Measurement of the chemical sputtering yields of CH4/CD4 and C2Hx/C2Dx at the carbon divertor plates of JT-60U

Abstract

The chemical sputtering yields of CH4/CD4 and C2Hx/C2Dx have been measured at the divertor plates of JT-60U. Spectroscopic measurements for CH/CD and C2 spectral bands are applied to estimate the CH4/CD4 and the C2Hx/C2Dx flux. At the surface temperatures of 380, 440 and 560 K, the CH4 yield is, respectively, ~0.8%, 1-2% and 2-3%, the C2Hx yield 1-2%, 3-4% and 4-5%, and the total sputtering yield by hydrogen ions 3-4%, ~8% and ~10%. With increasing ion flux to the divertor plates (Γion), the sputtering yields (Y) decrease, i.e. Y∝Γion(-0.05 to -0.40). With increasing electrontemperature (Te), the sputtering yields increase, i.e. Y∝Te0.5. It is concluded from the result of regression analysis of Y∝Te0.5 that the negative dependence of the yields on the ion flux is attributedto the incident ion energies to the carbon plates. The ratio of thesputtering yields by deuterium ions and hydrogen ions is estimated to be⩾1.5 based on the ion flux measurement by Hα/Dα intensity. The C2Hx/C2Dxsputtering yield accounts for ~80% of the total number ofsputtered carbon atoms.