BIFURCATION TO A MARFING DIVERTOR PLASMA GOVERNED BY CHARGE-EXCHANGE

A one-dimensional (1-D) model is used to investigate the effect of a high neutral density on the divertor plasma via charge exchange. A regime characterized by a strong plasma pressure decrease along the field line is obtained above a critical neutral density. The bifurcation is characterized by the location of the ionization front, which reaches 70% of the distance from the divertor plate to the X point at the threshold. At this position, the Mach number is large, which has a beneficial aspect on impurity control. The weak dependence of the bifurcation control parameter on the heat outflow from the core allows one to extrapolate this regime to the ignited plasma in the next step devices without requiring large modification of the divertor geometry with respect to that of present experiments.