Overview of helical systems

Recent experimental results, mainly from heliotron/torsatron devices and an advanced stellarator, are reviewed. An international helical system database on the energy confinement time has been compiled; the confinement time scaling is similar to the tokamak L mode. Improved confinement regimes, i.e. high Ti mode, pellet mode, H mode and reheat mode, have also been investigated. In L mode and improved confinement regimes the energy confinement scaling has a favourable density dependence, and the operating density limit by radiation collapse in helical devices has a much steeper magnetic field dependence than in tokamaks. The maximum attainable β of about 2% is not limited by MHD instabilities but by the absorbed heating power. Investigations of divertor configurations, including local island divertors and natural island divertors, showed that next generation devices are very promising for steady state operations. On the basis of these results, next generation devices (LHD and WVII-X) with a major radius in the range of 4 to 5.5 m, have been under construction or have been approved for construction. A small Heliac has started its operation, and another has been under construction. A small modular stellarator with quasi-helical symmetry has also been under construction. (author). 65 refs, 4 figs, 2 tabs