Experimental investigation of sputtering, onset of sublimation, and spallation threshold in carbonaceous materials*

A Frontier Science experiment was performed at the DIII-D tokamak to investigate heating conditions leading to sputtering, the onset of sublimation, and the spallation threshold for carbonaceous materials relevant to spacecraft heat shields and fusion plasma-facing components. Sputtering was assessed from spectroscopic data in experiments where graphite rods ~6 mm in diameter protruding ~1.5 cm above the DIII-D lower divertor were exposed using the Divertor Material Evaluation System (DiMES). When the heat flux to the plasma-facing rod surfaces was increased to ~30-40 MW/m2, the onset of fast sublimation (the solid-vapor phase transition) was determined after ~1.5 s of exposure from the appearance of Swan Bands in the carbon spectra, which indicates that high fluxes of C2 molecules are ejected from the surface. After ~5.2 s of plasma exposure, the rods were retrieved and the total mass lost was determined, corresponding to loss rates between 0.01-0.03 g/cm2/s, which will be compared to several ablation models. Finally, the spallation threshold (onset of surface fragmentation/degradation) was investigated using experiment video data where µm-sized glassy carbon pellets were injected in the core plasma, where heat fluxes are expected to exceed 500 MW/m2.