Low Noise Cold Head of a Four-Valve Pulse Tube Refrigerator
We have designed and constructed a split type of a four-valve pulse tube refrigerator (FVPTR) in order to cool high-Tc Superconducting Quantum Interference Devices1 (SQUIDs). A SQUID, in particular, places rigorous upper limits on the tolerable levels of magnetic interference, vibrations and temperature fluctuations. The pressure wave in the system is generated by means of a commercial He-compressor in combination with a rotary valve. In order to protect the cold head from disturbances generated by the valve gear and the compressor, it is useful to separate those parts spatially. Reduction of interference signals from the compressor and the rotary valve is accomplished using flexible tubes between the rotary valve and the cold head. Most parts of the cold head, including the regenerator, were made of non-metallic, electrically insulating, and non-magnetic materials to minimize eddy currents and local magnetic fields, which interfere the sensor directly.
In this contribution we show our setup of a low noise split FVPTR. Some fundamental changes have been done in this setup compared with the integral FVPTR2. These changes will be discussed in detail. Furthermore, we will present the non-metallic components of the cold finger. The thermodynamic consequences and the magnetic noise reductions which result from the material substitution will be considered.
KeywordsPulse Tube Regenerator Material Cold Finger Magnetic Noise Pressure Pipe
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