Observations on chemical erosion in DIII-D and PISCES

Abstract

The chemical erosion properties of DIII-D tokamak graphite tiles are examined using in-situ hydrocarbon visible spectroscopy, along with knowledge of hydrocarbon dissociation and excitation rates. We previously inferred a long-term (eight years) reduction in divertor chemical sputtering of carbon from divertor surfaces. Ex-situ analysis of sample DIII-D tiles from both the divertor and main wall is also performed. Boron is found to be present on all plasma-facing surfaces with 5–34% surface coverage. Periodic application of 100 nm thick boronization films provides the source of boron, while plasma erosion and transport determine self-consistently the surface composition of boron and carbon. Chemical erosion yield (C/D+) is uniformly ∼1% in regions where the edge plasma is attached (i.e. electron temperature Te > 10 eV), consistent with the yield expected from boron-doped graphite. The long-term four-fold reduction in chemical erosion yield at the outer divertor is consistent with boron-doping effects. Inferred yields in detached (Te < 2 eV) plasmas are much lower (∼0.01%) than the expected yield of boron-doped graphite. However, experimental evidence shows that spectroscopy may be influenced by the dissociation and prompt redeposition of hydrocarbons. DIII-D divertor tile samples are exposed in the PISCES-B linear plasma device. Results indicate that the high net erosion rate in the PISCES device greatly modifies the surface and erosion properties of the plasma-facing surfaces, making quantitative yield assessments problematic.