Dry Dilution Refrigerator with He-4 Precool Loop

TL;DR

This paper presents a simplified, integrated He-4 precooled dilution refrigerator design that reduces condensation time, eliminates the need for a compressor, and improves base temperature performance for sub-Kelvin research.

Contribution

The authors demonstrate how to combine He-3/He-4 circuits in a dilution refrigerator, enhancing efficiency and safety by removing the compressor and reducing condensation time.

Findings

Condensation time is halved compared to previous designs.

Base temperature achieved is as low as 4.1 mK at small He-3 flow.

No compressor needed, reducing risk of He-3 loss.

Abstract

He-3/He-4 dilution refrigerators (DR) are very common in sub-Kelvin temperature research. We describe a pulse tube precooled DR where a separate He-4 circuit condenses the He-3 of the dilution loop. Whereas in our previous work the dilution circuit and the He-4 circuit were separate, we show how the two circuits can be combined. Originally, the He-4 loop with a base temperature of ~ 1 K was installed to make an additional cooling power of up to 100 mW available to cool cold amplifiers and electrical lines. In the new design, the dilution circuit is run through a heat exchanger in the vessel of the He-4 circuit so that the condensation of the He-3 stream of the DR is done by the He-4 stage. A much reduced condensation time (factor of 2) of the He-3/He-4 gas mixture at the beginning of an experiment is achieved. A compressor is no longer needed with the DR as the condensation pressure remains below atmospheric pressure at all times; thus the risk of losing expensive He-3 gas is small. The performance of the DR has been improved compared to previous work: The base temperature of the mixing chamber at a small He-3 flow rate is now 4.1 mK; at the highest He-3 flow rate of 1.2 mmol/s this temperature increases to 13 mK. Mixing chamber temperatures were measured with a cerium magnesium nitrate (CMN) thermometer which was calibrated with a superconducting fixed point device.