An analytical model for retention of divertor impurities by forced flows

In this paper the retention of target produced impurities in a pumped divertor with 'externally recirculated' or 'forced flow' is studied using an analytical model. The results are compared with those from a detailed numerical study which has been previously reported. The dominant forces acting on impurity ions in a scrape-off layer (SOL) are the ion thermal force, directed away from the divertor, and the ion friction force, directed towards the divertor. In steady state, an equilibrium is established in which the impurity pressure gradient balances the difference between these two forces. (author) 4 refs., 3 figs., 1 tab