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

The chemical sputtering yields of CH 4 /CD 4 and C 2 H x /C 2 D x have been measured at the divertor plates of JT-60U. Spectroscopic measurements for CH/CD and C 2 spectral bands are applied to estimate the CH 4 /CD 4 and the C 2 H x /C 2 D x flux. At the surface temperatures of 380, 440 and 560 K, the CH 4 yield is, respectively, ~0.8%, 1-2% and 2-3%, the C 2 H x 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 (Γ ion ), the sputtering yields ( Y ) decrease, i.e. Y ∝Γ ion (-0.05 to -0.40) . With increasing electron temperature ( T e ), the sputtering yields increase, i.e. Y ∝ T e 0.5 . It is concluded from the result of regression analysis of Y ∝ T e 0.5 that the negative dependence of the yields on the ion flux is attributed to the incident ion energies to the carbon plates. The ratio of the sputtering yields by deuterium ions and hydrogen ions is estimated to be ⩾1.5 based on the ion flux measurement by H α /D α intensity. The C 2 H x /C 2 D x sputtering yield accounts for ~80% of the total number of sputtered carbon atoms.