METHOD OF CONTROLLING IMPURITY TRANSPORT

Abstract

We discuss a proposed method of impurity control, based on poloidally asymmetric injection of hydrogen gas into a tokamak discharge. This injection creates a localized source of protons, which then interact with the impurities by Coulomb collisions. In a toroidal device, each species has a flow pattern such that there is an upward or downward drift (depending on the toroidal field polarity) which is compensated for by a return flow along the field lines. The motion of impurities is slower due to their higher Z. Thus, collisional friction between species exists and can be shown to cause radial diffusion of impurities. The source of protons allows one to compensate the vertical drift, thus altering the collisional friction and the radial diffusion. Calculations have shown that the magnitude of the sources required can be attained in present machines. The effect will be sought in the first experimental investigation in the Impurity Study Experiment tokamak.