NEUTRAL BEAM DRIVEN IMPURITY FLOW REVERSAL IN TOKAMAKS (FUSION, PLASMA)

A theory for the effect of neutral beam injection on the transport of impurity ions in a tokamak plasma was extended to include the effect of temperature gradients. A theory for the effect of neutral beam momentum input on the radial heat conduction was also developed. Injection of neutral beam momentum in the direction of the toroidal magnetic field, called co-injection, was found to reverse the normally inward flow of impurities. The theory was found to provide a reasonable basis for interpretation of impurity flow reversal experiments performed in the Princeton Large Torus (PLT), when a multiplicative factor of two was applied to the predicted impurity fluxes.

The model that was adjusted to fit the experimental results in PLT was then applied to the Tokamak Fusion Test Reactor (TFTR) and to models based on designs of future tokamaks. Using the maximum available co-injected beam power (16 MW) in TFTR is predicted to lead to a substantial reduction (relative to balanced momentum injection) in the penetration of impurities to the center of the discharge and to substantially increase the impurity radiation from the plasma edge. This would possibly lead to a cold, radiating edge which would reduce sputtering erosion of the limiter. A modest amount (30 MW) of co-injected beam power is predicted to substantially reduce the penetration of impurities to the center of models of the Fusion Engineering Device (FED) and the STARFIRE commercial reactor. If these plasmas operate with a high edge density, as would be the case with a high rate of edge recycling, this amount of co-injected power is also sufficient to significantly increase the impurity radiation from the edge region, again possibly leading to a cold radiating edge and associated reduction in limiter erosion.