Studies of configuration control effects on dynamic behavior of Heliotron J plasmas

The transition to an improved confinement mode in NBI-only plasma is investigated in Heliotron J, focusing on the onset condition of the transition. The transition, identified by a drop in the Hα intensity and an increase in the stored energy, the line-averaged electron density, has been observed in the Co-injection case. The experiment shows (1) the delay time between the NBI turned-on and the drop of the Hα intensity decreases with the power, and (2) the transition occurs at a certain toroidal current (bootstrap current and NBCD current). The critical current depends on the vacuum rotational transform. The free-boundary equilibrium calculation predicts that the rotational transform and the shape of the magnetic surface, especially in the outer region, can be modified asymmetrically to the current direction. (author)