Change of the induced magnetic field and time constant along twisted superconducting cables with finite length

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.