Impact of ExB drifts on impurity distribution in the scrape-off layer of atokamak

As shown in recent years, a strong electrostatic field exists in the tokama k Scrape-Off-Layer (SOL) and divertor zone. In the present paper it is demo nstrated that the ExB drifts resulting from it, compete with the impurity v elocities caused by friction with the main ions, as well as with thermal fo rces. This effect can redistribute impurity flows significantly. Particular ly, it may lead to impurity flow reversal. Fluid equations for the test imp urities are derived using the two most representative ionization states mod el. As shown to first approximation, impurity motion does not depend on the ionization state. For a favorable magnetic-field direction, when the ion d el B drift is directed towards the X-point, in the private region the impur ities flow from the outer to the inner divertor plate. On the contrary, in the SOL the impurity flow is directed from the inner to the outer plate. Si gnificant impurity velocity shear exists near the separatrix, and creates t he effect of flow reversal. Diffusion across the flux surfaces transports i mpurities flowing away from the divertor plates to regions with the opposit e flow direction. A reduced visual model is presented. (C) 2000 American In stitute of Physics. [S1070-664X(00)01004-1].