Recycling impurity compression in Alcator C-mod divertor

Impurity retention is a key role of a divertor in a tokamak. One of the measurements of retention performance is the impurity compression ratio cz, defined as the ratio of the impurity neutral density in the divertor to the impurity ion density in the core plasma. DIVIMP Monte-Carlo modeling of argon recycling impurity compression in C-Mod has been performed to explore the physics behind impurity compression. It is found that the frictional force is dominant over the temperature gradient forces in determining the impurity compression in C-Mod divertor.