Hermanus J.M. ter Brake

Thermodynamic considerations on a microminiature sorption cooler

The sorption/Joule-Thomson cycle is a promising cycle for microscale cooling of low-temperature electronic devices because the cycle lacks moving parts. This facilitates scaling down to small sizes, eliminates interferences, and contributes to achieving a long life time. A thermodynamic analysis is presented in which the behaviour of compressor and cold stage are analysed separately, leading to a better understanding of sorption coolers. Some fundamental possibilities to improve the thermodynamic efficiency are discussed, and as a part of this a novel two stage compressor concept is proposed.

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.

Interference Characterization of Cryocoolers for a High-Tc SQUID-Based Fetal Heart Monitor

The FHARMON-project at the University of Twente aims at a high-Tc SQUID based fetal heart monitor for use in standard clinical environments. Besides the suppression of environmental magnetic noise, the cooling of this fetal heart monitor is an important issue. For maximum flexibility, we intend to apply a closed-cycle cryocooler instead of a liquid nitrogen cryostat. Because of the extreme sensitivity of SQUID magnetometers, the interference caused by the cryocooler is of major importance. This concerns electromagnetic interference (EMI), mechanical vibrations and temperature fluctuations. We have developed measuring techniques to characterize coolers in this respect. The characterization procedures were tested on a Signaal USFA 7058 Stirling cooler and a Leybold RGD210 GM-cooler. In the paper the measuring techniques are described along with interference characteristics of an APD Cryotiger and a Ricor/AirLiquide K535 Stirling cooler. Also, the impact on the design of the fetal heart monitor is considered.