Effect of Bumpy Magnetic Field on Energy Confinement in NBI Plasmas of Heliotron J

The energy confinement in the NBI plasmas has been investigated in the helical-axis heliotron device, Heliotron J. The configuration control experiments for bumpiness (εb), being one of the side-bands of the helical component, show that the plasma performance in NBI plasmas depends on not only the effective helical ripple (εeff) but also the bumpiness. The preferable enhancement factor of the experimental energy confinement time to the international stellarator scaling law ISS95 has been obtained in the high-εb configuration and in the low-εeff (i.e. medium-εb) condition. The improvement in the ion and electron temperature contributes to the enhancement of the plasma performance in the high-εb configurations since the fast ion confinement is found to be better in this configuration. The control of bumpiness is effective not only in the energetic particle transport but also in the global energy confinement in NBI plasmas of Heliotron J.