AVERAGE ION APPROXIMATION FOR MODELING IMPURITY TRANSPORT IN TOKAMAKS

A simple method is described to model impurity transport in magnetical ly confined plasmas. The model is based on the average ion approximati on for the impurity species, in which atomic rates are assumed to depe nd only on the local plasma parameters and on the ion average charge. Results obtained with this approximation in an idealized two-dimension al transport code, are compared with similar results obtained from det ailed atomic calculations. The agreement between the two calculations is found to be excellent for impurities in an ionizing regime. This co ndition is expected to prevail in high recycling divertors, where the average ionisation rate exceeds the average recombination rate. For st rongly recombining impurities, however, the radiative loss and cooling rates computed in the average ion approximation can be seriously in e rror.