D and D/He plasma interactions with diamond: Surface modification and D retention

Chemical vapour deposition (CVD) diamond is of interest as a plasma facing material in magnetically confined nuclear fusion devices. CVD diamond samples were exposed to ion fluences between 1.5 × 10²²ions m^{- 2}and 1 × 10²⁴ions m^{- 2}in D, D/He and He plasmas. A significant surface coverage of 10 nm-100 nm sized hemispherical and conical surface features resulted from chemical erosion and subsequent redeposition on samples exposed to ≤10²³ions m^{- 2}when exposed to D containing plasmas. Near edge X-ray absorption fine structure (NEXAFS) spectra showed strong diamond features following plasma exposure, indicating that the diamond crystal structure on the surface was largely retained. A small increase in the sp²fraction (from 1.2% to 4-7%), was observed suggesting some amorphous carbon formation. Elastic recoil detection analysis (ERDA) revealed that D retention saturates at ~ 5.4% averaged over the depth resolution of 20 nm from the surface, for an ion fluence of ~ 10²³ions m^{- 2}. The addition of He to the plasma resulted in a slower uptake of D, which was attributed to He ion induced desorption of D.