Dry demagnetization cryostat for sub-millikelvin helium experiments: refrigeration and thermometry
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 3He. 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 3He. We show that the fork oscillator can be considered as self-calibrating in superfluid 3He at the crossover point from hydrodynamic into ballistic quasiparticle regime.