Construction of 320 kJ superconducting force-balanced coil

Strong electromagnetic force caused by high magnetic fields and coil current is a serious problem in superconducting magnetic energy storage (SMES) systems. To cope with this problem, we propose the concept of force-balanced coil (FBC) applied to SMES. The FBC is a helical-winding torus coil that balances the centering force with the hoop force, both of which are exerted in the major radius direction. In the case of FBC, the toroidal current distribution is modulated so that one of the magnetic surfaces coincides with the torus surface in order to reduce the torsional force. This current distribution can be achieved by varying the helical winding pitch. However, the smaller the aspect ratio (major radius/minor radius), the more difficult it becomes to manufacture the varying helical windings. To overcome this difficulty, we developed a winding machine which can modulate the helical winding. We also designed an experimental superconducting FBC which is being manufactured using the winding machine.