The present invention relates to a superconducting magnet apparatus, more particularly, to leads for supplying power to a superconducting coil in a liquid helium container through said container and a surrounding vacuum insulation vessel.
FIG. 3 is a longitudinal section showing the upper part of a prior art superconducting magnet apparatus. The respective numerals in the figure denote the following: 1, superconducting coil; 2, liquid helium in which the superconducting coil 1 is submerged to be cooled to cryogenic temperature; 3, a liquid helium container for accommodating the superconducting coil 1 and the liquid helium 2; 4, a vacuum insulation vessel for thermally insulating the liquid helium container 3 by providing a vacuum layer around it; 5, a heat shielding plate provided between the liquid helium container 3 and the vacuum insulation vessel 4; 6, a wiring tube penetrating through the liquid helium container 3 and the vacuum insulation vessel 4; 7, power supply leads inserted into the wiring tube 6, which are formed of a hollow normal conductor; 8, a flange for assisting in the mounting of the power supply leads 7; 9, a connector to which the power supply leads 7 are connected; 10, a power source for exciting the superconducting coil 1; 11, an inlet for allowing helium gas vaporized from the liquid helium 2 to flow into the wiring tube 6, 12, a discharge port through which the helium gas that has flown through the inlet 11 is released into air atmosphere; 13, an inlet for allowing part of the helium gas flowing into the wiring tube 6 through the inlet 11 to flow into the hollow power supply leads 7; and 14, an outlet through which the helium gas flown through the inlet 13 is released into the wiring tube 6.
FIG. 4 is a cross section of FIG. 3 taken along line IV--IV; 7a in FIG. 4 denotes a power supply lead on the negative side; 7b denotes a power supply lead on the positive side; and 6a and 7c denote channels for the passage of helium gas vaporized from the liquid helium 2.
In the prior art superconducting magnet apparatus having the construction described above, the superconducting coil 1 in the liquid helium container 3 is cooled with liquid helium 2 to the cryogenic temperature at which it becomes superconductive. In order to excite (magnetize) or demagnetize the superconducting coil 1, the power supply leads 7 are connected to the connector 9 through the wiring tube 6, and the connector 9 is further connected to the external power source 10 for current application. After excitation or demagnetization is completed, the leads 7 are removed so as to prevent external heat from transmitting to the liquid helium container 3 through the leads 7.
Since the power supply leads 7 are formed of a hollow normal conductor, when current is applied for excitation or demagnetization purposes, the current flowing through the normal conductor will cause a resistance loss expressed by I.sup.2 R. Since cryogenic helium gas vaporized in the liquid helium container 3 flows through channels 6a and 7c shown in FIG. 4, part of the heat resulting from the resistance loss is dissipated into the helium gas which is released into air atmosphere through the outlet 12. The remainder of the heat is conducted from the leads 7 through the connector 9 to the liquid helium container 3, thereby promoting the evaporation of liquid helium 2.
As described above, the prior art superconducting magnet apparatus which employs a normal conductor in the power supply leads suffers the problem of the development of resistance loss in both excitation and demagnetization modes, which leads to accelerated consumption of liquid helium 2 on account of heat penetration into the liquid helium container 3.