Experimental Study of a 4 K Pulse Tube Cryocooler
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A prototype two-stage pulse tube cryocooler was previously developed that uses a double-inlet configuration. When the second-stage regenerator was filled with conventional magnetic regenerator materials, Er 3 Ni and HoCu 2 , the cryocooler achieved a minimum temperature of 2.89 K on the second stage and a maximum cooling capacity of 170 mW at 4.2 K. The rated input power of the compressor unit is 3.3 kW at 50 Hz.
In this study, to improve the cooling capacity at 4.2 K for this cryocooler, we used a new oxide regenerator material, GdAlO 3 , in the second-stage regenerator. This material has a magnetic transition temperature of about 3.8 K, and has a considerably larger heat capacity compared with that of Er 3 Ni and HoCu 2 below 4 K. When GdAlO 3 was placed in the lowest-temperature part of the second-stage regenerator, the cryocooler achieved a no-load temperature of 2.51 K at the second stage, and a cooling capacity at 4.2 K of 250 mW. By adjusting the pulse tube size for further optimization, a cooling capacity at 4.2 K of 288 mW and a no-load temperature of 2.42 K was achieved.
To further evaluate the effect of GdAlO 3 , we placed GdAlO3 in the second-stage regenerator of a 4 K Gifford-McMahon (GM) cryocooler. The cooling capacity below 4 K was improved, but that at 4.2 K was degraded.
By numerical simulation, we determined the effect of GdAlO3 on cooling performance both for the 4 K pulse tube cryocooler and the 4 K GM cryocooler. For the 4 K GM cooler, the numerical results roughly agreed with the experimental results. For the 4 K pulse tube cooler, the numerical results showed that, again, the cooling performance at 4.2 K was degraded by using GdAlO3 By numerical simulation, we also examined the temperature distribution in the second-stage regenerator and the temperature oscillation in the second-stage expansion space. The mechanism that makes GdAlO3 effective for improving the cooling performance at 4.2 K in the 4 K pulse tube cooler remains unclear. More detailed analysis is needed to clarify this mechanism.
KeywordsTemperature Oscillation Cooling Capacity Needle Valve Pulse Tube Regenerator Material
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