Erosion of graphite by high flux hydrogen plasma bombardment

Abstract

The total erosion yields of several types of graphites have been measured during high ion flux exposure in the PISCES plasma facility. Hydrogen and deuterium plasmas have been used to bombard Poco, ATJ, pyrolytic, and C-SiC-coated graphites, and a '4-D' C-C composite weave. Erosion experiments with ion fluxes of up to 2 × 10¹⁸ cm⁻² s⁻¹, at ion energies of 50 to 200 eV and sample temperatures of 50 to 950°C, are reported. In the absence of redeposition, the erosion rates of all the graphites tested is essentially the same. The C-C composite weave material showed equal or lower erosion yields compared to the other graphites over the range of temperatures tested. The maximum total carbon erosion yield at 600°C increased by a factor of two for 50 eV ion bombardment and by a factor of five for 200 eV ion bombardment, compared to the reference values found for room temperature samples. At temperatures above 800°C, the chemical erosion is suppressed and the erosion yield reaches values expected for physical sputtering. A direct comparison with two ion beam experiments with an ion flux three orders of magnitude lower than used in PISCES showed total erosion rates within a factor of two, indicating that chemical sputtering is not significantly suppressed by high ion fluxes. The results show that the chemical sputtering of graphite limiters or divertor plates in tokamaks will vary by a factor of only two to three over a limiter temperature range of 0 to 800°C if the incident ion energy is less than 100 eV as is observed in tokamak experiments. The net erosion is also strongly reduced by reionization in the plasma and redeposition of both hydrocarbons and physically sputtered carbon.