Construction of a large superconducting spectrometer magnet

A sector-type superconducting spectrometer magnet for nuclear physics experiments has been constructed. The central magnetic field is 3 T with a magnet gap of 50 cm and a stored energy estimated to be 11.2 MJ. Three-dimensional magnetic field calculations and a stress analysis have been performed to design the coil and supporting structures. The magnet has a flexible helium transfer line to measure angular distributions at nuclear physics experiments. To simplify the transfer line, a small-size G-M refrigerator, which is installed on a service port of the helium vessel, is used for thermal insulation at 80 K and 20 K in combination with a medium-size refrigerator for cooling the magnet at 4 K. The heat leak at 4 K is estimated to be 3 W at 4 K. The maximum operation current of 500 A is selected by considering the maximum temperature of the coil at quench. In preliminary cooling tests, the magnet has been cooled within 52 h as expected