Cryogenic Aspects of a Fetal Heart Monitor Based on High-Tc Squids

Abstract

A high-Tc SQUID-based magnetometer system for measuring fetal heart activity in standard clinical environments is the goal of the so-called FHARMON project. To lower the threshold for the application of this fetal heart monitor, it should be low-cost and, moreover, simple to operate. It is, therefore, advantageous to replace the liquid cryogen bath by a closed-cycle refrigerator. In this paper the requirements with respect to the cryogenic system are discussed for this specific application. These include operating temperature (55–80 K), temperature stability (<0.5 K drift in 10 hours), cooling power (<0.5 W), mechanical vibrations, electromagnetic interference and costs (<$US 5000). Next to the requirements, the paper also reviews the most relevant options for the realization of the cryogenic system. In this respect two categories are distinguished: systems incorporating coolers with ‘noisy’ cold heads (e.g. GM and Stirling) versus those using coolers with a low-noise cold head (e.g. pulse tube and JT) requiring respectively long and short thermal interfaces.