[en] Tungsten (W) and beryllium (Be) have been chosen as plasma-facing materials for the ITER reactor and the main fuel component will be deuterium (D). Due to plasma–wall interactions, these materials will immediately mix via erosion, transport and re-deposition. We present the first atomistic study on the effect of D co-implanted with Be into W, by modelling D plus Be irradiation of W surfaces, at projectile energies and compositions relevant for the plasma–wall interactions. The D implantation and Be sticking yields increased with the Be fraction in the system, especially at the lowest energies, as a Be layer was deposited on the surface. Tungsten was sputtered by Be, although the yield was partially suppressed by the deposited Be–D layer, and the Be erosion was determined by the balance between the Be concentration at the surface and projectile energy. Molecules were also sputtered: a large fraction of the D is reflected as D₂, and purely metallic molecules (Be₂, BeW) as well as different Be–D compounds were sputtered. On the other hand, the D clustered when implanted in or beneath a pre-deposited Be–W layer. (paper)

[en] In the recent ITER-Like Wall experiment at JET, tungsten (W) and beryllium (Be) are used as the first wall plasma-facing materials. Due to the plasma–wall interactions, these materials will erode, be transported, re-deposit and mix. We present the first computational, atomistic, systematic study on the W–Be material mixing under fusion-relevant conditions. To this end, W surfaces were irradiated by Be, varying the impacting energy and angle, followed by annealing the mixed W–Be layers. At low energies, a Be layer is deposited on W, suppressing the W erosion. The materials mix as the W atoms migrate towards the Be layer due to the heat of mixing. Be₂ and BeW molecules eroded, both physically (dimer sputtering) and chemically (sputter etching). All the mixed layers show an underlying hcp-like Be structure and the Be : W ratios are close to those in the intermetallic phases (Be₂W—Be₁₂W). However, no crystalline alloy structure formed, even after annealing. Further, we present a geometrical model for the angular dependence of the Be reflection, which strongly affects the W sputtering. (paper)

[en] The interaction of fusion reactor plasma with the material of the first wall involves a complex multitude of interlinked physical and chemical effects. Hence, modern theoretical treatment of it relies to a large extent on multiscale modelling, i.e. using different kinds of simulation approaches suitable for different length and time scales in connection with each other. In this review article, we overview briefly the physics and chemistry of plasma–wall interactions in tokamak-like fusion reactors, and present some of the most commonly used material simulation approaches relevant for the topic. We also give summaries of recent multiscale modelling studies of the effects of fusion plasma on the modification of the materials of the first wall, especially on swift chemical sputtering, mixed material formation and hydrogen isotope retention in tungsten. (paper)

[en] When helium (He) escapes a fusion reactor plasma, a tungsten (W)-based divertor may, under some conditions, form a fuzz-like nano-morphology. This is a highly undesired phenomenon for the divertor, and is not well understood. We performed molecular dynamics simulations of high fluence He and also C-seeded He (He+C) irradiation on W, focusing on the effect of the high fluence, the temperature and the impurities on the onset of the structure formation. We concluded that MD reproduces the experimentally found square root of time dependence of the surface growth. The He atomic density decreases when increasing the number of He atoms in the cell. A higher temperature causes a larger bubble growth and desorption activity, specially for the pure He irradiation cases. It also it leads to W recrystallization for the He+C irradiation cases. Carbon acts as a local He trap for small clusters or single atoms and causes a larger loss of crystallinity of the W surface

[en] Iron-based alloys are now being considered as plasma-facing materials for the first wall of future fusion reactors. Therefore, the iron (Fe) and carbon (C) erosion will play a key role in predicting the life-time and viability of reactors with steel walls. In this work, the surface erosion and morphology changes due to deuterium (D) irradiation in pure Fe, Fe with 1% C impurity and the cementite, are studied using molecular dynamics (MD) simulations, varying surface temperature and impact energy. The sputtering yields for both Fe and C were found to increase with incoming energy. In iron carbide, C sputtering was preferential to Fe and the deuterium was mainly trapped as D_2 in bubbles, while mostly atomic D was present in Fe and Fe–1%C. The sputtering yields obtained from MD were compared to SDTrimSP yields. At lower impact energies, the sputtering mechanism was of both physical and chemical origin, while at higher energies (>100 eV) the physical sputtering dominated.

[en] We present an atomistic study on the D irradiation on W–Be mixtures, including a comparison between molecular dynamics (MD) and binary collision approximation methods. We compared the D reflection and Be erosion yields after the non-cumulative D impacts, concluding that both methods agree qualitatively, but low-energy irradiation related chemical effects can be recognized in MD. We also followed the evolution of W–Be mixtures under cumulative D irradiation. At low energies, the surface deuterates, quickly saturating the D reflection and suppressing the Be erosion. (paper)

[en] Beryllium (Be) has been chosen as the plasma-facing material for the main wall of ITER, the next generation fusion reactor. Identifying the key parameters that determine Be erosion under reactor relevant conditions is vital to predict the ITER plasma-facing component lifetime and viability. To date, a certain prediction of Be erosion, focusing on the effect of two such parameters, surface temperature and D surface content, has not been achieved. In this work, we develop the first multi-scale KMC-MD modeling approach for Be to provide a more accurate database for its erosion, as well as investigating parameters that affect erosion. First, we calculate the complex relationship between surface temperature and D concentration precisely by simulating the time evolution of the system using an object kinetic Monte Carlo (OKMC) technique. These simulations provide a D surface concentration profile for any surface temperature and incoming D energy. We then describe how this profile can be implemented as a starting configuration in molecular dynamics (MD) simulations. We finally use MD simulations to investigate the effect of temperature (300–800 K) and impact energy (10–200 eV) on the erosion of Be due to D plasma irradiations. The results reveal a strong dependency of the D surface content on temperature. Increasing the surface temperature leads to a lower D concentration at the surface, because of the tendency of D atoms to avoid being accommodated in a vacancy, and de-trapping from impurity sites diffuse fast toward bulk. At the next step, total and molecular Be erosion yields due to D irradiations are analyzed using MD simulations. The results show a strong dependency of erosion yields on surface temperature and incoming ion energy. The total Be erosion yield increases with temperature for impact energies up to 100 eV. However, increasing temperature and impact energy results in a lower fraction of Be atoms being sputtered as BeD molecules due to the lower D surface concentrations at higher temperatures. These findings correlate well with different experiments performed at JET and PISCES-B devices. (paper)

[en] Penetration of low energy (2–12 keV) hydrogen molecular ions (H₂⁺) and single protons through thin (40 Å) carbon films is simulated using molecular dynamic approach. It is shown that the width of energy loss spectra for the case of H₂⁺ penetration is larger than that for H⁺ spectra (a “molecular effect”) as it was previously observed in experiments . This is explained by the molecular ions dissociation in the first few monolayers of the target. A simple semi-analytical model accounting for the molecular effect is provided. Results of simulations are compared with experiments.

[en] Current and future tokamak-like fusion reactors include the three elements Be, C, and W as the plasma-facing materials. During reactor operation, also mixtures of all these elements will form. Hence it is important to understand the atom-level mechanisms of physical and chemical sputtering in these materials. We have previously shown that athermal low-energy sputtering of pure C and Be can be understood by the swift chemical sputtering mechanism, where an incoming H (or D or T) ion enters between two atoms and pushes them apart. In the current article, we examine the model system of D impacting on a single dimer to determine the detailed mechanism of bond breaking and its probability for the Be₂, C₂, W₂, WC, BeW, and BeC dimers. The results are found to correlate well with recent experiments and simulations of sputtering of the corresponding bulk materials during prolonged H isotope bombardment.

[en] In this manuscript we introduce a simulation tool-suite for predicting plasma-surface interactions (PSI), which aims to predict the evolution of the plasma-facing surfaces that continually change due to exposure to fusion plasmas. A comprehensive description of PSI involves a wide range of physical phenomena, of which we include components for (a) the gas implantation and its dynamic evolution below the divertor surface; (b) erosion of wall material; (c) transport and re-deposition of the eroded impurities; and (d) the scrape-off layer plasma including fuel ions and extrinsic impurities. These components are integrated to predict changes in surface morphology and fuel recycling, and the effect of material erosion and re-deposition in fuel retention. Integrated simulations for ITER-like parameters in a helium plasma environment are presented, focused on the response of the tungsten divertor. The model is also applied to predicting the response of the tungsten surface pre-damaged by He plasma, to burning plasma operations. This case further demonstrates the capability to model the effect of sub-surface helium dynamics, which include helium nucleation, clustering and the bursting of over-pressurized bubbles, its impact on fuel recycling as well as the effect of sputtering on the surface evolution. (topical issue article)