Divertor plate biasing experiments on TdeV

Divertor plate biasing on TdeV (Tokamak de Varennes) is used to affect the impurity influx and possibly influence the transport in the edge region. Electrostatic microturbulence inside the separatrix is significantly reduced with either bias polarity in agreement with velocity shear stabilization. Biasing improves the divertor pumping efficiency for the top or bottom closed divertor chamber depending on the bias polarity, in agreement with the ExB/B2 drift direction. Respectively, negative/positive biasing improves/degrades the plasma by reducing/increasing the carbon and CO fluxes, the loop voltage and the X-ray emissivity without noticeable changes on the electron density or temperature profiles. The gradient density scale length near the separatrix, the poloidal rotation velocity near the edge, the heat pulse propagation speed and the sawtooth period and crash times are also affected by biasing. (author) 4 refs., 5 figs