In this paper, a pre-cooled two-stage 4K pulse tube refrigerator is presented. The phase shifter of the first stage is two-middle buffer type, that of the second stage is double inlet type. There are four 1.5 meter long connecting tubes between the rotary valve and the cold head for reducing the vibration. The cooling power of 0.62W and 0.72W at 4.2K were obtained with 5.5kW and 6.5kW of compressor input power, respectively. A simple method for partially solving the intrinsic low efficiency problem of 4K regenerative refrigerator is proposed. A heat exchanger, which is called inter-heat exchanger, is installed near the center of the second stage regenerator to get excess cooling power around 10-16K without any influence to the cooling power of the second stage. Theoretical analysis and numerical simulation shows that significant excess cooling power can be produced.
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