NEOCLASSICAL TRANSPORT IN THE PRESENCE OF FLUCTUATIONS

The usual argument for automatic ambipolarity of neoclassical particle fluxes in a tokamak is based on the averaged toroidal momentum equation. It does not apply when fluctuations are present, because they also contribute to the momentum. The finite Larmor radius pressure tensor produced by electrostatic fluctuations makes a small contribution to the averaged momentum balance and also gives rise to a small non-ambipolar radial flux. The [j x B(over arrow pointing right)]phi force from stochastic magnetic fluctuations can be much larger, as is the associated non-ambipolar particle flux. The ambipolar electric field is determined by the combined neoclassical and anomalous fluxes. Since the transport produced by electrostatic fluctuations is nearly ambipolar, the resulting change in the radial electric field from its neoclassical value is small. However, electron loss along a stochastic magnetic field is strongly non-ambipolar and can reduce or reverse the inward electric field produced by neoclassical transport alone. Impurity neoclassical transport is most sensitive to the electric field because of its high atomic charge. This can explain the observed pump-out of impurities during intense MHD activity.