Investigation of carbon chemical erosion with increasing plasma flux and density

The possible reduction of the chemical erosion yield of carbon Ychem withincreasing hydrogenic ion flux is a critical issue for the use ofcarbon based plasma facing materials in future high flux confinementdevices. The PISCES linear plasma device is used to assess carbonYchem through nearly an order of magnitude scan in flux (⩽ 1023 s-1 m-2) and plasma density, while controlling other exposureparameters that are known to affect Ychem; namely, incident ion energyand surface temperature. Two independent techniques are used tomeasure Ychem, CD band emission spectroscopy and sample massloss. Both techniques are carefully benchmarked using a combination ofmethane injection experiments and modelling. The key results of thisstudy are that the CD molecular band photon efficiency is sensitive tothe plasma density at fixed electron temperature and that theredeposition efficiency of hydrocarbons increases with density,thereby decreasing the measured net erosion. After taking these trendsinto account, no measurable flux dependence of Ychem is found fromeither of the two measurement techniques, with Ychem ∼ 3-5% for anincident deuterium ion energy of 30 eV. Independent scans of incidentenergy and surface temperature confirm previously measured (by ionbeams with relatively low fluxes) Ychem dependences on theseparameters. These results together suggest that apparent reductions ofYchem in plasma devices with increasing flux are more likely to beattributed to either increasing redeposition efficiency with density,changing dissociation/excitation/transport properties of thehydrocarbons near the surface, decreasing incident energy or somecombination thereof.