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Summary of the SEWG meeting, Aalto, 19-20 May 2011

The week of ITPA meeting was complemented by a 1.5-day Plasma Wall Interaction (PWI) Task Force meeting on material migration and mixed materials in ITER. Short presentations on 2011 priority tasks were followed by a lively discussion. Similar meetings will also be held by the other Special Expert Working Groups of PWI during the coming summer months, with the aim of identifying priorities for the 2012 PWI work programme; after the meetings, the work will be continued on the Wiki pages of EFDA.


In the mixed material front, probably the largest step forward was the inclusion of nitrogen and oxygen in the drama triangle of beryllium, carbon and tungsten when studying the formation and properties of mixed materials. Oxygen is an intrinsic impurity that tends to penetrate everywhere, while nitrogen will be used in ITER on purpose to improve radiation properties of the edge plasma for better handling of power loads. General consensus was that there is not enough data available on alloys and compounds that will be formed in the presence of these elements. Experiments in Pisces and Pilot-PSI, e.g., using nitrogen-seeded plasmas will be continued – supported by simulations using molecular dynamics, density functional theory, and the WallDyn method.

All this culminates in the question of sufficiently large lifetime of beryllium components in the first wall of ITER, particularly when beryllium has formed mixed materials. In addition to the well-known problem of tritium retention, restricted lifetime might be another potential showstopper in ITER. Erosion and migration of materials had been studied both numerically and experimentally using tracer injections in the ASDEX Upgrade (AUG) and TEXTOR tokamaks. The less common stable isotope of natural carbon,13C, is the main workhorse for these investigations, but also here nitrogen is gaining more and more interest. During nitrogen seeding, a 15N tracer injection was already carried out in TEXTOR, inspiring planning such an experiment also in AUG.

Antti Hakola & Markus Airila