Closed-cycle gas flow system for cooling of high Tc d.c. SQUID magnetometers

Abstract

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.