large specific heat at temperature lower than 7.5 K as a regenerating material.
The three-stage regenerative refrigerator 30 is operated as follows:
First, high pressure helium gas 24 compressed by a helium compressor 25 serving as a helium gas compression means is introduced into first, second and the third expansion chambers 22, 23 and 46 in a state where the first, second and the third displacer 16, 17 and 41 are at the lowermost positions, a suction valve 26 is opened and an exhaust valve 27 is closed so that a high pressure state is realized.
Then, the first, second and the third displacers 16, 17 and 41 are moved upwards, and the high-pressure helium gas 24 is passed through the first, second and the third regenerators 20, 21 and 45into a low-temperature and low-pressure gas.
The ensuing downward movements of the first, second and the third displacers 16, 17 and 41 cause the low-temperature and low-pressure helium gas 24 to be passed through the first, second and the third regenerators 20, 21 and 45 before it is exhausted from the exhaust valve 27. The low-temperature and low-pressure helium gas 24, at this time, cools the regenerating materials in the first, second and the third regenerators 20, 21 and 45 before it is returned to the helium compressor 25.
In an ensuing state where the capacities of the first, second and third expansion chambers 22, 23 and 36 have been minimized, the exhaust valve 27 is closed and the suction valve 26 is opened. As a result, the high pressure