We claim: 1. A microminiature heat exchanger for transferring heat from a high pressure gas stream to a low pressure gas stream within a cryogenic probe, said heat exchanger comprising: a laminated assembly of a plurality of plates of heat conductive material; a high pressure passageway through said laminated assembly, said high pressure passageway having an inlet port connectable to a cryogenic gas supply, said high pressure passageway having an outlet port connectable to an inlet of a gas expansion element; a low pressure passageway through said laminated assembly, said low pressure passageway lying substantially alongside said high pressure passageway, said low pressure passageway having an inlet port connectable to an outlet of a gas expansion element, said low pressure passageway having an outlet port; and a substantially annular insulating jacket integrally formed within said heat exchanger by formation of substantially annular channels through said plates, said substantially annular channels being positioned radially outwardly from said high pressure passageway and said low pressure passageway, said annular insulating jacket being evacuated, wherein the direction of flow from said inlet port to said outlet port of said high pressure passageway is substantially opposite to the direction of flow from said inlet port to said outlet port of said low pressure passageway; and wherein each said passageway is arranged to create a tortuous flow path, to ensure turbulent flow of cryogenic gas therethrough. 2. A microminiature heat exchanger as recited in claim 1, wherein: said plurality of plates are laminated in a stack along a longitudinal axis to form an elongate laminated assembly; said high pressure passageway comprises a first plurality of holes through each said plate; and said low pressure passageway comprises a second plurality of holes through each said plate. 3. A microminiature heat exchanger as recited in claim 2, wherein: said first plurality of holes in any one said plate are sufficiently transversely offset from said first plurality of holes in adjacent said plates to cause substantial portions of said high pressure passageway to be transverse to said longitudinal axis of said heat exchanger; said second plurality of holes in any one said plate are sufficiently transversely offset from said second plurality of holes in adjacent said plates to cause substantial portions of said low pressure passageway to be transverse to said longitudinal axis of said heat exchanger; and transfer of heat from said high pressure passageway to said low pressure passageway is substantially parallel to said longitudinal axis. 4. A microminiature heat exchanger as recited in claim 2, wherein said first plurality of holes and said second plurality of holes in adjacent plates are arranged so that each said passageway is substantially parallel to said longitudinal axis of said heat exchanger, and transfer of heat from said high pressure passageway to said low pressure passageway is substantially transverse to said longitudinal axis. 5. A microminiature heat exchanger as recited in claim 2, further comprising a plurality of spacers laminated alternatingly between said plates to form a high pressure flow chamber and a low pressure flow chamber between each pair of adjacent said plates. 6. A microminiature heat exchanger as recited in claim 5, wherein each said spacer includes a partition dividing said high pressure flow chamber from said low pressure flow chamber. 7. A microminiature heat exchanger as recited in claim 6, wherein said partitions on said spacers are located to align said high pressure flow chambers with each other, and to align said low pressure flow chambers with each other, to direct each said passageway substantially parallel to said longitudinal axis of said heat exchanger. 8. A microminiature heat exchanger as recited in claim 6, wherein said partitions on said spacers are located to transversely offset adjacent said high pressure flow chambers from each other, and to transversely offset adjacent said low pressure flow chambers from each other, to direct substantial portions of each said passageway substantially transverse to said longitudinal axis of said heat exchanger. 9. A microminiature heat exchanger for transferring heat from a high pressure gas stream to a low pressure gas stream within a cryogenic probe, said heat exchanger comprising: a laminated assembly of a plurality of plates of heat conductive material; a high pressure passageway through said laminated assembly, said high pressure passageway having an inlet port connectable to a cryogenic gas supply, said high pressure passageway having an outlet port connectable to an inlet of a gas expansion element; and a low pressure passageway through said laminated assembly, said low pressure passageway lying substantially alongside said high pressure passageway, said low pressure passageway having an inlet port connectable to an outlet of a gas expansion element, said low pressure passageway having an outlet port; wherein the direction of flow from said inlet port to said outlet port of said high pressure passageway is substantially opposite to the direction of flow from said inlet port to said outlet port of said low pressure passageway; wherein each said passageway is arranged to create a tortuous flow path, to ensure turbulent flow of cryogenic gas therethrough; and wherein: said plates are laminated together and then rolled about a longitudinal axis, to form a cylindrical laminated assembly; said high pressure passageway comprises a first channel formed in a first said plate; and said low pressure passageway comprises a second channel formed in a second said plate. 10. A microminiature heat exchanger as recited in claim 9, wherein: substantial portions of each said passageway in said laminated assembly are parallel to said longitudinal axis; each said portion of said high pressure passageway is adjacent to one of said portions of said low pressure passageway; and transfer of heat from said high pressure passageway to said low pressure passageway is substantially radial, relative to said longitudinal axis. 11. A microminiature heat exchanger for transferring heat from a high pressure gas stream to a low pressure gas stream within a cryogenic probe, said heat exchanger comprising: an axially stacked assembly of laminated flat plates and flat spacers, said plates being alternated with said spacers; a first plurality of holes formed through said plates to form a high pressure passageway through said laminated assembly, each said hole in said high pressure passageway in a said plate being transversely offset from a corresponding hole in each adjacent said plate, creating a tortuous high pressure flow path to cause turbulent flow of cryogenic gas; a second plurality of holes formed through said plates to form a low pressure passageway through said laminated assembly, each said hole in said low pressure passageway in a said plate being transversely offset from a corresponding hole in each adjacent said plate, creating a tortuous low pressure flow path to cause turbulent flow of cryogenic gas; and a substantially annular channel formed through each said plate and each said spacer to integrally form a substantially annular insulating jacket within said laminated assembly, said substantially annular channel being positioned radially outwardly from said high pressure passageway and said low pressure passageway, said annular insulating jacket being evacuated; wherein the direction of flow from said inlet port to said outlet port of said high pressure passageway is substantially opposite to the direction of flow from said inlet port to said outlet port of said low pressure passageway. 12. A microminiature heat exchanger as recited in claim 11, wherein: each said high pressure hole in any one said plate is sufficiently transversely offset from said high pressure holes in adjacent said plates to cause substantial portions of said high pressure passageway to be transverse to a longitudinal axis of said heat exchanger; each said low pressure hole in any one said plate is sufficiently transversely offset from said low pressure holes in adjacent said plates to cause substantial portions of said low pressure passageway to be transverse to a longitudinal axis of said heat exchanger; and transfer of heat from said high pressure passageway to said low pressure passageway is substantially parallel to said longitudinal axis. 13. A microminiature heat exchanger as recited in claim 11, wherein said first plurality of holes are formed through a central portion of each of said plates, and said second plurality of holes are formed in a substantially annular pattern around said central portion of each of said plates so that each said passageway is substantially parallel to a longitudinal axis of said heat exchanger, and transfer of heat from said high pressure passageway to said low pressure passageway is substantially transverse to said longitudinal axis. 14. A microminiature heat exchanger as recited in claim 11, wherein said spacers form a high pressure flow chamber and a low pressure flow chamber between each pair of adjacent said plates, said substantially annular channel being positioned radially outwardly from said high pressure flow chamber and said low pressure flow chamber. 15. A microminiature heat exchanger as recited in claim 14, wherein each said spacer includes a partition dividing said high pressure flow chamber from said low pressure flow chamber. 16. A microminiature heat exchanger as recited in claim 15, wherein said partition on each said spacer comprises a substantially annular partition surrounding a central portion of said spacer, to create substantially central high pressure flow chambers aligned with each other, and to create substantially annular low pressure flow chambers aligned with each other, to direct said high pressure passageway substantially along a longitudinal axis of said heat exchanger, and to direct said low pressure passageway substantially coaxially around said high pressure passageway. 17. A microminiature heat exchanger as recited in claim 15, wherein said partitions on said spacers are arranged to transversely offset adjacent said high pressure flow chambers from each other, and to transversely offset adjacent said low pressure flow chambers from each other, to direct substantial portions of each said passageway substantially transverse to a longitudinal axis of said heat exchanger. 18. A microminiature heat exchanger for transferring heat from a high pressure gas stream to a low pressure gas stream within a cryogenic probe, said heat exchanger comprising: an axially stacked assembly of laminated flat plates and flat spacers, said plates being alternated with said spacers; a first plurality of holes formed through said plates, for turbulent flow of high pressure cryogenic gas, each said high pressure hole being transversely offset from a corresponding high pressure hole in each adjacent said plate; a second plurality of holes formed through said plates, for turbulent flow of low pressure cryogenic gas, each said low pressure hole being transversely offset from a corresponding low pressure hole in each adjacent said plate; an end plate laminated to an end of said stacked assembly, said end plate having an orifice therethrough in flow communication with said high pressure holes; and a substantially annular insulating jacket integrally formed within said heat exchanger by formation of substantially annular channels through said plates, said substantially annular channels being positioned radially outwardly from said high pressure passageway and said low pressure passageway, said annular insulating jacket being evacuated; wherein the direction of flow from said inlet port to said outlet port of said high pressure passageway is substantially opposite to the direction of flow from said inlet port to said outlet port of said low pressure passageway. 19. A microminiature heat exchanger for transferring heat from a high pressure gas stream to a low pressure gas stream within a cryogenic probe, said heat exchanger comprising: an axially stacked assembly of laminated flat plates and flat spacers, said plates being alternated with said spacers; a first plurality of holes formed through said plates, for turbulent flow of high pressure cryogenic gas, each said high pressure hole being transversely offset from a corresponding high pressure hole in each adjacent said plate; a second plurality of holes formed through said plates, for turbulent flow of low pressure cryogenic gas, each said low pressure hole being transversely offset from a corresponding low pressure hole in each adjacent said plate; a plurality of spacers laminated to an end of said stacked assembly, to create a manifold for connection of high pressure and low pressure gas lines to said heat exchanger; and a substantially annular insulating jacket integrally formed within said heat exchanger by formation of substantially annular channels through said plates, said substantially annular channels being positioned radially outwardly from said high pressure passageway and said low pressure passageway, said annular insulating jacket being evacuated; wherein the direction of flow from said inlet port to said outlet port of said high pressure passageway is substantially opposite to the direction of flow from said inlet port to said outlet port of said low pressure passageway. 20. A microminiature heat exchanger for transferring heat from a high pressure gas stream to a low pressure gas stream within a cryogenic probe, said heat exchanger comprising: an axially stacked assembly of laminated flat plates and flat spacers, said plates being alternated with said spacers; a first plurality of holes formed through said plates, for turbulent flow of high pressure cryogenic gas, each said high pressure hole being transversely offset from a corresponding high pressure hole in each adjacent said plate; a second plurality of holes formed through said plates, for turbulent flow of low pressure cryogenic gas, each said low pressure hole being transversely offset from a corresponding low pressure hole in each adjacent said plate; a substantially annular channel formed through each said plate and each said spacer to integrally form a substantially annular insulating jacket within said laminated assembly, said substantially annular channel being positioned radially outwardly from said high pressure and low pressure holes, said annular insulating jacket being evacuated; a plurality of spacers laminated to a first end of said stacked assembly, to create a manifold for connection of high pressure and low pressure gas lines to said heat exchanger; and an end plate laminated to a second end of said stacked assembly, said end plate having an orifice therethrough in flow communication with said high pressure holes; wherein the direction of flow from said inlet port to said outlet port of said high pressure passageway is substantially opposite to the direction of flow from said inlet port to said outlet port of said low pressure passageway. 21. A microminiature heat exchanger for transferring heat from a high pressure gas stream to a low pressure gas stream within a cryogenic probe, said heat exchanger comprising: a first sheet having a first flow channel etched therein to form a high pressure passageway, said high pressure passageway having an inlet port and an outlet port, said inlet high pressure port being formed at a first edge of said first sheet and said outlet high pressure port being formed at a second edge of said first sheet; and a second sheet having a second flow channel etched therein to form a low pressure passageway, said low pressure passageway having an inlet port and an outlet port, said outlet low pressure port being formed at a first edge of said second sheet and said inlet low pressure port being formed at a second edge of said second sheet; wherein said first and second sheets are laminated together, with said first edge of said first sheet aligned with said first edge of said second sheet, and with said second edge of said first sheet aligned with said second edge of said second sheet; and wherein said first and second sheets are rolled into a cylindrical shape having said inlet high pressure port and said outlet low pressure port at a first end, and having said outlet high pressure port and said inlet low pressure port at a second end. 22. A microminiature heat exchanger as recited in claim 21, wherein: substantial portions of said first flow channel in said first sheet are parallel to a longitudinal axis of said heat exchanger; substantial portions of said second flow channel in said second sheet are parallel to said longitudinal axis of said heat exchanger; each said portion of said high pressure passageway is adjacent to one of said portions of said low pressure passageway; and transfer of heat from said high pressure passageway to said low pressure passageway is substantially radial, relative to said longitudinal axis. 23. A method of manufacturing a microminiature heat exchanger for use in a cryogenic probe, said method comprising: coating a plurality of sheets with a layer of photo resistive compound; creating a first design of a plurality of flat plates, each said plate having a first plurality of holes and a second plurality of holes and a substantially annular channel substantially surrounding said first plurality of holes and said second plurality of holes; creating a second design of a plurality of flat spacers, each said spacer having a first large opening and a second large opening divided by a partition and a substantially annular channel substantially surrounding said first large opening and said second large opening; transferring said first and second designs to said plurality of sheets by tracing said first and second designs with a laser to bond portions of said photo resistive compound to said plurality of sheets; washing said plurality of sheets in an etching compound to disintegrate non-bonded portions of said sheets to create said plurality of plates and said plurality of spacers arranged in said sheets; axially stacking said sheets to form a laminar assembly, said assembly comprising a plurality of laminar subassemblies, each said laminar subassembly comprising an axial stack of plates and spacers, said plates being alternated with said spacers, said first plurality of holes in each said plate being transversely offset from said first plurality of holes in adjacent said plates of said respective subassembly and aligned with said first large opening in said spacer therebetween, to form a high pressure passageway through said laminar subassembly, said second plurality of holes in each said plate being transversely offset from said second plurality of holes in adjacent said plates of said respective subassembly and aligned with said second large opening in said spacer therebetween, to form a low pressure passageway through said laminar subassembly, said annular channels in said plates and said spacers being aligned to form an insulating jacket surrounding said high pressure passageway and said low pressure passageway; placing said laminar assembly under a vacuum; diffusion bonding said laminated assembly together; maintaining said vacuum while performing said diffusion bonding to establish a vacuum in each said insulating jacket; and removing said plurality of laminar subassemblies from said laminar assembly, each said laminar subassembly comprising one said microminiature heat exchanger. 24. A method of manufacturing a microminiature heat exchanger as recited in claim 23, wherein: said first design of a plurality of flat plates is transferred to a first plurality of sheets; said second design of a plurality of flat spacers is transferred to a second plurality of sheets; and said first plurality of sheets are alternated with said second plurality of sheets in said laminar assembly. 25. A method of manufacturing a microminiature heat exchanger as recited in claim 24, wherein: said first plurality of sheets are constructed of a first material having a relatively higher coefficient of heat transfer; and said second plurality of sheets are constructed of a second material having a relatively lower coefficient of heat transfer. 26. A method of manufacturing a microminiature heat exchanger as recited in claim 23, further comprising: creating a third design of a plurality of first manifold plates, each said first manifold plate having an inlet to said high pressure passageway and an outlet from said low pressure passageway; creating a fourth design of a plurality of second manifold plates, each said second manifold plate having an outlet from said high pressure passageway and an inlet to said low pressure passageway; transferring said third design to a first manifold sheet by tracing said third design with a laser to bond portions of said photo resistive compound to said first manifold sheet; transferring said fourth design to a second manifold sheet by tracing said fourth design with a laser to bond portions of said photo resistive compound to said second manifold sheet; washing said first and second manifold sheets in an etching compound to disintegrate non-bonded portions of said sheets to create said plurality of first manifold plates arranged in said first manifold sheet, and to create said plurality of second manifold plates arranged in said second manifold sheet; laminating said first manifold sheet to a first end of said laminar assembly, each said first manifold plate being aligned with a first end of a said laminar subassembly; and laminating said second manifold sheet to a second end of said laminar assembly, each said second manifold plate being aligned with a second end of a said laminar subassembly. 27. A method of manufacturing a microminiature heat exchanger as recited in claim 23, wherein said design is computer aided. 28. A method of manufacturing a microminiature heat exchanger as recited in claim 23, wherein said laser is numerically controlled. 29. A method of manufacturing a microminiature heat exchanger as recited in claim 23, further comprising: establishing sufficient angular offset between said first plurality of holes in any one said plate and said first plurality of holes in adjacent said plates to cause substantial portions of said high pressure passageway to be transverse to a longitudinal axis of said heat exchanger; and establishing sufficient angular offset between said second plurality of holes in any one said plate and said second plurality of holes in adjacent said plates to cause substantial portions of said low pressure passageway to be transverse to said longitudinal axis of said heat exchanger. 30. A method of manufacturing a microminiature heat exchanger for use in a cryogenic probe, said method comprising: coating at least one flexible substrate with a layer of photo resistive compound; creating at least two designs of serpentine flow channels, with one said design being a high pressure gas flow channel, and another said design being a low pressure gas flow channel, said high pressure flow channel having an inlet at a first end and an outlet at a second end, said low pressure flow channel having an outlet at a first end and an inlet at a second end; transferring said designs to said at least one substrate by tracing said designs with a laser to bond portions of said photo resistive compound to said at least one substrate; washing said at least one substrate in an etching compound to etch high pressure and low pressure flow channels into non-bonded portions of said at least one substrate to create a high pressure flow sheet containing said high pressure flow channel and to create a low pressure flow sheet containing said low pressure flow channel, said high pressure flow sheet having said high pressure inlet at a first edge and said high pressure outlet at a second edge, said low pressure flow sheet having said low pressure outlet at a first edge and said low pressure inlet at a second edge; laminating said high pressure flow sheet to said low pressure flow sheet, with said high pressure flow channel lying substantially parallel to said low pressure flow channel, and with said first edge of said high pressure flow sheet adjacent to said first edge of said low pressure flow sheet, and with said second edge of said high pressure flow sheet adjacent to said second edge of said low pressure flow sheet; and rolling said laminated flow sheets about a third edge of each said flow sheet to form a cylindrical heat exchanger, said high pressure inlet and said low pressure outlet being located at a first end of said heat exchanger, and said high pressure outlet and said low pressure inlet being located at a second end of said heat exchanger. 31. A method of manufacturing a microminiature heat exchanger as recited in claim 30, wherein said designs are computer aided. 32. A method of manufacturing a microminiature heat exchanger as recited in claim 30, wherein said laser is numerically controlled.