Construction of Continuous Magnetic Cooling Apparatus with Zinc-Soldered PrNi$$_5$$ Nuclear Stages

Abstract

We report design and test results of a new thermal contact method for a PrNi $$_5$$ rod, a nuclear coolant for a compact and continuous nuclear demagnetization refrigerator (CNDR) which can keep temperature below 1 mK continuously, using Zn soldering rather than Cd soldering. By measuring a residual electrical resistance of a short test piece, the thermal contact resistivity between the PrNi $$_5$$ rod and an Ag wires thermal link was estimated as $$1.8\pm 0.1 \times 10^{-4}\, T^{-1}\,\mathrm {Km^2\,W^{-1}}$$ . Based on this value and 2D numerical and 1D analytical thermal simulations, the largest possible temperature gradient throughout the nuclear stage is calculated to be negligibly small ( $$\le$$ 2 %) at 1 mK under a 10 nW heat leak, the expected cooling power of the CNDR. The whole assembly of CNDR using two PrNi $$_5$$ nuclear stages designed in this way is also outlined.