Performance of the helical coils for the large helical device in six years' 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' 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.