Surface analysis of CVD diamond exposed to fusion plasma

Microcrystalline undoped and heavily boron-doped polycrystalline diamond layers have been deposited on various substrates by hot filament CVD and exposed to hydrogen plasma in a linear plasma reactor (Pilot-PSI, The Netherlands) that simulates the high flux and high density plasma conditions of tokamak divertors, as well as in the DIII-D tokamak (US). Pre- and post-exposure analysis by SEM and Raman spectroscopy characterised the surface appearance and the sp3 and sp2 components of the diamond films respectively. Surface roughness variation was measured by AFM. Hardness and Young's modulus were assessed by nanoindentation in order to characterise the effect of the plasma on the mechanical properties. HRTEM and EELS have been used to evaluate the nature of the modification induced at the diamond surface by the plasma exposure. The measurements have shown that, despite some surface amorphisation of the exposed layers, further long-term exposure studies are warranted as neither delamination, dramatic film failure nor entire erosion of the film were observed.