Long Period of Time Cooling Technique Using Liquid Nitrogen for HTS-SQUID System

We examined the design method of a glass Dewar for liquid nitrogen as a cooling apparatus of an HTS-SQUID system for field use. Cooling technique without stressing operators is very important. Although cooling technique using liquid nitrogen is suitable to cool HTS-SQUIDs fast at an operation site and operate them without additional noise, the operation time of SQUIDs is often limited by the holding time of liquid nitrogen. To design a system with adequate size and operation time, it is important to estimate the holding time of the cryostat. We calculated the heat inflow due to thermal conduction through the glass Dewar, probe rod, wirings to the SQUID, silver coating on the glass, heat insulator, and nitrogen gas, as well as the heat radiation from the surface of the heat insulator. We compared these results with the measured data for three types of glass Dewar. We successfully develop a simple model to calculate the holding time of the Dewar, and achieved the holding time of more than 100 h for 0.8 L of liquid nitrogen.