Suppression of MHD activity with limiter biasing in the HT-7 tokamak

We report here the first preliminary results of our limiter biasing experiments performed in the HT-7 tokamak. For this study, a limiter biasing system was designed, constructed and installed on the HT-7 tokamak. When a positive voltage was applied to the poloidal limiter, we observed a significant improvement in gross global particle confinement, as confirmed by the associated changes of the ratio between the line-averaged electron density and the Hα radiation level. A significant decrease in the ECE emission and significant increases in the plasma density, XUV bolometer emission and soft X-ray (SXR) emission are also observed. Furthermore the amplitude of Magnetohydrodynamic activity can be totally suppressed when a large enough biasing is applied to the limiter in the phases of current flat-top, current ramp-down and disruptions. (authors)