The effective sensing area of a high-T c dc SQUID depends on temperature. As a consequence, fluctuations in the operating temperature result in apparent magnetic field noise if the SQUID is placed in a background magnetic field. An analysis of this effect for two SQUID types, the square-washer 'Ketchen' type and the inductively shunted type, is performed. For magnetocardiography, the temperature fluctuations (peak to peak) of the latter SQUID type should be below 0.3 mK at 77 K, and below 2 mK at 55 K, with an earth's field suppression of 40 dB. For the square-washer SQUID the requirements are about 8 times less stringent.
A heart scanner based on high-T, SQUIDS is currently under development at the University of Twente. It is intended to be used in standard clinical environments without a magnetically shielded room. In order to make the application simple to use, the SQUIDS will be cooled by small cryocoolers, thus realizing a turnkey apparatus. The aimed field resolution is 50 ffRMs HZ-'/~ in a measuring band of 0.1-100 Hz. The mechanical cooler interference is reduced by incorporating two coolers and operating them in counter phase. The magnetic cooler interference is reduced by positioning the coolers and the SQUIDS in a coplanar arrangement, and by separating the SQUIDS from the cold tips with a solid conducting thermal interface. A design is presented in which a temperature of 55 K is expected with a cool-down time of less than 1 h. 0 1997 Elsevier Science Limited Keywords: high-T, d&QUID magnetometer; Stirling cryocooler; magnetocardiogrwhy SuperconductingQuantum Interference Devices (SQUIDS) are the most sensitive magnetic flux-to-voltage converting sensors. At present, their main application lies in the field of biomagnetic research where multichannel low-T, dc-SQUID based magnetometer systems are used. These systems are usually cooled by liquid helium and operated in magnetically shielded rooms to obtain an extremely lownoise measuring environment.They are expensive, require helium refills and cannot be transported in a simple manner.Because of the higher operating temperature, a much more flexible magnetometer system can be realized with high-T, SQUIDS. The workhorse of current high-T, superconducting electronics, YBa,Cu,O,, with its critical temperature (T,) of 92 K opened the possibility of operating SQUIDS in liquid nitrogen at 77 K. Also, small-scale tumkey cryocoolers, originally developed for cooling infrared sensors, are available and are very reliable'. The advantages of cryocoolers compared to a liquid-nitrogen cryostat are the lower operating temperature (which gives a potentially better SQUID performance*), the turnkey operation (no refills required), and the possibility of operating the system in all directions.In this paper the design of a heart scanner is presented that can be equipped with up to 25 high-T, SQUIDS cooled by two small Stirling-type cryocoolers. In the next section the design goals and constraints are reviewed, in which special attention is paid to the heart signal and the required sensor resolution. After that, the specific coolers that are used in this project are discussed. Because of the interference at the SQUID positions arising from the cryocoolers, a separation in space or time through a thermal interface is demanded. A separate section deals with possible thermal interfaces which can transfer heat from the SQUID unit to the coolers, and with special measures that have to be taken to reduce the magnetic and mechanical cooler interference. Then, the thermodynamic aspects of the cooling system for the heart scanner using a conductivestrip thermal interface are considered. The paper con...
A high T, d.c. SQUID based magnetometer for magnetocardiography is currently under development at the University of Twente. Since such a magnetometer should be simple to use, the cooling of the system can be realized most practically by means of a cryocooler. A closed-cycle gas flow cooling system incorporating such a cooler has been designed, constructed and tested. The aimed resolution of the magnetometer is 0.1 pT HZ-'/~. The required operating temperature for the SQUIDS is 30 to about 77 K with a stability of 2 x 10" K HZ-'/*. After a cool-down time of l-2 h, a stationary cooling power of at least 0.2 W is required. In the design, helium gas is cooled by a Leybold Heraeus RG 210 cryocooler, transported through a gas line, and subsequently passed through a heat exchanger on which SQUIDS can be installed. The lowest obtainable SQUID heat exchanger temperature is 31+2 K. This can be reached in roughly 2-3 h with an optimal mass flow with respect to the cooling power of 6 x 1O-6 kg s-l. At this mass flow the cooling power at the SQUID heat exchanger is 0.2 W at 42 K and roughly 1.2 W at 77 K. A temperature stability of 0.05 K was measured at a SQUID heat exchanger temperature of 54 K and a mass flow of 3 x 10e5 kg s-l. The experience gained with this large cooling system will be used in the design of a smaller configuration cooling system, incorporating miniature Stirling cryocoolers. In this paper the design and the construction of the present closed-cycle system are described and test results are presented.
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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