Cryogenic refrigerator

In a cryogenic refrigerator in which a movable displacer means defines within an enclosure first and second chambers of variable volume, and in which a refrigerant fluid is circulated in a fluid path between the first chamber and the second chamber by movement of the displacer means, a regenerator associated with said displacer means, chamber means for guiding a slide connected to the displacer means, a motor connected to said slide for reciprocating said slide, a valve having a reciprocable valve member for controlling the flow of high and low pressure fluid, said valve member having a peripheral groove, said motor being arranged to move said valve member in timed relation with movement of said slide so that the valve member will introduce high pressure fluid via said groove into said first and second chambers when the displacer means is at one of the extremities of its movement, and means for controlling said flow so that the flow from the groove to or from the displacer is at a substantially constant mass flow rate to thereby increase the dwell time of the fluid within said regenerator.