AVERAGE ION APPROXIMATION FOR MODELING IMPURITY TRANSPORT IN TOKAMAKS

A simple method is described to model impurity transport in magnetically confined plasmas. The model is based on the average ion approximation for the impurity species, in which atomic rates are assumed to depend only on the local plasma parameters and on the ion average charge. Results obtained with this approximation in an idealized two-dimensional transport code, are compared with similar results obtained from detailed atomic calculations. The agreement between the two calculations is found to be excellent for impurities in an ionizing regime. This condition is expected to prevail in high recycling divertors, where the average ionisation rate exceeds the average recombination rate. For strongly recombining impurities, however, the radiative loss and cooling rates computed in the average ion approximation can be seriously in error.