Parameter Regime of Ion Internal Transport Barrier Formation in the Large Helical Device

Improvement of ion heat transport was observed in the core plasma heated by neutral beam injection (NBI) in the Large Helical Device (LHD). A peaked ion temperature profile with a steep gradient (an ion internal transport barrier, ion ITB) formed in the plasma. To investigate the formation conditions of the ion ITB, a simple definition of ITB based on the reversal of the temperature gradient between the core and the edge is proposed. An ion ITB formed in the low-collisionality regime of 1/ν with low density and high ion heating power in the LHD.