In the past several years, we have investigated two-stage high frequency pulse tube coolers (PTC) for operation at 20 K to 40 K by simulation and experiment. Initially, a computer code, based on a thermal physics method, was developed to optimize the geometrical parameters of our first u-tube two-stage PTC, built and tested at the University of Giessen. In 2003, this PTC achieved the lowest temperature of 19.6 K with a 250 W electrical input, using a stainless steel mesh for the regenerator material. Here, we report on further tests conducted with this cooler. The experiments use lead spheres of different size or lead-plated mesh in the 2nd regenerator and then by use of a larger 2nd-stage pulse tube. There is no evident improve- ment in the cooling performance by the use of the lead materials. With the larger pulse tube, the refrigera- tion temperature has been reduced and the cooling capacity has been increased. A new version of this PTC with a coaxial configuration for the 1 st stage was built at CAS Beijing. It resulted in a low temperature of 16.1K with 250 W electrical input power.
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