Plasma wall interaction (PWI) issues move strongly into the focus of magnetic
confinement fusion research, because it can sensitively influence the plasma performance and reactor availability. Low-Z armour materials undergo very large erosion, which at least in the case of carbon can lead to strong tritium co-deposition. High-Z materials are expected to exhibit lowest erosion yields but their high atomic number can give rise to excessive radiation losses in the central plasma and the formation of melt layers during transient heat loads can lead to an unacceptable modification or even destruction of the affected plasma facing component (PFC). The ITER design tries to optimize the performance by using Be, C and W as armour materials at positions where they should perform best.
However, a future reactor cannot rely on low-Z PFCs due to the high erosion and a solution with high-Z armours have to be developed. Therefore the W programme at ASDEX Upgrade was initiated, to investigate the PWI and its implications in an all W divertor tokamak. Starting in 1999 from a full carbon device, ASDEX Upgrade was steadily converted to a full W device in 2007. During this transition, the evolution of the impurity influxes and concentrations was monitored closely. The carbon influxes - even from W surfaces - and its concentrations were only marginally reduced during the first steps and only for a W surface fraction of 80% a reduction in the concentration and deposited C layers was observed. This is attributed to the fact that the eroded carbon can form volatile species allowing it to be strongly recycled from PFCs. Conversely, strong W influxes were only observed from regions with direct contact to a hot plasma or with significant load from fast (non-thermal) particles. The W concentrations in the core plasma are strongly affected by plasma transport and variations up to a factor of 50 can be observed for similar W influxes. The experience gathered so far allowed to develop integrated scenarios, with confinement, edge plasma parameters and central W concentrations (< 1e-5) similar to that which would be required in a reactor. |