A multichannel high-To dc-SQUID based heart-magnetometer is currently under development in our laboratory. The system is cooled by a cooler that, due to its magnetic interference, has to be separated from the SQUID unit. In the present prototype system a closed-cycle gas flow was chosen as the interface between the SQUID unit and the cooler (a Leybold Heraeus RG 210). In this paper the prototype system is shortly described, its thermodynamic behaviour is considered and simulations are compared with experimental results. INTRODUCTIONNowadays high-To dc-SQUIDs are available that operate at relatively high temperatures. Therefore, smallscale cryocoolers can be applied for cooling a simple-to-use SQUID-based magnetometer. We constructed a closed-cycle gas flow system for cooling a SQUID-based magnetometer for heart measurements incorporating a Leybold Heraeus RG 210 cooler [1]. We plan to use the experience obtained with this configuration in the design of smaller future systems, based on one or more miniature Stirling cryocoolers [2]. For that purpose a thermodynamic model of the system is under development and in this paper we present the first results. The cold head unit and the rest of the system are modelled separately. Simulations are compared with experimental results. SYSTEM DESCRIPTIONThe closed-cycle gas flow system incorporates a LH RG 210 cooler that, due to its magnetic interference, has to be separated from the SQUID unit. Helium gas is cooled by the cooler, transported through a 2.5 m long coaxial gas transfer line, and after that through a heat exchanger on which SQUIDs can be installed (see Fig. 1). Because the gas flow pump has to operate at room temperature, a counterflow heat exchanger was incorporated. Five separate parts can thus be distinguished: the cryocooler unit (consisting of the cooler with heat exchangers mounted on the two cold heads), the gas line unit (four coaxial tubes for the supply and return gas and for thermal insulation), the SQUID unit (a SQUID-plate heat exchanger and a radiation-shield heat exchanger), the counterflow heat exchanger (two 1 m long coaxial tubes), and the gas flow controller (a pump, mass flow controllers and buffers). The length of the gas circuit of the system without the gas flow controller is about 11 m. This system has been constructed and tested, and extensively described elsewhere [1]. The temperatures are measured at ten positions for different mass flows (between 2.10 -6 and 3.2.10 -5 kgs-~). The lowest obtainable SQUID plate temperature is 31 4-2 K that can be reached in roughly 2-3 hours with an optimal mass flow of 6.10 .6 kg/s. This lowest temperature is for a large part determined by the counterflow heat exchanger. THERMODYNAMICS AND SIMULATIONSIn all system elements heat is transferred between the gas and the surrounding material and between the latter material and its environment. The energy balances (in Wm 1) for the elements can be written as a Cryogenics 1994Vol 341CEC Supplement 143ICEC 15 Proceedings pair of coupled differential equations...
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