EFDA-JET-CP(04)07/37

Overview of Recent Work on Carbon Erosion, Migration and Long-Term Fuel Retention in the EU-Fusion Programme and Conclusions for ITER

Recent results on carbon erosion, short and long range carbon transport and fuel retention are summarised. Existing data on carbon chemical erosion from erosion dominated areas in fusion devices and beam experiments show a consistent dependence on impact energy, target temperature and flux, indicating low carbon chemical erosion in ITER at the strike zones. The campaign averaged overall carbon deposition rates in the all carbon devices JET and DIII-D and in AUG, which is covered to a large extent with W in the main chamber, show remarkable similarities with values of 3-7x1020 C/s. While the inner divertor is deposition dominated in general, no uniform characteristic of the erosion/deposition behaviour of the outer divertor can be stated for AUG and JET. The majority of carbon species have a high sticking probability, leading to line of sight deposition with only a minority of the carbon with low sticking (<1%) that can travel long distances in regions shadowed from plasma. Deposition on the Quartz Micro Balance (QMB) occurs only for discharges with the strike point in the vicinity of the QMB location. Massive deposition (10nm/s) can occur in discharges, where the strike point was freshly moved to the vicinity of the QMB, accompanied by the simultaneous appearance of strong C2 band emission in these shots. Be is deposited in JET mainly at the location of primary deposition at the upper part of the inner vertical tile. The deposited layer is Be -rich (Be/C 2/1) with a significant D inventory although these layers are heated regularly to temperatures above 1200oC, indicating the formation of a stable Be -C compound that is able to retain hydrogen up to high temperatures.
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