Generally, a compressor together with a rotary valve system generates the pressure oscillation in GM-type cryocoolers. The timing of the rotary valve, which is one of the key operating parameters for cryocoolers, determines the relationship between intake andexhaust processes. A systematic investigation of valve timing effects on cooling performance of a two-stage 4K pulse tube cooler (PTC) is reported. The experiments show that the optimization of valve timing can considerably improve the cooling performance for both stages. For the same PTC, a performance comparison for operation on different compressors with various input powers ranging from 0.5 to 6.0kW is also presented.

Cryogen-free operation of Josephson voltage standards requires a low-noise cryocooler that provides a cooling power of 100mW or less at 4.2K, and about 1W of precooling at an intermediate temperature near 70K. Due to its intrinsic low level of mechanical vibrations a pulse tube cooler (PTC) appears to be a suitable candidate for such an application. In this work, a two-stage pulse tube cooler has been developed and optimized for cooling of a 1V or 10V Josephson voltage standard near 4K. The performance of the PTC, which is driven by a compressor with a rated input power of 1.7kW, is optimized under three different conditions. So far, a minimum temperature of 2.66K and a maximum cooling power of 202mW at 4.2K with coefficient of performance (COP) of 1.15xlO-4 have been been achieved. The performance of the crycooler is expected to fully satisfy the cooling requirement of the volatge standard. In addition, the experimental results are also compared with those obtained by driving the same cooler by use of a 6kW compressor.

The cooling power and coeffcient of performance of a two-stage pulse tube cooler below 4K have been increased greatly by using the newly developed ceramic magnetic regenerative material GdAlO3 (GAP). Cooling powers of 200mW at 2.8K, 300mW at 3.13K, and 400mW aat 3.70K have been achieved with a compressor imput power of about 4.8kW. The results show that the cooling power near 3.0K incrases by 150% compared that of the same pulse tube cooler (PTC) employing only conventional HoCu2 and ErNi regenerator materials

A bench consisting of a pulse tube refrigerator driven by a standing-wave thermoacoustic prime mover has been set up to study the relationship among stack, regenerator and working fluids. The stack of the thermoacoustic prime mover is packed with dense-mesh wire screens because of their low cost and easy manufacture. The effect of the packing factor in the stack on onset temperature, refrigeration temperature and input power is explored. The optimum packing factor of 1.15 pieces per millimeter has been found experimentally, which supplies an empirical value to satisfy a compromise for enhancing thermoacoustic effect, decreasing heat conduction and fluid-friction losses along the stack. The pulse tube cooler driven by the thermoacoustic prime mover is able to obtain refrigeration temperatures as low as 138 and 196K with helium and nitrogen, respectively. Copyright

As the thermoacoustic conversion process occurring in its regenerator is considered reversible, a traveling wave thermoacoustic engine can theoretically produce and transmit acoustic power with high effi-ciency. A large scale multifunction thermoacoustic engine has been designed, constructed and tested. Compared with the results of standing wave thermoacoustic heat engines, the experimental results show the engine has a lower onset temperature, much higher pressure ratio and higher efficiency. With the filling nitrogen of 0.9MPa, the maximal pressure ratio reached 1.21, and the operation frequency was steady at 25Hz. The single stage U type orifice pulse tube cooler driven by the engine reached 153.7K when the working pressure was 1.5MPa.