Research On Cool-down Methods Of A Small Superconducting Magnet System Cooled By Pulse Tube Cooler

A crycocooler cooled superconducting magnet system will gradually replace conventional superconducting magnet system cooled by liquid helium. However, one of the important problems for the new system is that the cool- down process lasts too long. For our system, the magnet is immerged in a dewer and the upper surface of the dewer is thermally anchored to the second cold end of a two-stage pulse tube cooler with cooling power of 0.6W at 4.2K. During the cool-down process, helium gas is first liquefied by the pulse tube cooler and filled into the dewer. On steady state, the evaporated helium gas is re-liquefied by convectional heat transfer with fins soldered to the upper surface the dewar.In order to accelerate the cool down process, two methods, using a thermal switch or liquid nitrogen, are investigated and compared in detail. The corresponding work is as follows:1) Cryogenic system for a superconducting magnet cooled by a two-stage pulse tube cooler is introduced in detail. The magnet system is immerged to liquid helium, and the evaporated helium gas is re-condensed by the pulse tube cooler.2) A detailed heat load analysis and calculation is presented for each stage of the pulse tube cooler. The cool-down time without any accessorial facilities is obtained.3) Performance of two kinds of gas-gap thermal switches is investigated and compared. A numerical analysis model is established to research on the effects of length, length of fin, thickness of fin, gas clearance between fins on the performance of the switches. The cool-down rate of the system with these two switches is presented, respectively.4) The superconducting magnet system pre-cooled by the liquid nitrogen is analyzed. The magnet system is first cooled down to 77K by vaporizing the liquid nitrogen, and then it is cooled down to 4.2K by the pulse tube cooler.5) The improved cool-down performance of the system with the pre-cooled methods: by thermal switch and by liquid nitrogen, is compared, and liquid nitrogen is finally adopted.