Concept of a Powerful Cryogen-Free Dilution Refrigerator with Separate 1 K Stage
Helium-3,4 dilution refrigerators (DR) are the workhorses for ultralow temperature scientists. DRs can be continuously operated for unlimited periods of time, and compared to other cooling techniques offer high cooling capacities. Base temperatures are well below 10 mK for well designed DRs. Cryogen-free (CF) DRs precooled by pulse tube refrigerators (PTR) have become standard in recent years. In a typical CF-DR, the second stage of the PTR runs at temperatures between 2.5 K and 4 K. Its cooling capacity at 4 K is about of 1 W. The next cooling stage is the still of the dilution unit with a typical temperature near 0.7 K. Its cooling capacity is proportional to the He-3 flow and usually well below 20 mW. For many modern applications this is too little to cool and heatsink cold amplifiers, coax lines and electric cables. For these experiments an additional refrigeration stage at an intermediate temperature of about 1 K would be desirable. Several suggestions have been made in the past to address the task.1 In our CF-DR, we plan to implement a continuous He-4 refrigeration stage with a base temperature of ~ 1.2 K. The components of the cryostat have been tested separately, but have not been combined in our cryostat so far. The cooling capacity of a newly built 1K-stage has been measured as a function of temperature; it is about 100 mW, whereas the cooling power of the still of the DR is 17 mW. The circulation rates of the DR stage and of the new He-4 stage are about of 1 mmol/s (22 std. cm3/s) each. We present details of the planned CF-DR with 1K-stage.