Impact of surface enrichment and morphology on sputtering of EUROFER by deuterium

Abstract

• A fluence-dependent reduction of the sputter yield was observed when bombarding EUROFER steel with 200 eV D ions. • The sputter yield was reduced to 29% ± 5% of the initial value at a fluence of 10 24 D/m 2 . • The impact on the sputter yield of both W surface enrichment and surface morphology were investigated with the aid of the SDTrimSP and SDTrimSP-3D codes. • The combined effect of W surface enrichment and surface morphology is an estimated reduction of the sputter yield of 27% ± 4%, in agreement with the measured values and data from literature. • The heterogeneous formation of a spiked surface morphology was investigated with the aid of dynamic 3-D simulations with SDTrimSP-3D, finding qualitative agreement with the experimental results. EUROFER is a Reduced Activation Ferritic Martensitic (RAFM) steel developed as structural material for future fusion power plants and is considered as first-wall material in recessed areas of DEMO. Recent work has shown a fluence-dependent decrease of the sputter yield for bombardment with deuterium ions in the energy range of 100 to about 500 eV. This decrease was previously attributed to preferential sputtering of the lower mass constituents in EUROFER, such as Fe and Cr, compared to the higher mass alloying elements. This leads to an increase of the surface tungsten concentration. However, it was also observed that, after sputtering, the samples had developed a very rough surface morphology. In this work, the combined influence of surface roughness and W surface enrichment on the sputter yield of EUROFER under bombardment with 200 eV deuterium ions was studied. The influence of surface roughness was determined with the aid of Scanning Electron Microscopy (SEM) and SDTrimSP-3D simulations. W surface enrichment was investigated applying sputter X-Ray Photoelectron Spectroscopy (XPS) depth-profiling and Rutherford backscattering spectrometry (RBS). After bombardment to a fluence of 10 24 D m −2 (at 200 eV per deuterium) a reduction of the sputter yield to 29% ± 5% of the initial value was measured by weight-loss measurements. This reduction is in good agreement with published values. Two distinct surface morphologies, consisting of smooth and spiked surfaces, were observed on the EUROFER sample after sputtering. Based on the experimental results, the combined effect of the two factors, surface roughness and W surface enrichment, is estimated to be responsible for a reduction in the sputter yield to 27% ± 4% of the initial value, which is in excellent agreement with the measured value. Our assessment shows that both surface morphology and W surface enrichment contribute significantly to the reduction of the sputter yield of EUROFER under the given experimental conditions, and are sufficient to fully explain the experimentally observed reduction in the sputter yield.