A zero boil-off dewar for use with a cryogenic current comparator

We describe the initial performance of our CCC / SQUID insert with a cryocooled dewar based on a two-stage pulse tube that maintains a pool of liquid helium within a sample space. The heat-leak into the liquid helium pool was calculated at ~ 180 mW. When the unit was actively cooling, microphonic signals were seen in the SQUID output mostly below 20 kHz, however, the RF SQUID remained stable and locked at its operating point. The cryocooler may be switched off for periods of several hours during sensitive measurements, if necessary.