DIVERTOR PLASMA STUDIES ON DIII-D - EXPERIMENT AND MODELING

In a magnetically diverted tokamak, the scrape-off layer (SOL) and div ertor plasma separates the first wall from the core plasma, intercepti ng impurities generated at the wall before they reach the core plasma. The divertor plasma can also,serve to spread the heat and particle fl ux over a large area of divertor structure wall using impurity radiati on and neutral charge exchange, thus reducing peak heat and particle f luxes at the divertor strike plate. Such a reduction will be required in the next generation of tokamaks, for without it the divertor engine ering requirements are very demanding. To successfully demonstrate a r adiative divertor, a highly radiative condition with significant volum e recombination must be achieved in the divertor, while maintaining a low impurity content in the core plasma. Divertor plasma properties ar e determined by a complex interaction of classical parallel transport, anomalous perpendicular transport, impurity transport and radiation, and plasma-wall interaction. In this paper we will describe a set of e xperiments on DIII-D designed to provide detailed two-dimensional docu mentation of the divertor and SOL plasma. Measurements have been made in operating modes where the plasma is attached to the divertor strike plate and in highly radiating cases where the plasma is detached from the divertor strike plate. We will also discuss the results of experi ments designed to influence the distribution of impurities in the plas ma using enhanced SOL plasma flow. Extensive modelling efforts will be described which are successfully reproducing attached plasma conditio ns and are helping to elucidate the important plasma and atomic physic s involved in the detachment process.