Cooling arrangement for a superconducting coil

A superconducting device comprising:

• a) a vacuum enclosure having an interior wall surrounding a generally-annular cavity, wherein said cavity has a generally longitudinal axis, and wherein said cavity contains a vacuum having a pressure not exceeding generally one-thousandth of a torr;
• b) a superconductive coil disposed within said cavity, generally coaxially aligned with said longitudinal axis, generally spaced-apart from said interior wall, having a circumferential outward-facing surface, and having a circumferential inward-facing surface;
• c) a generally-annularly-arranged sheet of thermally conductive material having a coefficient of thermal conductivity at least equal to that of copper at a temperature of generally 50 Kelvin, disposed within said cavity, generally coaxially aligned with said longitudinal axis, generally spaced-apart from said interior wall, having a circumferential outward-facing surface, and having a circumferential inward-facing surface, wherein the inward-facing surface of said sheet contacts generally the entire outward-facing surface of said superconductive coil;
• d) a generally-annularly-arranged coolant tube containing a cryogenic fluid having a cryogenic temperature, disposed within said cavity, generally coaxially aligned with said longitudinal axis, and generally spaced-apart from said interior wall, wherein said coolant tube contacts a generally-circumferential portion of the outward-facing surface of said sheet;
• and
• e) a generally-annularly-arranged coil overwrap of thermally insulative material having a coefficient of thermal conductivity no greater than that of fiberglass at a temperature of generally 50 Kelvin, disposed within said cavity, generally coaxially aligned with said longitudinal axis, generally spaced-apart from said interior wall, having a circumferential outward-facing surface, and having a circumferential inward-facing surface, wherein said coil overwrap generally circumferentially surrounds the outward-facing surface of said sheet, wherein said coolant tube and the inward-facing surface of said coil overwrap together contact generally the entire outward-facing surface of said sheet, and wherein said superconductive coil, said sheet, and said coil overwrap are disposed completely outside said coolant tube.