On transport, in particular of impurities, in a stochastic magnetic field

The transport coefficients in a stochastic magnetic field are computed by taking into account analytical relationships for the distribution of plasma parameters along divergent lines of force. In the case of impurities, in contrast to what was believed before, stochastization primarily influences their convection but not their diffusive motion. At the plasma periphery, this convection is mainly caused by friction with the flow of background particles sustained by recycling and is directed toward the plasma boundary. This leads to an enhancement of the impurity exhaust, which provides a plausible explanation for the plasma decontamination observed in tokamaks with peripheral stochastic layers.