ABSTRACT

The features and characteristics of 4 K cryocoolers, such as coldhead vibration, temperature oscillation, magnetic distortion, effects of external magnetic field and intercept cooling, have been theoretically analyzed and experimentally investigated.

Pulsation of gas pressure generates the coldhead vibrations and the 2nd stage temperature oscillations near 4 K. It is measured that the cryostat vibration introduced by the 4 K pulse tube cryocooler without moving parts is one order of magnitude less than that by the 4 K GM cryocooler. The temperature oscillation on the 4 K cooling station can be reduced from few hundreds mK to <±3 mK with different temperature damping methods.

Small magnetic distortions of 20–200 nT generated by temperature dependence of magnetization of the rare earth materials are discussed. Experimental results show that strong external magnetic field has impact to the 4 K cryocooler performance due to the reduction of the specific heat of the rare earth materials. The latest 4 K cryocoolers work very well under the axial magnetic field of 1 T, which is parallel to the displacer. However, the lifetime of GM cryocooler could be affected by strong radial magnetic fields. Investigations demonstrate that the 2nd stage pulse tube and regenerator in a 4 K pulse tube cryocooler are able to provide intercept cooling at the temperatures of 4 K to ~40 K with penalties of small acceptable losses on the 1st and 2nd stages.