Wolfgang Ullrich
Heliumtransport in der Plasmarandschicht an ASDEX Upgrade
Betreuer: Prof. Dr. Kurt Behringer [Experimentalphysik IV]
Datum der mündlichen Prüfung: 10.12.1999
90 Seiten, deutsch , ISBN 3-89820-059-0
In fusion plasmas consisting of deuterium and tritium the helium production is directly coupled to the energy production. For the stationary operation of a future fusion reactor, the efficient exhaust of helium ash is of extraordinary importance, to avoid the dilution of the plasma. In ASDEX Upgrade, which is working with pure deuterium plasmas, the helium transport is investigated by short gas puffs, injected during the otherwise steady-state phase of a discharge. For the interpretation of these investigations the understanding of the scrape-off layer physics and processes affecting the helium transport is necessary. One-dimensional models describing the relations between central plasma parameters, scrape-off layer, divertor plasma and neutral gas are discussed. Based on the physics of the deuterium background the one-dimesional force balance of impurity ions, e.g. helium, is deduced. The significance of frictional forces and thermal forces influencing the motion of helium ions is emphasized. The experimental database of ASDEX Upgrade discharges with helium gas puffs shows the main dependencies of the helium compression from discharge parameters, mainly an increase in compression with neutral gas flux density and with power flowing over the separatrix. Further evaluations demonstrate the role of divertor geometry by variing the strike point position and they show a small positive effect of externally induced deuterium flows (so-called "puff and pump" technique). The importance of the mean free path of helium neutrals in the divertor plasma is clearly seen in discharges at the transition to the detachment regime, where the helium compression decreases.