High-field, high-current-density, stable superconducting magnets for fusion machines

Designs for large fusion machines require high-performance superconducting magnets to reduce cost or increase machine performance. By employing force-flow cooling, cable-in-conduit conductor configuration, and NbTi superconductor, it is now possible to design superconducting magnets that operate at high fields (8-12 T) with high current densities (5-15 kA/cm2 over the winding pack) in a stable manner. High current density leads to smaller, lighter, and thus less expensive coils. The force-flow cooling provides confined helium, full conductor insulation, and a rigid winding pack for better load distribution. The cable-in-conduit conductor configuration ensures a high stability margin for the magnet. The NbTi superconductor has reached a good engineering material standard. Its strain-insensitive critical parameters are particularly suitable for complex coil windings of a stellarator machine. The optimization procedure for such a conductor design, developed over the past decade, is summarized here. If desired, a magnet built on the principles outlined in this paper can be extended to a field higher than the design value without degrading its stability by simply lowering the operating temperature below 4.2 K. (author). 11 refs, 3 figs