Interference characterisation of a commercial Joule–Thomson cooler to be used in a SQUID-based foetal heart monitor
✍ Scribed by M.R Bangma; A.P Rijpma; E de Vries; H.A Reincke; H.J Holland; H.J.M ter Brake; H Rogalla
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 243 KB
- Volume
- 41
- Category
- Article
- ISSN
- 0011-2275
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✦ Synopsis
At the University of Twente, a foetal heart monitor based on a high-T C SQUID magnetometer system is under development. The purpose of this system is to measure a foetal heart signal in a clinical environment. For cooling a ®rst demonstrator version, a closed-cycle Joule±Thomson cooler from APD Cryogenics ± the Cryotiger â ± was selected. In this work, the Cryotiger is characterised with respect to three noise generating mechanisms: electromagnetic interference, mechanical vibrations and temperature ¯uctuations. The electromagnetic interference of the cold tip was below the resolution of our 3-axis ¯uxgate magnetometer (7 pT= Hz p ). The interference from the compressor, however, requires it to be placed 2 m 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 ®eld ± the earth magnetic ®eld ± will result in an apparent ®eld at the frequency of vibration, we require the rotation of the cold tip to remain below 200 lrad in the frequency band of interest (0.5±100 Hz). This was checked by applying a magnetic ®eld to a SQUID magnetometer mounted on the cooler's cold tip. In this way, rotations of 3 lrad 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 ¯uctuation 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.