Abstract: A system for preventing freezing of crops within a volume includes a plurality of RF radiators configured to radiate RF energy into the volume. A height, a spacing, an output power, a vertical beam angle, a vertical beam width, and a horizontal beam width of each one of the plurality of RF radiators is selected to result in an average RF power density taken about three dimensions within the volume sufficient to prevent freezing of a substantial portion of the crops, and also to result in a peak-to-peak variation of RF power density taken about two dimensions in a horizontal plane within the volume and at heights below a predetermined height to be less than a predetermined percentage of an average RF power density taken about the two dimensions.

Abstract: A cryogenic refrigerator operating on the Split-Sterling cycle principle is disclosed. The refrigerator has a seal-less expander and achieves efficient operation without the use of external control apparatus. This is accomplished by sizing the expander so that two pressure differential forces developed across a displacer in the expander are approximately equal in magnitude. For efficient operation, the expander's gas flow rate is selected to provide the proper time delay between the two pressure differential forces.