[en] In this thesis, the behaviour of a material situated in a fusion reactor was studied using molecular dynamics simulations. Simulations of processes in the next generation fusion reactor ITER include the reactor materials beryllium, carbon and tungsten as well as the plasma hydrogen isotopes. This means that interaction models, i.e. interatomic potentials, for this complicated quaternary system are needed. The task of finding such potentials is nonetheless nearly at its end, since models for the beryllium-carbon-hydrogen interactions were constructed in this thesis and as a continuation of that work, a beryllium-tungsten model is under development. These potentials are combinable with the earlier tungsten-carbon-hydrogen ones. The potentials were used to explain the chemical sputtering of beryllium due to deuterium plasma exposure. During experiments, a large fraction of the sputtered beryllium atoms were observed to be released as BeD molecules, and the simulations identified the swift chemical sputtering mechanism, previously not believed to be important in metals, as the underlying mechanism. Radiation damage in the reactor structural materials vanadium, iron and iron chromium, as well as in the wall material tungsten and the mixed alloy tungsten carbide, was also studied in this thesis. Interatomic potentials for vanadium, tungsten and iron were modified to be better suited for simulating collision cascades that are formed during particle irradiation, and the potential features affecting the resulting primary damage were identified. Including the often neglected electronic effects in the simulations was also shown to have an impact on the damage. With proper tuning of the electronphonon interaction strength, experimentally measured quantities related to ion-beam mixing in iron could be reproduced. The damage in tungsten carbide alloys showed elemental asymmetry, as the major part of the damage consisted of carbon defects. On the other hand, modelling the damage in the iron chromium alloy, essentially representing steel, showed that small additions of chromium do not noticeably affect the primary damage in iron. Since a complete assessment of the response of a material in a future full-scale fusion reactor is not achievable using only experimental techniques, molecular dynamics simulations are of vital help. This thesis has not only provided insight into complicated reactor processes and improved current methods, but also offered tools for further simulations. It is therefore an important step towards making fusion energy more than a future goal. (orig.)

[en] We study the initial state of irradiation damage in WC, an alloy with a large mass difference between the constituents, using molecular dynamics computer simulations. We find that a vast majority of the resulting isolated defects are carbon. Moreover, an in-cascade defect recombination effect similar to that in metals is observed. Both effects are shown to be related to the high formation energy of W defects

[en] Understanding of sputtering by ion bombardment is needed in a wide range of applications. In fusion reactors, ion impacts originating from a hydrogen-isotope-rich plasma will lead, among other effects, to sputtering of the wall material. To study the effect of plasma impurities on the sputtering of the wall mixed material tungsten carbide molecular dynamics simulations were carried out. Simulations of cumulative D cobombardment with C, W, He, Ne or Ar impurities on crystalline tungsten carbide were performed in the energy range 100-300 eV. The sputtering yields obtained at low fluences were compared to steady state SDTrimSP yields. During bombardment single C atom sputtering was preferentially observed. We also detected significant W_xC_y molecule sputtering. We found that this molecule sputtering mechanism is of physical origin.

[en] Considerable quantitative uncertainty has remained regarding the amount and structure of defects produced in molecular dynamics simulations of collision cascades in Fe. The problem is most likely related to the description of interstitial energetics in the interatomic potentials. Three potentials have recently been developed for Fe, which, even though they have different physical motivations and functional forms, describe the interstitial energetics well. Using these potentials, we simulate recoil collision cascades in Fe in the recoil energy range 0.5-20 keV. Prior to the cascade simulations a realistic high-energy repulsive part was added to two of the potentials, adjusting the fit to reproduce the experimentally obtained threshold displacement energies. The results show that the total Frenkel pair production, as predicted by the three potentials, is the same within the statistical uncertainty, but also that some differences remain in the fraction of clustered defects. However, these differences are smaller than those predicted by previous potentials

[en] A Monte-Carlo code called SURO has been developed to study the influence of surface roughness on the impurity deposition characteristic in fusion experiments. SURO uses the test particle approach to describe the impact of background plasma and the deposition of impurity particles on a sinusoidal surface. The local impact angle and dynamic change of surface roughness as well as surface concentrations of different species due to erosion and deposition are taken into account. Coupled with 3D Monte-Carlo code ERO, SURO was used to study the impact of surface roughness on "1"3C deposition in "1"3CH_4 injection experiments in TEXTOR. The simulations showed that the amount of net deposited "1"3C species increases with surface roughness. Parameter studies with varying "1"2C and "1"3C fluxes were performed to gain insight into impurity deposition characteristic on the rough surface. Calculations of the exposure time needed for surface smoothing for TEXTOR and ITER were also carried out for different scenarios. (author)

[en] As a step in the process of assessing the reliability of interatomic potentials for iron, we compare experimental measurements of ion beam mixing with values obtained from molecular dynamics simulations. We include the electron-phonon coupling (EPC) model by Hou et al. [Q. Hou, M. Hou, L. Bardotti, B. Prevel, P. Melinon, A. Perez, Phys. Rev. B 62 (2000) 2825] in the simulations and consider a range of coupling strenghts. Three different iron interatomic potentials are used. We discuss the effect of the coupling on the primary damage and how the damage is influenced by different velocity minima for applying electron stopping.

[en] While covalently bonded materials such as carbon are well known to be eroded by chemical sputtering when exposed to plasmas or low-energy ion irradiation, pure metals have been believed to sputter only physically. The erosion of Be when subject to D bombardment was in this work measured at the PISCES-B facility and modelled with molecular dynamics simulations. During the experiments, a chemical effect was observed, since a fraction of the eroded Be was in the form of BeD molecules. This fraction decreased with increasing ion energy. The same trend was seen in the simulations and was explained by the swift chemical sputtering mechanism, showing that pure metals can, indeed, be sputtered chemically. D ions of only 7 eV can erode Be through this mechanism.

[en] One of the objectives of the International Thermonuclear Experimental Reactor (ITER) is to demonstrate prolonged fusion power production in deuterium-tritium plasma. The selection of plasma facing materials (PFMs) is a key issue for this objective, and multiple factors have to be taken into account. These include the lifetime of the materials (shortened by e.g. erosion and thermal fatigue), safety requirements (tritium retention and activation) and engineering aspects. Due to the ITER tokamak plasma design, the thermal load and particle flux are divided between different areas in the reactor. Consequently, the material requirements vary with location and the current choice for first wall material is beryllium and the divertor region is to be composed of carbon-fibre-composites (CFC) (strike point tiles) and tungsten (baffle and dome). When energetic atoms or ions escape from the hot plasma in a fusion reactor and hit a wall material, they can cause the material to erode. The erosion is well understood if the ion energy is high enough that the erosion is caused by physical sputtering, i.e. when the ion collides with a sample atom and directly kicks it out of the sample. Alternatively ions with thermal energies can also cause erosion if a chemical etching reaction can take place. In the particular case of hydrogen escaping from a fusion reactor plasma and hitting a carbon-based wall material, high carbon erosion has been observed for hydrogen ion energies which are so low (10-30 eV) that physical sputtering is impossible. On the other hand, no chemical etching reaction has been able to explain the erosion either. Using classical and quantum mechanical atomistic simulations of the ion-sample collision dynamics, we have shown that the observed erosion can be explained by a chemical sputtering mechanism, where the incoming ion attacks a chemical bond between two carbon atoms, and causes the bond to break. This mechanism requires an ion energy of only about 3 eV, and occurs on femtosecond time scales, whence we call it swift chemical sputtering. Sputtering and redeposition will unavoidably lead to formation of surfaces which are made up of a mixture of the three PFMs. The properties of these mixed materials can differ strongly from those of their constituents, hence, an understanding of their effects is considered to be crucial for the reactor operation. The understanding of the retention and recycling of hydrogen isotopes in Be is also of particular interest, since this will give insight into the undesired trapping of tritium and also into the plasma cooling effect by the release of the isotopes. When exploring the consequences of mixed materials and the influence of H isotopes on PFMs, realistic experiments with ITER relevant conditions are preferred, but not yet feasible. An excellent tool is, however, computer simulations and especially the sub-part of this field, the simulation technique based on Molecular Dynamics (MD), which allows for modelling of tens of millions atoms. The reliability of MD is proportional to the accuracy of the interatomic potentials used in the simulations, hence, great efforts must be made in developing proper potentials. We have developed interatomic potentials to be used in MD simulations to study the interplay between the fusion reactor materials beryllium, carbon and hydrogen. These potentials are so called analytical bond-order potentials (ABOP), which were initially developed by Tersoff (1988) to describe covalent solids and extended to metals by Brenner (1990). They are able to describe variations of the local chemical environment, such as bond-breaking, yet at the same time they are computationally efficient. In this contribution, we will present the swift chemical sputtering and show how this mechanism can explain many of the observed features of carbon erosion in fusion reactors. We will also present the development and performance of the recent Be-C-H potentials. (au)

[en] The Embedded Atom Model (EAM) Derlet-Nguyen-Manh-Dudarev tungsten and vanadium potentials were modified to correctly reproduce the experimentally obtained defect threshold energies. This was done by letting the interactions at short distances be dictated by the universal screened Coulomb potential. Both the repulsive part and the electron density function of the potentials were modified. The potentials were then used in collision cascade simulations and the resulting defects were compared with the corresponding defects in iron. Based on this comparison, factors affecting the outcome of a cascade were identified.

[en] Ion irradiation causes damage in semiconductor crystal structures and affects charge carrier dynamics. We have studied the damage production by high-energy (100 keV-10 MeV) H, He, Ne, and Ni ions in GaAs and GaAs₉₀N₁₀ using molecular dynamics computer simulations. We find that the heavier Ne and Ni ions produce a larger fraction of damage in large clusters than H and He. These large clusters are either in the form of amorphous zones or (after room-temperature aging or high-temperature annealing) in the form of vacancy and antisite clusters. The total damage production in GaAs and GaAs₉₀N₁₀ is found to be practically the same for all the ions. A clearly smaller fraction of the damage in GaAs₉₀N₁₀ compared to GaAs is in large clusters, however. Our results indicate that experimentally observed differences in charge carrier lifetimes between light and heavy ion irradiations, and before and after annealing, can be understood in terms of the large defect clusters. An increasing amount of damage in large clusters decreases the carrier decay time