EFFECTS OF DIVERTOR PLATE BIASING ON RADIAL AND POLOIDAL EDGE FLUXES IN THE TDEV

The divertor plates of TdeV, a tokamak with a double-null divertor and closed divertor chambers, have been electrically biased with respect to the walls. The paper discusses the resulting effects on the edge electron density profile, on the neutral pressures and impurity fluxes in the main vacuum chamber and the divertor chambers, and on the plasma flow to the divertors. As a function of the bias voltage, which was varied between -180 V and +160 V, the electron density scrape-off width and the wall impurity influxes increase monotonically; the flows to the top and bottom divertors vary strongly, in qualitative agreement with an E over arrow pointing right x B over arrow point right/B2 rotation, but not symmetrically. With negative biasing, the electrostatic barrier and the rotation combine to give a strong improvement of the divertor efficiency.