Research on a solid Nitrogen-Cooled cryogenic system for MgB₂ superconducting magnets used in gyrotrons

A cryogenic cooling system for a superconducting magnet used in gyrotrons has been studied. The superconducting magnet was fabricated using MgB₂ wire and was designed to achieve a maximum magnetic flux density of ∼1.3 T. Firstly, the electromagnetic design was carried out, based on which the mechanical design and manufacturing were completed. In preliminary tests, the MgB₂ superconducting magnet was cooled by direct conduction cooling using four cryocoolers, and magnetization and demagnetization tests were performed at several different temperatures between 12 K and 20 K. Subsequently, the magnet was integrated into a carefully designed cryostat. Detailed heat transfer analyses and calculations were conducted during the design of the cryogenic system. The superconducting magnet is welded into a sealed reservoir and cooled through contact with nitrogen coolant (including both convection and conduction). The large heat capacity of solid nitrogen at low temperatures is utilized to maintain the temperature of the magnet over extended periods of time. The solid nitrogen reservoir is cooled by the second stage of a two-stage GM cryocooler and is enclosed within a cold shield cooled by the first stage. Specifically, to extend the holding time of solid nitrogen at low temperatures, a cryocooler chamber device is employed, which enables the detachment and repositioning of the cryocooler's cold head while the magnet is in normal operation. Experimental results show that the cryogenic cooling system can cool the MgB₂ superconducting magnet to nearly 4.5 K. With the cold head detached, the magnet operates stably at a working current of 84 A for over 4 h, during which the magnet's temperature increased from approximately 5 K to around 14 K. The maximum magnetic field strength achieved is 1.295 ± 0.01 T, meeting the design requirements.