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In a cryogenic split Stirling refrigerator, the compressor and displacer are driven by linear drive motors. Each compressor linear drive motor has a plurality of radial permanent magnets 44, 46 mounted to each compressor piston 22, 24. A tapered collet 56 expands an expanding sleeve 42 against the m

In a cryogenic split Stirling refrigerator, the compressor and displacer are driven by linear drive motors. Each compressor linear drive motor has a plurality of radial permanent magnets 44, 46 mounted to each compressor piston 22, 24. A tapered collet 56 expands an expanding sleeve 42 against the magnets and compresses a sleeve 38 against the piston. A housing 34 hermetically seals the gas about the armature and supports the coil assembly 76, 78. The displacer drive motor includes an axial permanent magnet 202 mounted to the displacer drive piston 192. It is surrounded by a housing 210 which hermetically seals the spring volume surrounding the magnet. The drive coils 216, 218 are supported on that housing. The coils are surrounded by flux returns 222, 224 which can be adjusted to minimize radial forces on the drive piston.

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1. A cryogenic refrigerator comprising a gaseous working fluid which is alternatively compressed and expanded to cool a portion of the working fluid to cryogenic temperatures and a linear drive motor for driving a piston element in the refrigerator which acts on the working fluid in a thermodynamic

1. A cryogenic refrigerator comprising a gaseous working fluid which is alternatively compressed and expanded to cool a portion of the working fluid to cryogenic temperatures and a linear drive motor for driving a piston element in the refrigerator which acts on the working fluid in a thermodynamic refrigeration cycle, the linear drive motor comprising: a permanent magnet armature fixed to the piston element; a housing surrounding the permanent magnet armature and hermetically sealing the volume in which the armature reciprocates; and a coil assembly surrounding the housing. 2. A cryogenic refrigerator as claimed in claim 1 further comprising means for adjusting the magnetic axis of either or both of the moving or fixed parts of the linear drive motor to minimize radial forces on the driven piston element. 3. A cryogenic refrigerator as claimed in claim 1 further comprising a generally cylindrical outer flux path surrounding the coil assembly and formed of sections, the angular positions of which can be adjusted. 4. A cryogenic refrigerator as claimed in claim 1 wherein the driven piston has a close tolerance relative to a surrounding cylinder to form a clearance seal and the piston element within the surrounding cylinder is the sole mechanical support of the armature. 5. A cryogenic refrigerator as claimed in claim 1 wherein the permanent magnet is an axial magnet joined by a screw threaded connector to the piston element. 6. A cryogenic refrigerator as claimed in claim 5 wherein the screw threaded connector is a flux return plate. 7. A cryogenic refrigerator comprising a gaseous working fluid which is alternately compressed and expanded to cool a portion of the working fluid to cryogenic temperature and a linear drive motor for driving, along a piston axis, a piston element in the refrigerator which acts on the working fluid in a thermodynamic refrigeration cycle, the linear drive motor comprising: a plurality of circumferentially spaced segments of a flux path encircling the piston axis to form an annular portion of the flux path by which the linear motor drives the piston, the spaced segments being mounted for adjusting the relative circumferential positions of the segments and fixing the relative circumferential positions of the segments during operation of the linear drive motor for adjusting a magnetic center of the linear drive motor so that the center matches the mechanical center of the piston element. 8. A cryogenic refrigerator as claimed in claim 7 further comprising a generally cylindrical outer flux return surrounding a motor coil assembly and formed of sections, the angular positions of which can be adjusted. 9. A cryogenic refrigerator comprising a gaseous working fluid which is alternately compressed and expanded to cool a portion of the working fluid to cryogenic temperatures and a linear drive motor for driving a piston element in the refrigerator which acts on the working fluid in a thermodynamic refrigeration cycle, the linear drive motor armature comprising: a screw threaded extension of the piston element extending into a volume within a motor stator; an annular, axially magnetized magnet surrounding and supported by the piston extension; and a screw threaded connector fixed to the end of the piston extension by a screw threaded coupling to retain the magnet on the piston extension. 10. A cryogenic refrigerator as claimed in claim 9 wherein the screw threaded connector is a flux return plate. 11. A cryogenic refrigerator having a gas displacer which reciprocates in a housing to displace gas in a working volume of gas through a regenerator and a linear drive motor for driving the the displacer in a thermodynamic refrigeration cycle, the linear drive motor comprising: an axial permanent magnet mounted to a drive piston of the displacer; a housing hermetically sealing a spring volume of gas about the axial magnet; and a drive coil assembly surrounding the housing. 12. A cryogenic refrigerator as claimed in claim 11 further comprising a generally cylindrical outer flux return surrounding the coil assembly and formed of sections, the angular positions of which can be adjusted. 13. A cryogenic refrigerator as claimed in claim 11 wherein the permanent magnet is joined by a screw threaded connector to the piston element. 14. A cryogenic refrigerator as claimed in claim 13 wherein the screw threaded connector is a flux return plate.