Erosion and Redeposition of Graphite by Hydrogen Plasmas

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Abstract
The results of erosion and redeposition studies of graphite by hydrogen plasma bombardment in the PISCES facility are reviewed. The total erosion yields of several types of graphites have been measured during plasma exposure with ion fluxes of up to 2 × 1018 cm−2·s−1, ion energies of 50 to 200 eV, and sample temperatures of 50 to 950°C. Hydrogen and deuterium plasmas have been used to bombard Poco, ATJ, and pyrolytic graphites, and a “four-directional” carbon-carbon (C-C) composite weave. The erosion rates of all the graphites tested are about equal, suggesting that surface damage by the ion bombardment results in similar erosion yields. The C-C composite weave material showed an increased weight loss during initial exposure, and then equal or lower erosion yields compared to the other graphites. Graphite has a strong ion energy dependence in the maximum chemical erosion yield at a temperature of 500 to 600°C and no energy dependence for the erosion at room temperature. At temperatures above 800°C, the chemical erosion is suppressed and the erosion yield reaches values expected for physical sputtering with thresholds of ∼40 eV for both hydrogen and deuterium. The measured erosion rates demonstrate that chemical sputtering is not significantly suppressed by high-ion fluxes. The net erosion is also reduced by reionization in the plasma and redeposition of hydrocarbons and physically sputtered carbon.

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