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

Abstract

We report design details of the whole assembly of a compact and continuous nuclear demagnetization refrigerator (CNDR) with two PrNi$_5$ nuclear stages, which can keep temperature below 1 mK continuously, and test results of a new thermal contact method for the PrNi$_5$ stage 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 $1.8\pm0.1 \times 10^{-4} T^{-1}~\rm{Km^2W^{-1}}$. Based on this value and 2D numerical and 1D analytical thermal simulations, the largest possible temperature gradient throughout the nuclear stage was calculated to be negligibly small ($\leq$ 2 %) at 1 mK under a 10 nW heat leak, the expected cooling power of the CNDR.