Survey of Cooling Options for Application in a Low‐TC Squid System for Fetal Magnetocardiography

As part of the development of a low‐Tc SQUID‐based magnetometer system for measuring fetal heart activity, the means of cooling is evaluated. To lower the threshold for the clinical application of this fetal heart monitor, it should be simple to operate. It is, therefore, deemed necessary to replace the liquid helium by a closed‐cycle refrigerator. In this paper, the requirements with respect to the cryogenic system are defined. These include operating temperature (4 K), temperature stability ( 0.1 W) and requirements on magnetic and mechanical interference. The paper also reviews the most relevant options for the realization of the cryogenic system. After comparison, we selected a 4‐K mechanical cooler. To reduce the interference, it is placed at several meters from the magnetometer. The cooling power is to be transferred by circulation of helium.