Thermal sensitivity of an RF SQUID

A well known limitation to the resolution of practical SQUID systems is set by the sensitivity of the SQUID output to temperature fluctuations. This is particularly serious in non-stationary systems where agitation of the helium bath inevitably results in temperature fluctuations of the SQUID. We have undertaken to characterize the thermal response of commercial RF SQUIDS and tried to understand the causes of this sensitivity. The measurements are done with the SQUID package mounted in a thermally isolated can with He exchange gas. The input coil is open-circuited, and the SQUID unit is placed inside a Nb shield, resulting in negligible sensitivity to external fields. The ambient magnetic field is reduced to less than 10-2 G with mu-metal shields. Our measurement procedure is to slowly cool the SQUID and Nb shield through their superconducting transition temperature in the presence of various externally applied magnetic fields, then observe the temperature dependence of the SQUID output Δ /phi/ from 2K to the highest temperature at which it will operate. It is of course necessary to readjust the RF drive level to its optimum level at each temperature since the critical current of the weak link in the SQUID has a strong temperature dependence. We find that near the optimum setting the SQUID output depends only weakly on RF level, so this is not a crucial adjustment