P.J. van den Bosch

Magnetic Noise of Small Stirling Coolers

We measured the magnetic noise generated by three small Stirling coolers (cooling powers 0.5 to 1.5 W at 80 K). Such a cooler will be used for cooling a high-Tc SQUID magnetometer. The measurements were performed with a fluxgate magnetometer and a 3-axis low-Tc SQUID magnetometer, the latter in a magnetically shielded room. The measuring setup is shortly described, and results on the coolers are given and compared to a simple dipole model. Consequences for SQUID-cooling are shortly discussed.

Active noise compensation for multichannel magnetocardiography in an unshielded environment

A multichannel high-T/sub c/-SQUID-based heart scanner for unshielded environments is under development, Outside a magnetically shielded room, sensitive SQUID measurements are possible using gradiometers. However, it is difficult to realize large-baseline gradiometers in high-T/sub c/ materials, Therefore, the authors developed two active noise compensation techniques. In the Total Field Compensation technique, a Helmholtz type coil set is placed around the sensors. One magnetometer is used as a zero detector controlling the compensation current through the coil set. For Individual Flux Compensation, the reference signal is sent to the separate SQUIDs (or their flux transformer circuits) to compensate the local environmental noise fluxes, The latter technique was tested on low-T/sub c/ rf-SQUID magnetometers, each sensor set to a field resolution SQUID magnetometers, i.e. 0.1 pT/sub RMS///spl radic/Hz. The authors were able to suppress the environmental disturbances to such an extent that magnetocardiograms could be recorded in an ordinary environment. Here the two suppression techniques are described and experimental results are presented.<<ETX>>

Reduction of the Vibration Generated by Stirling Cryocoolers Used for Cooling a High-Tc SQUID Magnetometer

Recently a research project was started to realize magnetocardiography in clinical conditions by means of high-Tc SQUID magnetometry (SQUID for Superconducting Quantum Interference Device). The cooling of the SQUID-devices involved will be performed by a pair of dual opposed Stirling type cryocoolers each consisting of separate compressor and displacer modules. Since the magnetic signals from the human heart are quite small (10 – 100 pT), important problems to cope with are interferences from external magnetic fields and magnetic disturbances due to the vibratory motion of the cryocooler components. In the paper we consider the reduction of the vibrations generated by the coolers. It is shown that an effective vibration reduction is achieved by 1) a dual opposed arrangement of the two displacer modules, mounted in a rigid construction; 2) a flexible mounting of the compressors and displacer unit; and 3) manual tuning of the currents of the compressor coils leading to highly balanced motions of the various parts. As a result the axial acceleration of a single compressor was reduced by a factor of 20 down to 0.05 m/s2, whereas for the displacer unit a reduction by a factor of 25 down to 0.02 m/s2 was obtained.

The Application of Cryocoolers for Cooling a High-Tc SQUID Magnetometer

A multichannel high-Tc dc-SQUID based heart-magnetometer is currently under development in our laboratory. Since this system has to be simple to use, the cooling of the device should be established by means of a turn-key apparatus incorporating a cryocooler. Because of its magnetic interference, the cooler has to be separated from the SQUID unit. Therefore, an interface between the cooler and the SQUIDs is needed. Possibilities are a gas flow system or a conductive strip. A prototype closed-cycle gas flow system has been constructed and tested, in which helium gas is cooled by a Leybold Heraeus RG 210 Gifford-McMahon cryocooler. Then it is transported through a gas line of 2.5 meter length, and after that through a glass-epoxy heat exchanger on which the SQUIDs can be installed. With this system a temperature of 30 K can be established in about 2 hours (depending on the gas flow rate). Based on the results obtained with this configuration, a smaller system was designed incorporating two Signaal Usfa UP 7058 Stirling cryocoolers. Compared to the prototype the dimensions were reduced by roughly a factor 5.