Condensation stage of a pulse tube pre-cooled dilution refrigerator

摘要

In our article, experiments with a pulse tube (PTR) pre-cooled dilution refrigerator (DR) are presented, where an upgraded ~3He condensation stage has been tested. The DR had a ~3He flow rate of up to 1.1 mmol/s. The ~3He gas entering the refrigerator was first pre-cooled to a temperature of approx 50 KAPPA at the first stage of the PTR. In the next cooling step, the ~3He was run through a recently installed heat exchanger, which was attached to the regenerator of the second stage of the pulse tube cryocooler; at the outlet of this heat exchanger the temperature of the ~3He was as low as approx 4 KAPPA. Due to the non-ideality of the helium gas, the second regenerator of a two stage PTR has excess cooling power which can be made use of without affecting the base temperature of this stage, and it is this effect which was put to work, here. Finally, the ~3He was further cooled in a heat exchanger, mounted at the second stage of the PTR, before it entered the dilution unit of the cryostat. The installation of a heat exchanger at the regenerator of the second stage of the PTR is especially important for the construction of DRs with high refrigeration capacities; in addition, it allows for a plain design of the subsequent Joule-Thomson (JT) stage, and herewith facilitates considerably the construction of "dry" DRs. The condensation rate of the ~(3,4)He mash prior to an experiment was increased. The pressure during condensation could be kept near 1 bar, and thus a compressor was no longer necessary with the modified apparatus.