Single-crystal and polycrystalline diamond erosion studies in Pilot-PSI

Diamond is a promising candidate for enhancing the negative-ion surface production in the ion sources for neutral injection in fusion reactors; hence evaluation of its reactivity towards hydrogen plasma is of high importance. High quality PECVD single crystal and polycrystalline diamond samples were exposed in Pilot-PSI with the D + flux of (4-7)$\times$10 24 m-2 s-1 and the impact energy of 7-9 eV per deuteron at different surface temperatures; under such conditions physical sputtering is negligible, however chemical sputtering is important. Net chemical sputtering yield $Y = 9.7\times 10^{-3}$ at/ion at 800$^\circ$C was precisely measured ex-situ using a protective platinum mask (5x10x2 $\mu$m) deposited beforehand on a single crystal followed by the post-mortem analysis using Transmission Electron Microscopy (TEM). The structural properties of the exposed diamond surface were analyzed by Raman spectroscopy and X-ray Photoelectron Spectroscopy (XPS). Gross chemical sputtering yields were determined in-situ by means of optical emission spectroscopy of the molecular CH AX band for several surface temperatures. We observed a bell shape dependence of the erosion yield versus temperature between 400$^\circ$C and 1200$^\circ$C, with a maximum yield of ~1.5$\times$10-2 at/ion attained at 900$^\circ$C. The yields obtained for diamond are relatively high $(0.51.5)\times 10^{-2}$ at/ion, comparable with those of graphite. XPS analyses show amorphization of diamond surface within 1 nm depth, in good agreement with molecular dynamics (MD) simulation. MD was also applied to study the hydrogen impact energy threshold for erosion of [100] diamond surface at different temperatures.