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 transit ions is investigated for highly radiative divertor discharges in the A SDEX Upgrade tokamak. The transitions between H- and L-mode depend on the net heat Bow across the separatrix, P-sep, calculated from the hea ting power and bolometric radiation profiles. For typical radiative bo undary conditions in ASDEX Upgrade, the radiation distribution is domi nated 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 sep aratrix. Argon seed results in a higher fraction of core radiation, wh ile the line emission is shifted further outside for nitrogen. The rad iation-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 t ypically 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 tr ansition, a general alignment of H and L mode is observed with regard to global energy confinement time and edge density and temperature pro files. The H --> L transition itself exhibits a smooth evolution in ti me. Reduction of target plate power load down to about 10% of the tota l heating power is easily achieved by edge radiation in the CDH-mode f or low p(thres)(H --> L) conditions. However, this reduction is attrib uted mainly to radiation from inside the separatrix and is connected t o relatively high values of the core Z(eff). These results emphasize t he importance of the development of more closed divertor concepts, all owing for higher divertor radiation levels in connection with lower co re Z(eff).