Performance of the helical coils for the large helical device in six years\u27 operation

A pair of helical coils of the large helical device is the largest pool-cooled superconducting magnet. The first excitation test up to 1.5 T was carried out at the end of March, 1998. In the first trial to charge up to the design value of 3 T, wide propagation of a normal-zone was induced at 11.45 kA, and the quench detection system acted. It was revealed that a normal-zone could propagate below the cold-end recovery current by additional heat generation due to the slow current diffusion into the thick pure aluminum stabilizer. In these six years\u27 operation, propagation of a normal-zone has been observed 17 times at almost the same current. The normal-zones were recovered within several seconds except for the wide propagation. By a detection system of the propagation with pick-up coils along the helical coils, it was disclosed that the recovered normal-zones propagated in only one side. In order to attain plasma experiments near 3 T, higher excitations were tried by the current grading method, in which the current of the innermost block is decreased and those of the other two blocks are increased. The average current of 11.67 kA was attained. The excitation tests up to the highest currents have been carried out after each cool-down. Degradation is not observed in the coil performance, and the stable operation has been demonstrated

Improvement in Cryogenic Stability of the Model Coil of the LHD Helical Coil by Lowering the Temperature

Helical coils of the Large Helical Device are pool-cooled superconducting magnets, in which propagation of a normal-zone has been observed several times at about 86% of the nominal current of 13.0 kA. It is planned to improve the cryogenic stability by lowering the inlet temperature. In order to estimate the effect, the cryogenic stability of a model coil of the helical coil was examined in saturated and subcooled helium. Liquid helium is supplied from the bottom of the model coil, and it is exhausted through the winding to the current-leads tank. The inlet helium is subcooled by a pre-cooler. A normal zone was initiated by a heater on the conductor at the bottom of the coil. In saturated helium of 4.4 K and 0.12 MPa, the minimum current to propagate over the next turn varies from 10.7 to 11.2 kA in the four cases that are without or with additional thermal shields, and before or after being subcooled. The difference is considered to be caused by the change of quality of saturated helium inside the winding or by the change of the wetted condition of the conductor surface. The minimum currents are higher at the lower temperatures in subcooled helium. It is raised up to 11.7 kA at 3.5 K of the temperature inside the winding. The propagation velocity at each minimum current is almost same. Namely, the propagation velocities at the same current are slower at the lower temperature in subcooled heliu

Stability tests of module coil (TOKI-MC) wound with an aluminum stabilized superconductor

The module coil TOKI-MC is a twisted solenoid coil wound with an aluminum stabilized superconductor developed as an R&D program for the Large Helical Device (LHD). The TOKI-MC can simulate the conductor and winding structure cooled by pool boiling helium, the twisted winding and the large electromagnetic force of the helical coils for LHD. The TOKI-MC was designed as a cryostable coil at an operating current of 20 kA, but the coil quenched around 17 kA in excitation tests. The cause of quenches was thought to be the result of wire movement. Stability tests were also carried out and the measured recovery current was less than the designed value. The degradation of recovery current was due to the excess magnetoresistivity of the copper clad aluminum stabilizer. The stability of TOKI-MC was evaluated and compared with the data of short sample test