Enhanced room temperature erosion of ultra-thin carbon films on beryllium, titanium and tantalum by deuterium ions

Most present fusion devices use carbon at least partially as a first wall material. Due to high particle loads, especially hydrogen isotopes from the plasma, wall material is eroded and redeposited at different positions. This leads to the formation of surface layers, which are themselves subject to erosion. It is known that carbon exhibits chemically enhanced erosion by low energy hydrogen ions or elevated temperatures. However, room temperature erosion of carbon by keV ions is considered to be governed by a purely kinematic sputtering process. We found that this is not necessarily the case for carbon surface layers with thicknesses of a few nanometers. To investigate the basic mechanisms governing the erosion of carbon, carbon layers in the range of several nm thickness are evaporated on clean metal substrates. The films are irradiated with 1 or 1.3 keV deuterium ions and the surface layer thickness and composition are analysed by X-ray photoelectron spectroscopy (XPS). For our studies we choose carbon films on tantalum, titanium and beryllium to compare the influence of a wide range of substrate atomic masses. Our experimental data is compared to Monte Carlo calculations using the SDTRIM code which takes into account kinematic interactions and adjusts the sample composition dynamically. We discuss the results with respect to the kinematic collision interaction and the ion-induced chemical phase formations.