Physics of helical confinement systems

Recent stimulating studies that contributed to develop the physics of the helical systems are surveyed. In addition to exploring the advantage of the potential steady state operation, resolutions for the problem, e.g., the trapping of energetic particle, existence of stable high-beta equilibrium, ripple transport, etc., have been investigated. The optimization directions in magnetic structure are (i) the high rotational transform, the high shear and moderate well, and (ii) the reduced secondary current and the magnetic well. Both of them try to utilize the radial electric field for the improvement. Representatives of these two are the LHD and W7-X devices. Further optimizing approaches, like quasi-symmetry, are subject to studies. Experimental observations have been piled up. In this article it is emphasized that, based on these active works on the helical systems, the understanding of the general toroidal plasmas has become deeper, even including the tokamak plasma as a special example. The investigations for developing the helical systems have deepened the understanding of the nature of plasmas, and are some of the most important achievements in the physics of high temperature plasmas. (author)