Upgrading program for improving the cryogenic stability of LHD helical coils by lowering the operating temperature

Helical coils of the large helical device are pool-cooled superconducting magnets. Although they were designed to be cryostable at 13.0 kA in saturated helium at 4.4 K, propagation of a normal zone has been observed several times at higher currents than 11.0 kA. The main cause is additional heat generation due to the slow current diffusion into a thick pure aluminum stabilizer. Methods for improving the cryogenic stability of the coils have been examined. Considering the withstanding voltage of the coil and the possibility of air mixing, the method of being subcooled has been selected. Design values of the mass flow and the inlet temperature are 50 g/s and 3.1 K, respectively, by utilizing the surplus power of an existing helium refrigerator. The average temperature of the helical coil is expected to be lowered to 3.5 K. In order to estimate the effect, cryogenic stability tests have been carried out with a model coil. According to the test results and comparison of the difference of the magnetic field between the model coil and the helical coil, the operating current is expected to be increased to 12.0 kA that corresponds to 3.0 T at the major radius of 3.6 m.