A hybrid cryocooler achieving 1.8 K with He-4 as the only working medium and its application verification

With the rapid development of the quantum information technology and deep space exploration. there is an increasing demand for cryocoolers featuring high reliability, long operation lifetime, small scale, lightweight and high cooling efficiency, while the hybrid cooling cycle formed by the multi-stage high frequency pulse tube and the Joule-Thomson (JT) is an important approach to the problem. To date, over the world, the temperatures below 2 K with the above cycle have all been achieved by the use of He-4 in the pulse tube subsystem while He-3 in the JT subsystem. However, He-3 is rare and expensive, which is the key bottleneck of hindering the wide application of the cooling cycle. This paper makes theoretical and experimental investigations on the hybrid cooling cycle formed by the four-stage high frequency pulse tube and the JT, in which the pulse tube subsystem acts as the precooler while the JT subsystem as the final cooling stage. The distinctive feature of the hybrid cooling cycle is that in the entire system the He-4 is the only working medium. The key difficulty and feasibility of achieving temperatures below 2 K with the above hybrid system have been analyzed. Based on the suction pressure of the first stage compressor and counter-flow heat exchanger (CHEX) pressure drop on the low pressure side, it was predicted that with the charge pressure of 40 kPa, the suction pressure and the gross pressure drop can reach 1.1 kPa and 438.6 Pa, respectively, and thus a saturated vapor pressure of 1.54 kPa is obtained. which makes it feasible to achieve a no-load temperature of 1.78 K. Meanwhile, we theoretically analyzed the effect of the Kapitza conductance for a small interfacial temperature difference on the heat transfer in the cold head evaporator with He II, and then achieved the limiting condition of the parameter improvement for the JT cycle. The no-load temperature of the developed hybrid cryocooler dropped from 300 K to 1.8 K in 16.5 hours, and the temperature fluctuation was no higher than +/- 6 mK during the 360 hours of continuous operation time, which validates the above theoretical analyses and the satisfactory temperature stability of the superfluid helium medium. In order to comprehensively and objectively evaluate the performance of the developed hybrid cryocooler in practical applications, the experiment of coupling it with the actual superconducting nanowire single-photon detector (SNSPD) was conducted subsequently, in which the SNSPD performance was evaluated by measuring the system detection efficiency (SDE) and dark count rate (DCR). The results indicate that the developed cryocooler can provide effective cooling at 1.84 K and the favourable electrical environment, which ensures the SNSPD to work stably and reliably. It is the first time that an operating temperature below 2 K was reported with the similar hybrid cooling cycle using He-4 as the only working medium. especially when an actual SNSPD was coupled. This breakthrough will not only make it feasible for the future space application of the SNSPD, but also pave the way to break the bottleneck of its wide application in a variety of fields.