Spectroscopic characterization and simulation of chemical sputtering using the DiMES porous plug injector in DIII-D

A self-contained gas injection system for the Divertor Material Evaluation System (DiMES) on DIII-D has been employed for in situ study of chemical erosion in the tokamak divertor environment. The porous plug injector (PPI) releases methane into the plasma at a controlled rate through a porous graphite surface flush to a tile. In this way, the perturbation to the local plasma can be minimized, while also simulating the immediate environment of methane molecules released from a solid graphite surface. Photon efficiencies of CH4 for measured local plasma conditions are reported. The contribution of chemical vs physical sputtering to the source of C+ at the target can, in principle, be assessed through measurement of CII and CD/CH band emissions during release of CH4 from the PPI, and due to intrinsic emission. These first results from this new experimental tool demonstrate the potential for the PPI to provide definitive results in future applications in DIII-D and indicate the improvements required to obtain firm quantitative conclusions.