Cryogen-free operation of Josephson voltage standards requires a low-noise cryocooler that provides a cooling power of 100 mW or less at 4.2 K, and about 1 W of precooling at an intermediate temperature near 70 K. 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 1 V or 10 V Josephson voltage standard near 4 K. The performance of the PTC, which is driven by a compressor with a rate input power of 1.7 kW, is optimized under three different conditions. So far, a minimum temperature of 2.66 K and a maximum cooling power of 202 mW at 4.2 K with coefficient of performance (COP) of 1.15 * 10~(-4) have been achieved. The performance of the cryocooler is expected to fully satisfy the cooling requirement of the voltage standard. In addition, the experimental results are also compared with those obtained by driving the same cooler by use of a 6 kW compressor.
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