTi magnet would be 65-80K, if quenched at 102A. It has been compared with the experimental result. A detailed comparative studies have been done between different types of quench situation. Variable temperature inserts (VTI) is one of common feature integrated with each commercial CFMS to vary temperature on the sample space. The experimental studies have been performed on the gas gap heat switch (GGHS) integrated with CFMS. The feasibility of using GGHS as VTI has been studied up to 3T magnetic field.