Cryogen-free dilution refrigerator precooled by a pulse-tube refrigerator with non-magnetic regenerator

We present a cryogen-free He-3/He-4 dilution refrigerator (DR) which was precooled by a 10 K pulse tube refrigerator (PTR), CRYOMECH, model PT805. The PT805 has no Tare-earth materials in the second stage regenerator which are very expensive and normally generate oscillating magnetic fields with amplitudes of similar to 20 nT at low temperatures. The challenge for our application was that the base temperature of our PTR was 5.5 K which is similar to 3 K higher than that of a standard 4 K-PTR (similar to 2.5 K). After being precooled by the PTR, the He-3 of the DR was further precooled and liquefied in a Joule-Thomson stage before it entered the dilution unit. The He-3 throughput which determines the cooling capacity of the dilution unit was up to 250 mu mol/s, and its base temperature was 5.4 mK. "Dry" DRs are very reliable and user-friendly to operate, cool downs are automatic, and experimental conditions remain unchanged during sustained continuous operation of the cryostat. In our DR there are no magnetic field distortions because of the non-magnetic regenerators of the PTR.