Role and Contribution of the Open Field Line Region in the Large Helical Device

Open field line region plays the key role for steady state operation of the Large Helical Device (LHD) and greatly contributes to the high-performance plasma confinement in the LHD. Chaotic field line region, produced by high magnetic shear and nonaxisymmetry of the magnetic field, is present in open field line re gion outside the last closed flux surface (LCFS) of the LHD. The chaotic field line layer can sustain ambient plasma due to the long connection length of lines of force, presence of the embedded magnetic islands and mirror confinement effect of helical ripple nature of the magnetic field. This ambient plasma plays a role of an impregnable barrier for the core plasma, which suppresses both the MHD instabilities and the cooling of the core plasma due to charge exchange processes. Slow and small periodic sweeping of magnetic axis po sition can control the deconcentration of divertor heat flux in the LHD.