IMPURITY CONTROL BY NEUTRAL-BEAM INJECTION

A recently developed generalization of neoclassical theory is applied to study the use of neutral-beam injection to drive impurities out of a tokamak plasma. The theory accounts for the direct momentum exchange between beam and plasma, which has been studied previously, and also for the effects of the beam in altering the first-order particle flows and the ambipolar potential. Conditions are established for which co-injection drives impurity fluxes outward. It is estimated that order-unity changes of the radial impurity flow should be found in current experiments and that modest levels of neutral-beam injection should suffice for impurity control in reactors. In the course of this work, the general theory of particle transport in the presence of particle and momentum sources is evaluated for a two-species plasma with arbitrary plasma geometry and beta. © 1979 IOP Publishing Ltd.