PURPOSE: To prevent the welding parts of an outer tube from damage without spoiling the flow of liquid helium necessary for cooling a coil, by a method wherein the outer tube, having axial grooves or spiral grooves on the inner peripheral surface thereof, is engaged with a coil mounting shaft at both ends of the outer tube while the engaging parts are welded.

CONSTITUTION: Both ends of a helium outer tube 6 are fitted to a coil mounting shaft 2 through shrink fitting while both ends are welded at places 6b in order to prevent liquid helium from leaking into an evacuated part 14. The helium outer tube 6 is provided with axial grooves or spiral grooves, which serve as helium flow passages, on the inner peripheral surface thereof. According to this constitution, liquid helium, flowing out of a liquid helium reservoir 15, arrives at said grooves on the inner peripheral surface of the helium outer tube through a shaft end helium flow passage hole, provided on the mounting shaft 2, thereafter, flows into a superconducting field coil through these grooves to cool the superconducting field coil 3. The liquid helium, whose temperature has risen by cooling the coil 3, flows out to a liquid helium reservoir 15.