Thermal studies on cryocooler thermal masses from 4 K to 300 K for MRI applications

Abstract

Gifford–McMahon (GM) cryocoolers are pivotal for maintaining the temperature of MRI magnets, necessitating regular servicing for optimal performance and longevity. This study explores the geometric dimensions and shapes of superconducting magnets utilized in MRI for system cooling. The investigation demonstrates that warm-up time is contingent on heat capacity rather than geometric shapes. As a result, the design of magnets can be customized to specific shapes and sizes in accordance with favorable conditions, given that warm-up time is solely dependent on heat capacity. The primary focus is to deliberately prolong the warm-up time of the magnet relative to the GM cryocooler, consequently minimizing service intervals. Laboratory evaluations explore the use of thermal masses equivalent to MRI magnets, employing typical materials such as copper and aluminum. These masses are cooled to 4 K and then subjected to different heating powers to evaluate warm-up characteristics. Various configurations and shapes of thermal masses are numerically studied for warm-up analysis. Comparative assessments between experimental results and simulations reveal that, for cylindrical thermal masses, 80 W heating power facilitates a 76% reduction in warm-up time compared to 20 W heating across all cases. In the exploration of different geometric thermal masses, aluminum demonstrates a remarkable 35% decrease in warm-up time compared to copper. A noteworthy finding emerges, indicating that warm-up time is contingent on heat capacity rather than geometric shapes. These insights hold substantial relevance, particularly in systems with large thermal capacities, such as superconducting magnets. The study contributes valuable knowledge to the optimization of GM cryocoolers for enhanced MRI magnet performance and extended operational life.