Impurity transport in tokamaks in the plateau to Pfirsch–Schlüter regimes

Neoclassical transport coefficients for ion and impurity species are computed in a large aspect ratio toroidal magnetic confinement system for intermediate-to-high collision frequencies. A large range of values of the relevant density and charge parameters is explored and terms of order (mi/mI)1/2 are retained. The behavior of the transport coefficients is found to depend on the collisionality of both the ions and the impurities. A simple approximate analytic representation of these results which incorporates the correct asymptotic plateau and Pfirsch–Schlüter behavior is presented. In the plateau regime, when (nI/ni)(mI/mi)1/2≪1, the inward impurity flux is found to be suppressed for experimentally obtainable plasma parameters, implying that the time scale for impurity concentration could be too long to be observable.