~3He/~4He dilution refrigerator with high cooling capacity and direct pulse tube pre-cooling

摘要

In the article, a ~3He/~4He dilution refrigerator (DR) is described which is pre-cooled by a commercial two-stage pulse tube refrigerator (PTR); cryo-liquids are not necessary with this type of milli-kelvin refrigerator. The simple design of the condensation stage of this so-called dry DR is novel and explained in detail. In most dry DRs the circulating ~3He gas is cooled by a two-stage PTR to a temperature of about 4 K. In the next cooling step, the ~3He flow is cooled and partially liquefied in a Joule-Thomson circuit, before it is run to the dilution refrigeration unit. The counterflow heat exchanger of the Joule-Thomson circuit is cooled by the cold ~3He gas pumped from the still of the DR. In the DR described here, the heat exchanger of the Joule-Thomson stage was omitted entirely; in the present design, the ~3He gas is cooled by the PTR in three different heat exchangers, with the first one mounted on the first stage of the PTR, the second one on the regenerator of the second stage, and the third one on the cold end of the second stage. The heat load caused by the ~3He flow is mostly absorbed by the first two heat exchangers. Thus the ~3He flow presents only a small heat load to the second stage of the PTR, which therefore operates close to its base temperature of 2.5 K at all times. A pre-cooling temperature of 2.5 K of the ~3He flow is sufficiently low to run a DR without further pre-cooling. The simplified condensation system allows for a shorter, compacter and more economical design of the DR. Additionally, the pumping speed of the turbo pump is no longer obstructed by the counterflow heat exchanger of the Joule Thomson stage as in our earlier DR design.