Prospects Toward an Integrated Heliotron Fusion Reactor

Heliotron reactors have several features suitable for a fusion power plant, such as no need for current drive, no plasma current disruptions, suitability for steady-state operation, and a wide space between helical coils useful for maintenance of in-vessel components. According to recent reactor studies based on the experimental results in the Large Helical Device (LHD), the plasma major radius of a heliotron reactor is set to 14 to 16 m in order to install shielding and breeding blankets with total thickness of 1 m. The central toroidal field for the self-ignition is 5 to 6 T under the assumption that the confinement enhancement factor is 1.2 to 1.4 with respect to the LHD. The stored magnetic energy is estimated to be 120 to 150 GJ. Both the major radius and the magnetic energy are three times larger than those of ITER. Its large helical windings, however, can be realized by steady extension from the ITER technology, because cable-in-conduit conductors similar to those for ITER toroidal field coils can be adopted. Improvement of plasma confinement is essential to reduce the number of magnet systems. A roadmap to a heliotron DEMO is discussed.