Overview of Helical Systems

"Recent experimental results, mainly from heliotron/torsatron devices and an advanced stellarator, are reviewed. An international helical system data base 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 T_i mode, pellet mode, H-mode, and re-heat mode, have also been investigated. In L-mode and improved confinement regimes the energy confinement scaling has a favorable 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 beta 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 will have a good prospect for steady state operations. Based on these results next generation devices (LHD and WVII-X) with a major radius in the range, 4 - 5.5m, have been either under construction or approved for construction. A small Heliac has started its operation, and another has been under construction. A small modular stellarator with a quasi-helical symmetry has also been under construction.