Pulse Tube Refrigerator for High-TC Squid Operation
We report on a pulse tube refrigerator (PTR) designed for cooling an YBCO RF— SQUID electronic gradiometer and on measurements of the magnetic flux noise of the system. To reduce the size of the cooler, a coaxial configuration of regenerator and pulse tube is used. For higher performance the PTR can be operated in orifice and double-inlet configuration. A motor-driven rotary valve serves to control the helium gas flow between compressor and refrigerator inlets. In order to reduce vibrations and interference noise at the cold head, the rotary valve and refrigerator inlets are connected by a flexible tube. Using a tube with a length and inner diameter of 4 m and 3 mm, respectively, an average pressure of 18 bar, a frequency of f = 2 Hz and a high to low pressure ratio of 1.5, a minimum temperature of 53 K has been achieved. The gradiometer with a base line length of 45 mm consists of two YBCO microwave RF-SQUIDs which are attached to a mounting block connected with the cold heat exchanger of the PTR. The two SQUIDs were successfully operated in flux locked loop in laboratory environment without any magnetic shielding. The flux noise spectrum exhibited interference signals at f and f/2, which decreased rapidly upon increasing the distance between cold head and the rotary valve. At a distance of 2 m and more, a white noise of the order of 6 × 10-5 Ф0/√Hz was measured in gradiometer configuration.
KeywordsVacuum Vessel Pulse Tube Rotary Valve Cold Head Pulse Tube Refrigerator
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