MODIFICATION OF THE PLASMA EDGE PROPERTIES DURING DIVERTOR PLATE BIASING EXPERIMENTS ON TDEV

A laser-ablated fast particle diagnostic has been used for the measurement of n(e)(r) from the wall to 50 mm inside the TdeV separatrix. Divertor plate biasing has been found to modify substantially the n(e)(r) profile. The e-folding length at the separatrix, lambda(n), has been evaluated as a function of the biasing potential, and the results suggest a modification of the radial transport as a function of the radial electric field. The ''LaBombard model'' using different diffusivities (D(perpendicular-to)) and mobilities (mu(perpendicular-to)) for ions and electrons cannot reproduce the dependence of lambda(n) as a function of biasing. A simple model using the same D(perpendicular-to) and mu(perpendicular-to) for electrons and ions has been developed which gives a reasonable fit to the experimental measurements for D(perpendicular-to) = 3.2 m2/s and mu(perpendicular-to) = 0.039 m2/s V. Also, the particle confinement time tau(p) has been estimated with the help of the model, suggesting that negative biasing of the divertor plates increases tau(p).