Analytical impurity transport model: Coupling between particle and charge state transports in tokamak plasmas

Interpretation and understanding of a set of coupled continuity equations commonly used in impurity tokamak transport studies are found to be syncretic and inadequate, since they do not distinguish between the cross-field fluxes of impurity particles and of their charge states (cs). An analytical approach to impurity transport studies previously developed is generalized to the case of diffusive-convective cs dynamics and provides a set of dimensionless parameters to describe the impurity behavior more correctly than using empirical coefficients D and V, which, in turn, have to be interpreted in terms of their cs, rather than of particles. An analytical transport model (ATM) with underlying interpretation is proposed. It is based on the solutions of two separate transport problems for impurity cs and particles instead of a single one. It is shown that ATM consistently explains two groups of available empirical dependences regarding the scaling D proportional to n(e)(-1) and the profiles of V/D, which are shown to be the density profile gradients, del n(Z)/n(Z) and suggests the neoclassical scale of the particle diffusion coefficient.