Operational limits during high power long pulses in Tore Supra

The Tore Supra tokamak has a unique long pulse capability in an actively cooled machine, with injected power provided exclusively by radiofrequency (RF) waves. Due to its good diagnostic coverage, in particular the infrared (IR) imaging system, Tore Supra is a highly relevant device for studying issues related to power handling of RF antennas and plasma-wall interaction in true steady state conditions. The last two years experimental campaigns have been particularly intense in terms of injected power, energy and accumulated plasma time (10 hours plasma time in both 2006 and 2007, with 65 GJ injected RF energy in 2006 and 40 GJ in 2007). Large experience has been gained in the understanding of localised heat loads due to RF sheath effects and interaction by fast particles. However, an increasing operational difficulty has emerged, limiting the high power and long pulse performance. The analysis undertaken suggest that this limitation could be linked to the growth and flaking of re-deposited carbon layers on the main plasma facing component. This subject is presented in this paper, together with issues related to RF coupling and antenna heat loads. (author)