Gas - gap thermal switch for precooling of cryocooler-cooled superconducting magnets

The authors have developed a heat switch for use with superconducting magnets that are cooled by two - stage GM cryocoolers. In developing cryocooler-cooled 6T and 10T superconducting magnets, which weigh some tens of kilograms, the authors have found that shortening cooldown times of this type of magnets is of vital importance for their wide-spread use. To shorten the magnet cooldown time, a gas-filled thermal switch having a gas-gap configuration was developed; the gap between heat transfer cylinders inside the switch is designed to be I mm and is filled with nitrogen. The switch thermally bridges the first and second stages of the cryocooler above 70 K when the switch is open-quotes ON.close quotes It de-couples the two stages below 50 K when the switch is open-quotes OFF.close quotes It was observed that in the temperature range 70 to 300 K, the switch provides a 80 W refrigeration capacity at the second refrigeration stage, which is about four times higher capacity than the second stage alone can provide. Using a copper block of 40 kg, they demonstrated that the cooldown time from 300 K to 4 K was only a quarter of the time required without the switch