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

TL;DR

This paper details the design and testing of a compact continuous nuclear demagnetization refrigerator with zinc-soldered PrNi$_5$ stages, achieving sub-millikelvin temperatures with minimal thermal gradients.

Contribution

It introduces a new thermal contact method using Zn soldering for PrNi$_5$ stages and provides thermal contact resistivity measurements and simulations demonstrating effective low-temperature operation.

Findings

Thermal contact resistivity estimated at $1.8 ext{e-4} T^{-1}~ m{Km^2W^{-1}}$.

Thermal gradients in the nuclear stage are negligibly small ($ extless$ 2%) at 1 mK.

The apparatus can maintain temperatures below 1 mK continuously.

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.