Impurity transport in tokamaks in the banana-plateau regime

Abstract

Transport coefficients, relating ion and impurity fluxes, and their fluxes of energy, to the density and temperature gradients, are derived from neoclassical theory. The ions are assumed to have mean free paths much longer than the torus connection length. The banana-plateau transition is shown to depend upon additional parameters which are not present in the electron-ion problem. Only two different ion masses are considered, but the heavier ions may have an arbitrary number of different possible ionization states. The mass ratio is arbitrary; the small mass-ratio limit is considered in detail, however. In that limit, the critical value of the collisionality parameter υ1*, for the transition from banana regime to plateau regime for the fluxes due to ion-impurity collisions, is shown to be ∼(n1/ni)(m1/mi)1/2, where i denotes light ions and I denotes impurities. The plateau regime values of the corresponding transport coefficients are proportional to this parameter, which may be much smaller than unity, in typical cases. The modification of electron transport properties due to impurities is also discussed.