Investigation of carbon chemical erosion with increasing plasma flux and density

The possible reduction of the chemical erosion yield of carbon Ychem with increasing hydrogenic ion flux is a critical issue for the use of carbon based plasma facing materials in future high flux confinement devices. The PISCES linear plasma device is used to assess carbon Ychem through nearly an order of magnitude scan in flux (≤ 1023 s-1 m-2) and plasma density, while controlling other exposure parameters that are known to affect Ychem; namely, incident ion energy and surface temperature. Two independent techniques are used to measure Ychem, CD band emission spectroscopy and sample mass loss. Both techniques are carefully benchmarked using a combination of methane injection experiments and modelling. The key results of this study are that the CD molecular band photon efficiency is sensitive to the plasma density at fixed electron temperature and that the redeposition efficiency of hydrocarbons increases with density, thereby decreasing the measured net erosion. After taking these trends into account, no measurable flux dependence of Ychem is found from either of the two measurement techniques, with Ychem ~ 3-5% for an incident deuterium ion energy of 30 eV. Independent scans of incident energy and surface temperature confirm previously measured (by ion beams with relatively low fluxes) Ychem dependences on these parameters. These results together suggest that apparent reductions of Ychem in plasma devices with increasing flux are more likely to be attributed to either increasing redeposition efficiency with density, changing dissociation/excitation/transport properties of the hydrocarbons near the surface, decreasing incident energy or some combination thereof.