Dry demagnetization cryostat for sub-millikelvin helium experiments: refrigeration and thermometry

TL;DR

This paper presents a successful dry cryogenic system capable of reaching 0.16 mK for quantum fluid experiments, utilizing a copper nuclear demagnetization stage cooled by a pulse-tube refrigerator, with innovative vibration minimization.

Contribution

The study introduces a novel dry refrigeration setup for sub-millikelvin temperatures that minimizes vibrations and heat leaks, suitable for superfluid helium experiments.

Findings

Achieved temperature of 0.16 mK with minimal heat leak of 4.4 nW.

Implemented a vibration-free design to enhance thermal stability.

Demonstrated quartz tuning fork as a self-calibrating thermometry method.

Abstract

We demonstrate successful "dry" refrigeration of quantum fluids down to $T=0.16$\,mK by using copper nuclear demagnetization stage that is pre-cooled by a pulse-tube-based dilution refrigerator. This type of refrigeration delivers a flexible and simple sub-mK solution to a variety of needs including experiments with superfluid $^3$He. Our central design principle was to eliminate relative vibrations between the high-field magnet and the nuclear refrigeration stage, which resulted in the minimum heat leak of $Q=4.4$\,nW obtained in field of 35\,mT. For thermometry, we employed a quartz tuning fork immersed into liquid $^3$He. We show that the fork oscillator can be considered as self-calibrating in superfluid $^3$He at the crossover point from hydrodynamic into ballistic quasiparticle regime.