Results of the first excitation of helical coils of the Large Helical Device

The helical coils of the Large Helical Device are large scale pool-cooled superconducting coils. A conductor made of NbTi-Cu compacted strands and a pure Al stabilizer was developed to attain high cryostability. The design current of the conductor is 13.0 kA at 4.4 K, which produces a toroidal magnetic field of 3 T at a major radius of 3.9 m. The first excitation test up to 6.5 kA was conducted successfully is the beginning of the first cooling period. The higher excitations were tried in the second cooling period. The first propagation of a normal zone was observed when reaching 11.2 kA, and it recovered within 5 s. In the next step, a wide propagation occurred at 11.4 kA, and the quench detection system worked. The coils were designed to satisfy 'cold-end' stability by using the recovery current measured in short samples, but a normal zone propagates at lower than the recovery current in the composite conductor stabilized by very low resistive metal.