Extended Range of Stable Radial Position of Tokamak Plasma in TOKASTAR-2

In TOKASTAR-2, the tokamak plasma position was radially unstable in the outer region due to a large decay index (n-index) of the vertical field. To improve the radial position stability, the position of pulsed vertical field coils was moved toward the midplane. After the modification, improvement in the decay index profile and extension of the range of the stable radial plasma position toward the outer region were observed by the internal magnetic field measurement. The variation in the outermost position of the plasma center was about half of the radial shift of the decay index curve.