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The local magnetic induction was measured by Hall sensors on different places above Rutherford-type superconducting cables with NbTi/Cu strands, used for the helical coils of the LHD project. The conductor was several times the twist pitch of the cable and was bent into a circular form to assure constant applied magnetic field from the external split coil. Both the local magnetic moment of the cable and the local time constant τ showed clear oscillatory behaviors at ramping down the applied field from 4 T. The periodicity corresponds to the twist pitch of the cable. However, the amplitudes of both parameters are smaller than those expected from previous theories. Three effects are mentioned for explaining the differences between experimental results and theory. On some places, the magnetic moment changed unexpectedly its polarity during the discharge. This suggests some type of “paramagnetic” contribution in the induced currents. We show that in such cases τ can be deduced from the time tm at which the magnetic moment has maximum. The derived relation between tm and τ is tested in such cases when the determination of τ from the experimental results is possible. Very good agreement is achieved between experimental results and theory.

The Large Helical Device (LHD) is a heliotron-type toroidal fusion experimental device which will provide useful and reliable data sets for high temperature plasmas with an equivalent Q value of 0.1–0.35. One of the crucial tasks of LHD is to demonstrate steady-state operations by taking the advantage of currentless plasmas. In this respect, the coil systems are fully superconducting (SC); consisting of a pair of helical coils, three pairs of poloidal coils and nine bus-lines, all of which are presently in the final stage of assembly. The helical coils and the Outer Vertical (OV) poloidal coils are the world largest SC coils among the existing fusion devices. The flexible SC bus-lines are one of the innovative components newly applied through this project. The technological development of the SC magnets took key roles in LHD.

Helical coils for LHD are pool-cooled superconducting coils. In order to produce a fine magnetic surface, highly accurate manufacturing tolerances within ±2 mm and high rigidity against large electro-magnetic forces are required. Additionally, high current densities of over 50 A mm−2 are necessary to keep enough distance between the helical coil and plasma. The helical coil is designed to enhance cryogenic stability by optimizing the wetted surface fraction of each conductor in considering both the magnetic field and stress in the insulator between conductors. For attaining highly accurate helical winding and cryogenic stability, composite conductors with pure aluminum stabilizers were developed and directly wound on highly accurate thick cases. The actual winding was carried out on-site from January 1995 to May 1996. After that, the top covers of the case were set on the coils and welded very carefully. The entire assembly was installed into an outer supporting shell.

Irregular magnetic fields resonate with the rational surface of the magnetic confinement systems, form magnetic islands and ergodic layers, and destruct the plasma confinement. To avoid this confinement destruction the requirement of an accuracy of 10 −4 in the magnetic field is adopted as the magnetic-accuracy design criterion for the LHD machine. Following this criterion the width of the undesirable magnetic island is kept less than one tenth of the plasma radius. The irregular magnetic field from the superconducting (SC) helical and poloidal coils is produced by winding irregularity, installing irregularity, cooling-down deformations and electromagnetic deformations. The local irregularities such as feeders, layer connections, adjacent-conductor connections of the coils also produce an error field. The eddy currents on the supporting shell structure of SC coils, the cryostat, etc. are also evaluated. All irregular effects are analyzed using Fourier decomposition and field mapping methods for the LHD design, and it is confirmed that the present design of the superconducting coil system satisfies the design criterion for these field irregularities.

Enhancement of magnetoresistance due to a Hall current is discussed. If an inerface between different materials exists in a cable, a non-uniform Hall effect may yield a Hall current across the interface. Such a current causes an additional resistive loss, and an enhanced magnetoresistance should be observed. Since the polarity of the Hall effect in copper is opposite to that in aluminium, the effect might be conspicuous in an Al-Cu composite. Enhanced resistivity is calculated in a short circuit model, where the basic mechanism is easily understood, and next in a coaxial model, which is for a particular but realistic model of a cable. A basic method for evaluation of enhanced resistivity is established for the composite conductor. The reduction of the Hall current is particularly important for the application of a highly conductive material to stabilizers of a superconducting cable.