An experimental overview on pumping and particle exhaust studies for deuterium as well as for helium and neon in the ASDEX Upgrade divertor tokamak is presented. Strong turbomolecular pumps connected to the divertor region allow effective pumping of all these gases.Deuterium, however, is pumped effectively by the carbon walls in ASDEX Upgrade which dominate over the external pumping.Noble gases are hardly pumped by the walls, and their pumping efficiency and exhaust rate depends strongly on their respective density in the divertor chamber, i.e. on the effectivity of scrape-off layer transport and divertor retention. Compression of helium and neon in the divertor chamber increases with the divertor neutral gas density, and in high-power, high-density discharges in H-or CDH-mode pumping is very efficient. Helium exhaust in these scenarios is sufficiently fast to fulfill the requirements for ITER or a fusion reactor.

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 flow 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 L→H sep = 0.125 n 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 H→L sep ≈ P L→H sep . 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 H→L thres 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 .

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