Superconducting magnets for gyrotron tubes

Six superconducting magnet systems have been designed and built for gyrotron tubes furnished by Varian. The systems are being installed on the General Atomic Doublet III Tokamak Fusion Device for radio frequency heating. They will initially operate at 60 GHz requiring a 2.5 T field, but the magnets can be operated to 5 T for possible upgrade to 120 GHz. Key design features include high precision coil winding, high precision coil positioning (concentric with bore), low helium boil-off rate, and a one week operating time without refilling. A low operating current of 7 A for 60 GHz operation was selected for reducing the heat leak through the power leads. Aluminum tape and superinsulation are utilized to decrease the radiation heat leak. The overall dimensional constraint makes it difficult to keep the helium boil-off rate very low. Two systems have been operated with 60 GHz gyrotron tubes. The tubes have been brought up to full power and the performance of the magnet is satisfactory. The average liquid helium boil-off rate is 0.4 l/hr, i.e., liquid helium will last for six days without refilling. The liquid nitrogen cooled thermal shield must be refilled every three days. The detailed design, construction, and testing results are presented.