Thermal shunt for quick cool-down of two-stage closed-cycle refrigerator

An efficient and easy-to-build gas circuit is described which acts as a thermal shunt between the two stages of a Gifford–McMahon cooler (GMC) during the cool-down period of an experiment. With this circuit the high cooling power of the first stage is used to cool objects with a large heat capacity attached to the second stage like a superconducting magnet or, as in our case, a dilution refrigerator (DR) with a vacuum can. The circuit is driven by a small fraction of the helium gas of the Gifford–McMahon refrigerator and precools the DR from room temperature to a temperature of 50 K. The shunt circuit reduces the cool-down time by a factor of two in our cryostat.