Radiative boundary discharges with impurity injection and the H-L transition in ASDEX Upgrade

The influence of impurity radiation on the H --> L and L --> H transitions is investigated for highly radiative divertor discharges in the ASDEX Upgrade tokamak. The transitions between H- and L-mode depend on the net heat Bow across the separatrix, P-sep, calculated from the heating power and bolometric radiation profiles. For typical radiative boundary conditions in ASDEX Upgrade, the radiation distribution is dominated by line emission in the vacuum-ultraviolet region and peaks near the separatrix. For the case of neon used as seed impurity, about 2/3 of the main chamber radiation is emitted inside, but close to the separatrix. Argon seed results in a higher fraction of core radiation, while the line emission is shifted further outside for nitrogen. The radiation-corrected L --> H threshold is not affected by gas puffing and is described by P-sep(L --> H) = 0.125 <(n)over bar (e)>B-t(2/A(plasma)) [MW, 10(19) m(-3), T, amu]. The H --> L threshold power, which is typically lower by a factor of two without strong deuterium puffing, is increased by heavy gas puffing leading to P-sep(H --> L) approximate to p(sep)(L --> H) In the vicinity of the radiation-induced H --> L transition, a general alignment of H and L mode is observed with regard to global energy confinement time and edge density and temperature profiles. The H --> L transition itself exhibits a smooth evolution in time. Reduction of target plate power load down to about 10% of the total heating power is easily achieved by edge radiation in the CDH-mode for low p(thres)(H --> L) conditions. However, this reduction is attributed mainly to radiation from inside the separatrix and is connected to relatively high values of the core Z(eff). These results emphasize the importance of the development of more closed divertor concepts, allowing for higher divertor radiation levels in connection with lower core Z(eff).