Interference characterisation of a commercial Joule-Thomson cooler to be used in a SQUID-based foetal heart monitor

At the University of Twente, a foctal heart monitor based on a high-T-c SQUID magnetometer system is under development. The purpose of this system is to measure a foctal heart signal in a clinical environment. For cooling a first demonstrator version, a closed-cycle Joule Thomson cooler from APD Cryogenics the Cryotiger((R))- was selected. In this work, the Cryotiger is characterised with respect to three noise generating mechanisms: electromagnetic interference, mechanical vibrations and temperature fluctuations. The electromagnetic interference of the cold tip was below the resolution Of Our 3-axis fluxgate magnetometer (7 pT/rootHz). The interference from the compressor, however, requires it to be placed 2 in or more from the sensor head in order to remain below the environmental power line disturbances. As mechanical vibrations of a magnetometer in a background field the earth magnetic field with result in an apparent field at the frequency of vibration, we require the rotation of the cold tip to remain below 200 murad in the frequency band of interest (0.5 100 Hz). This was checked by applying a magnetic field to a SQUID magnetometer mounted on the cooler's cold tip. In this way, rotations of 3 murad and translations of 40 nm of the Cryotiger were measured; both at a frequency of 49 Hz. A further issue with respect to Our application is the fluctuation in operating temperature. Under no-load operation, the temperature occasionally increased from around 70 K to about 82 K, With a load of roughly 2 W, a temperature of about 74 K was obtained, which increased about 2 K over a 20 day period. (C) 2001 Elsevier Science Ltd. All rights reserved.