Effect of divertor bias on plasma flow in the DIII-D scrape-off layer

A toroidal ring bias electrode in contact with the single-null divertor scrape-off layer in the DIII-D tokamak achieved electric field control of plasma flow in the layer. Bias strongly altered the static pressure of neutralized gas in the new divertor chamber (designed for future pumping) during Ohmic, L-mode and ELMing H-mode plasma operation. The pressure increased or decreased, depending on the signs of the applied electric potential and toroidal magnetic field, in agreement with the E*B drift direction. The variation of the core plasma density was opposite to the variation of the pump chamber gas pressure and was proportionally much smaller. A large bias induced pressure rise appeared even when the scrape-off layer was not directed at the pump entrance aperture-a surprising effect that is interpreted qualitatively in terms of radial potential discontinuities. Bias also affected divertor heat fluxes, and preliminary observations are discussed qualitatively.