Current understanding of divertor detachment: Experiments and modelling

A qualitative as well as quantitative simulation of experimentally observed plasma parameters in the detached regime proves to be difficult for several tokamaks. A series of ohmic discharges have been performed in ASDEX Upgrade and DIII-D at as similar as possible plasma parameters and at different line averaged densities, n¯e. The experimental data represent a set of well diagnosed discharges against which numerical simulations are compared. For the numerical modelling the fluid-code B2.5 coupled to the Monte Carlo neutrals transport code EIRENE is used. Only the combination of effects, such as geometry, drift terms, neutral conductance, increased radial transport and divertor target composition explains a significant fraction of the experimentally observed ion fluxes, ΓtΓt, to the inner and outer target plates as a function of n¯e in ASDEX Upgrade. The relative importance of the mechanisms leading to detachment differ in DIII-D and ASDEX Upgrade.