低ノイズ単段パルス管冷凍機の開発

Abstract

A low-noise, pulse-tube cryocooler has been developed. It is suitable for cooling variable semiconductor or superconducting devices. The cryocooler's electro-magnetic noise and inclination characteristics are improved, and the cooling capacity is 4.9 W at 77 K with an input power of 800 W. The minimum temperature was less than 49 K. The cooling performance at 77 K was within 5% degradation for all orientations. The low-noise pulse-tube cryocooler was installed in a cryostat designed for a high-Tc SQUID sensor. The electro-magnetic noise was measured by the magnetometer-type SQUID sensor cooled in the cryostat. The noise level was influenced by the direction of the sensor. When the sensor was parallel to the cryocooler cylinder, the white noise was about 300 fT/√Hz and the cyclic noise produced by the cryocooler's magnetic material or mechanical vibration was not observed. When the sensor was vertical to the cylinder, the white noise was about 2 pT/√Hz and the cyclic noise was about 10 pT rms. A real-time magnetocardiography was successfully measured using a high-Tc SQUID magnetometer cooled by the pulse-tube cryocooler. The signal-to-noise ratio was better than the conventional cryocooling system.