The recently proposed continuous nuclear demagnetization refrigerators (CNDR), which are based on multiple nuclear demagnetization stages, would allow ultra-low temperature researchers to take full advantage of cryogen-free technology, making it much easier to reach T<1 mK. Our calculations implied that we could construct a CNDR with a cooling power of tens of nW at 1 mK [D. Schmoranzer et al., Cryogenics 110, 103119 (2020)]. However, building a CNDR is challenging, in part, because the two nuclear stages must be linked by superconducting heat switches that have very low thermal resistance when closed. This thermal resistance limits the rate at which the CNDR can be cycled and consequently limits its cooling power. We had demonstrated a heat switch whose thermal resistance when closed is five times better than the previous state of the art and, unlike the latter, does not rely on a cyanide-based plating process [J. Butterworth et al., Review of Scientific Instruments (2022)]. An essential fabrication step was the efficient removal of the native oxide of the aluminum superconducting element followed by gold deposition without breaking vacuum.
The ERC PoC project NewCooler was dedicated to building on these achievements to construct the first CNDR. Our successful use of aluminum in our heat switch motivated us to experiment with aluminum nuclear refrigerant. This material is more abundant and workable than the brittle rare earth-based conventional refrigerant PrNi5. Furthermore, the minimum temperature of a few microkelvin that can be achieved with Al is far below that of PrNi5, which is limited to 0.4 mK due to nuclear magnetic ordering. The first step was to demonstrate that an aluminum nuclear demagnetization refrigerator in "single-shot" mode would meet our design criteria for the CNDR. With the support of the ERC PoC project NewCooler, we achieved this and reported the results in M. Raba et al., Physical Review Applied 22, 024027 (2024). The results included measurements of the minimum temperature, heat leak and thermal time constant of the refrigerator. We confirmed that our design is effective, efficient and sustainable, avoiding toxic substance such as cadmium solder used in previous nuclear refrigerators.
Following the demonstration of the "single-shot" refrigerator, we focused on the design and construction of the continuous system, which is presently being manufactured.
