Reaching 96 mK by a Pulse-Tube Precooled Adiabatic Demagnetization Refrigerator

We report on the first operation of an adiabatic demagnetization refrigerator (ADR) that employs a two-stage 4 K pulse tube refrigerator (PTR) for precooling. The ADR stage consists of a ferric ammonium alum (FAA) salt pill located in the bore of a commercial superconducting magnet. Precooling of the FAA pill is accomplished by means of a mechanical heat switch thermally connected to the 2nd stage cold end of the PTR. The PTR, which was developed at the University of Giessen, simultaneously provides cooling powers of 0.43 W at 4.2 K and 10 W at 57 K at the 2nd and 1st stage, respectively. The PTR is operated by use of a 6 kW GM-compressor in combination with a rotary valve. The magnet is conductively cooled by thermally connecting the Al-bobbin to the 2nd stage cold end of the PTR. The part of the current leads extending from the 1st stage cold end to the magnet terminals consists of high-Tc Bi-2223 tapes with low thermal conductance. After a cool-down period of about 13 h, the temperatures of the 1st and 2nd stage were about 59 K and 2.8 K. Using a sweep rate of 10 A/minute the magnet was successfully charged with a current of 55 A, corresponding to a field of 4.4 T. After demagnetization the FAA pill attained a minimum temperature of 96 mK. Then the pill slowly warmed-up to 100 mK after l 1/2 hours and to 140 mK after 4 hours. Lower warm-up rates and lower temperatures could be expected after reducing the heat leak to the ADR stage.