Ion flows in the scrape-off layer with biased limiter: implications for Tore Supra toroidal pumped limiter design

A simple analytic fluid model based on particle and momentum conservation describes the response of the scrape-off layer to an applied radial electric field. The model explains why the variation of plenum pressure during limiter biasing in TEXTOR shows the opposite behaviour as divertor biasing in Tokamak de Varennes: the mechanical baffles that define the throats of the pump limiter are aligned along the magnetic field, thus preventing collection of the induced perpendicular flux when the drift is directed towards the scoop. Rather, the optimal pumping is expected when the drift is directed away from the scoop due to the increase of the parallel flux needed to satisfy the Bohm-Chodura criterion. Qualitative agreement is obtained with measurements of the parallel flow near the limiter throat, and with measurements of the plenum pressure during negative biasing for both directions of toroidal magnetic field. The results, extrapolated to Tore Supra, predict that the actual design with throats should provide close to optimal pumping with minor modification of the natural radial electric field, but greater performance could be obtained with negative biasing if a toroidally symmetric neutralizer geometry were adopted.