Erosion and co-deposition of thick SiC gradient coatings on doped graphite as limiter materials in the HT-7 device

Carbon-based materials have long been recognized as possessing many properties that make them attractive candidates for applications in experimental fusion devices. In particular, low atomic number (Z), high operating temperatures, superior thermal shock resistance, and potentially high thermal conductivities are all properties which suggest that carbon materials in some form would be an attractive choice for plasma facing components. However, pure carbon materials shows enhanced erosion duo to chemical interaction with hydrogen and oxygen resulting in a relatively high oxygen impurity concentration in the plasma and a large hydrocarbon erosion yield even under low-energy particle impact. The chemical erosion rate could be reduced by dopants, e.g. boron carbide, silicon or titanium. Many efforts have been made to improve the surface characteristics of graphite by use of boron or silicon containing materials which have a strong affinity for elemental oxygen. To meet the requirements of high power and steady state operation, one kind of multi-element doped graphite GBST1308 (1%B4C, 2.5%Si, 7.5%Ti) with thick SiC gradient coatings were chosen as the main PFM in HT-7 device. This moves to the new carbon limiter turned out to be very successful, the hard x-ray and high-Z impurities central radiation problem have been alleviated, and plasma performance has been significantly improved. From the last experimental campaign of year 2003, a new belt limiter plus upper and down toroidal limiter coated SiC with total area of more than 2-3 m2 has been installed in HT-7, which withstood several thousand shots of ohmic and auxiliary heated plasma discharges. It has been found in HT-7 device that the thick SiC gradient coatings on graphite exhibit superior surface characteristics and thermal shock resistance. In the above experiments, the surface of carbon tiles were examined with respect to erosion and co-deposition, changes of surface morphology and the surface atomic composition using SIMS, EDAX, X-ray crystallography, profilometer and Auger spectroscopy. The erosion and co-deposition pattern were carefully investigated, the detail research results will be presented in this paper. The obtained results indicated GBST1308 with high thermal conductivity as substrate will improve the durability of thick SiC gradient coatings significantly. (authors)