Effect of Direction of External Magnetic Field on Minimum Propagation Current of a Composite Conductor for LHD Helical Coils

The conductor for helical coils of the Large Helical Device consists of a Rutherford-type NbTi/Cu cable, a pure aluminum stabilizer, and a copper sheath. The dimensions of the conductor and the stabilizer cross-sections are 18.0 mm × 12.5 mm and 12.4 mm × 5.2 mm, respectively. The measured cold-end recovery current in the magnetic field parallel to the shorter side ( B //12.5) is clearly lower than that in the field parallel to the longer side ( B //18.0) because of the difference in magnetoresistance by Hall currents. Since the minimum propagation current I mp is important to determine the upper limit of operation current, I mp has been measured for two types of one-turn coil samples, which were bent flatwise ( B //18.0) and edgewise ( B //12.5) with the inner radius of 0.14 m to extend the length in the uniform background field of the test facility. The measured I mp at B //12.5 is almost the same as that at B //18.0 in spite of the large difference in the steady-state resistance. I mp is considered to be determined by the heat balance before the current diffuses deeply into the stabilizer.