Fusion Science and Technology / Volume 56 / Number 3 / October 2009 / Pages 1334-1352
Technical Papers / Tore Supra Special Issue
Fuel retention in carbon plasma-facing components (PFCs) is such a major concern for next-step operation that it could prevent the use of this material in the D-T phase of ITER. Because of its complete set of actively cooled PFCs, Tore Supra offers a unique opportunity to study this phenomenon in conditions where the plasma exposure time is much longer than the thermal equilibration time of the PFCs. In addition to the main characteristics of permanent retention measured during long-discharge operation, this paper discusses the different mechanisms possibly at work in the continuous increase of the in-vessel inventory and describes the morphology and physical properties of the deposits found at several locations in the vacuum chamber. The main results are (1) that D retention mainly depends on the lower hybrid power coupled to the plasma and, to a lesser extent, on the edge temperature and fueling method, (2) that permanent D retention is mainly due to codeposition, and (3) that the hydrogenated carbon deposits present at the surface of the different PFCs are strongly disorganized graphite carbons when they are exposed to high heat fluxes, whose formation occurs through a heterogeneous growth involving both codeposition of nanoparticles and basic structural unit vapor condensates.