Study on a high frequency pulse tube cryocooler capable of achieving temperatures below 4 K by helium-4

•3.6 K was achieved by a three-stage high frequency pulse tube cryocooler with helium-4.•6 mW/4.2 K is achieved with 250 W input power and 12.1 W/77 K precooling power.•The inertance tube-gas reservoir structure can be replaced by cold inertance tube alone.•Double-inlet can still play an effective role around 20 K for HPTC.•HoCu2-Er3Ni mixed filling can achieve a better cooling performance than Er3Ni or HoCu2 alone. High-frequency pulse tube cryocooler (HPTC) has advantages of compact structure, low vibration, high reliability and long operation time. In this study, Theoretical analysis and experimental tests have been conducted in four aspects based on a developed 4 K HPTC. Firstly, a compressor with larger power output capability was employed and the impedance match between the cold head and the compressor was discussed. Secondly, simply using inertance tube configuration to replace the traditional inertance tube-gas reservoir structure. Then, the type and the size of the regenerator materials working at 4–20 K have been experimentally optimized. Finally, the performance of double-inlet working at as low as 20 K has also been tested for the first time for the HPTC. The present prototype achieved a no-load temperature of 3.6 K, which is the lowest temperature record ever reported for HPTC using helium-4 as working gas. A cooling power of 6 mW/4.2 K was also obtained with 250 W input power and a precooling power of 12.1 W/77 K.