Influence of surface roughness on sputtering of Mo under keV D ions irradiation

An important issue in the development of thermonuclear fusion is the lifetime of the reactor walls. Irradiation by ions and neutrals will lead to erosion of the wall materials, contaminating the plasma and reducing the lifetime of plasma facing components. For perfectly smooth surfaces a theoretical model predicts sputter yields with great accuracy [1] showing a distinct angular dependence of the sputter yield [2]. However, when dealing with rough surfaces the experimental results deviate from model predictions and also the angle dependence is less distinct. Within the Eurofusion work package PFC a dedicated task was launched to quantitatively determine the influence of roughness on the sputter yield [3].
To address this issue erosion of molybdenum (Mo) thin films on graphite substrate by deuterium (D) ions was investigated. Such composition of samples was chosen on the basis that same studies ants to be carried out in ASDEX Upgrade. For this purpose we constructed an experimental setup where samples are exposed to D ions generated by an electron cyclotron resonance ion gun. The dominating ion species coming out from the ion gun are D3+ ions, which are accelerated by applying 3.1 kV voltage in this case and decelerated in front of the sample to 3.0 keV, therefore yielding 1 keV energy per D.
120 nm thick Mo films deposited by pulsed laser deposition on graphite substrate of different roughness were produced for this purpose. The surfaces roughness (Ra) ranged from mirror polished surfaces (Ra= 5 nm) to rough surface (Ra= 2-3 µm). Samples were exposed to 1.5×1023 D ions/m2 at different impact angles with respect to the surface (0°, 40°, 60° and 70°). The erosion was determined by measuring Mo areal density before and after the exposure by Rutherford Backscattering Spectroscopy (RBS). The RBS measurements were performed by 2.5 MeV 4He ion beam. In the presented contribution results of the effect of surface roughness on sputter yields and its angular dependence will be discussed in details as we observed clear angular dependence for Ra~5 nm which is smeared out at higher surfaces roughness.