A 3.27 K High-Frequency Pulse Tube Cryocooler Using Adsorbed ⁴He as Part of the Regenerator Material

Abstract

As a crucial temperature range for space applications as well as the pre-cooling temperature of mK-level refrigeration, the acquisition of the liquid-helium temperature has become a key research topic in recent years. Benefiting from the merits of no moving parts at the cold end, compact structure, high reliability, and low weight, high-frequency pulse tube cryocoolers (HPTCs) have become a conspicuous alternative for directly obtaining temperatures below 4 K. However, due to insufficient specific heat capacity of the commonly used regenerator materials at low temperatures, the large heat transfer losses of HPTCs limit its access to the liquid-helium temperature. Considering that the working gas ⁴He itself has a higher specific heat capacity than the commonly used rare earth materials below 10 K, an adsorptive regenerator scheme was proposed using non-magnetic, low-cost porous activated carbon (AC) to adsorb a part of working gas ⁴He as the low-temperature regenerator material to cool the remaining helium. Firstly, an adsorption model was established to simulate the dynamic adsorption characteristics of ⁴He with AC, and it was pointed out that low temperature and high pressure were beneficial to the adsorption of helium, which improves the cold storage capacity of the regenerator. Then, a three-stage HPTC was built for experimental verification. By filling AC in the low-temperature regenerator to substitute a part of the conventional rare earth material HoCu₂, the cryocooler obtained a record-breaking no-load temperature of 3.27 K using ⁴He as the working gas. The cooling power at 4.0 K increased by 41.71%, verifying the feasibility of this adsorption scheme.