Performance of all high-Tc superconducting magnets generating 4 T and 7 T at 20 K

Refrigerator-cooled all high-Tc superconducting magnets, i.e. coils and current leads, 4 T and 7 T types, have been developed. The 4 T type successfully produced a central magnetic field of 4.0 T over 30 min and 3.5 T for 48 h. High ramp rate excitation up to 0.35 T s-1 was achieved. The 7 T type magnet stably generated 7.1 T over 24 h and was excited at a rate of 7 T min-1 . These ramp rates were 20-100 times faster than that of the refrigerator-cooled magnets using metallic superconductors. A new cooling scheme and stability criterion for conduction-cooled magnets are proposed. (author)