FORMATION OF ELECTRIC-POTENTIAL IN A TOKAMAK EDGE BY LIMITER BIASING

It was found in the HYBTOK-II tokamak that the negative change of the electric potential of the plasma by a limiter biasing is influenced sensitively by the magnetic configuration in the SOL, but the positive change is not. The concept of "effective area" is introduced to explain the potential change due to a horizontal shift of plasma column in the negative biasing. Employing such a concept, a simple model for the formation of electric potential in a plasma for both a cold and an electron-emitting limiter is presented. It explains qualitatively the experimental results. The positive biasing was found to enhance the radial electric field at the scrape-off layer, and also at the inside, close to the last-closed magnetic flux surface, and seems to increase the particle confinement as measured by an increased line-averaged electron density with decreased H(alpha) and impurity line intensities, along with a stabilization of the magnetic and electrostatic fluctuations.