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.

The pulse tube cooler (PTC) driven by a thermoacoustic engine can completely eliminate mechanical moving parts, and then achieves a simpler and more reliable device. A Stirling thermoacoustic heat engine has been constructed and tested. The heat engine can generate a maximal pressure ratio of 1.19, which makes it possible to drive a PTC and get good performance. Frequency is one of the key operating parameters, not only for the heat engine but also for the PTC. In order to adapt to the relatively low design frequency of the PTC, the operating frequency of the thermoacoustic heat engine was regulated by varying the length of the resonance tube. Driven by the thermoacoustic engine, a single stage double-inlet PTC obtained the lowest refrigeration temperature of 80.9K with an operating frequency of 45Hz, which is regarded as a new record for the reported thermoacoustically driven refrigerators.

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.

Frequency effects on the performance of a two-stage 4K pulse tube cooler (PTC) have been extensively explored. The dependence of refrigeration temperature, cooling power and efficiency on the operating frequency of a 4K PTC are investigated and compared with those of G-M and Stirling coolers. The cooling performance can be considerably improved by frequency optimization. Cooling powers of 30W at 70K, 500mW at 4.2K (1.2Hz) and 20W at 65K, 590mW at 4.2K (1.1Hz) are obtained.

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.