Development of a High Efficiency 0.5 W Class 4K GM Cryocooler

The effect of displacer stroke on the performance of a 4K Gifford-McMahon (G-M) cryocooler has been investigated. Displacer strokes of 30mm, 25mm, 20mm and 15mm are examined. The largest cooling capacity at 4.2K and also at 7K is nearly equal for all the strokes, but the first stage cooling capacity at 50K is much improved by optimization of the stroke. The optimum stroke is 20mm from the experiment. The cycle speed at which the maximum 4.2K cooling capacity is obtained is different for each stroke, and is smaller when the stroke is longer. The reciprocal of the optimum cycle speed is in proportion to the stroke.