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Now showing items 49 - 64 of 90

  • Dynamic Monte-Carlo modeling of hydrogen isotope reactive-diffusive transport in porous graphite

    Schneider, R.   Rai, A.   Mutzke, A.   Warrier, M.   Salonen, E.   Nordlund, K.  

    An equal mixture of deuterium and tritium will be the fuel used in a fusion reactor. it is important to study the recycling and mixing of these hydrogen isotopes in graphite from several points of view: (i) impact on the ratio of deuterium to tritium in a reactor, (ii) continued use of graphite as a first wall and divertor material, and (iii) reaction with carbon atoms and the transport of hydrocarbons will provide insight into chemical erosion. Dynamic Monte-Carlo techniques are used to study the reactive-diffusive transport of hydrogen isotopes and interstitial carbon atoms in a 3-D porous graphite structure irradiated with hydrogen and deuterium and is compared with published experimental results for hydrogen re-emission and isotope exchange. (c) 2007 Published by Elsevier B.V.
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  • Multiscale modeling of BeD release and transport in PISCES-B

    Bjorkas, C.   Borodin, D.   Kirschner, A.   Janev, R. K.   Nishijima, D.   Doerner, R.   Nordlund, K.  

    Observation of BeD sputtering in several different fusion plasma devices demonstrates the need to include these molecules into predictive modeling codes. In this work, Molecular Dynamics simulations are used to obtain information about sputtered BeD molecules and the results are implemented into the plasma impurity modeling code ERO. Rates for dissociation and ionization of BeD molecules in the plasma are also estimated. The modeling results are compared with PISCES-B experimental spectroscopic observations, validating the used models as a good agreement on the shape of the light emission profiles is found. The amount of released BeD is, however, somewhat overestimated. (C) 2013 Elsevier B.V. All rights reserved.
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  • Effects of defect clustering on optical properties of GaN by single and molecular ion irradiation

    Ullah, M. W.   Kuronen, A.   Nordlund, K.   Djurabekova, F.   Karaseov, P. A.   Karabeshkin, K. V.   Titov, A. I.  

    The effects of irradiation by F, P, and PF4 on optical properties of GaN were studied experimentally and by atomistic simulations. Additionally, the effect of Ag was studied by simulation. The irradiation energy was 0.6 keV/amu for all projectiles. The measured photoluminescence (PL) decay time was found to be decreasing faster when irradiation was done by molecular ion compared to light ion irradiation. The PL decay time change is connected with the types of defect produced by different projectiles. Simulation results show that the light ions mainly produce isolated point defects while molecular and heavy ions produce clusters of point defects. The total amount of defects produced by the PF4 projectile was found to be very close to the sum of all defects produced in five individual cascades started by one P and four F single ions. This and the similar depth profiles of damage produced by molecular and light ion irradiations suggest that the defect clusters are one of the important reasons for fast PL decay. Moreover, the simulations of irradiation by Ag ions, whose mass is close to the mass of the PF4 molecule, showed that the produced defects are clustering in even bigger conglomerates compared to PF4 case. The latter has a tendency to split in the pre-surface region, reducing on average the density of the collision cascade. (C) 2013 AIP Publishing LLC.
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  • MD and SCA simulations of He and H bombardment of fuzz in bcc elements

    Klaver, T. P. C.   Zhang, S.   Nordlund, K.  

    We present results of MD simulations of low energy He ion bombardment of low density fuzz in bcc elements. He ions can penetrate several micrometers into sparse fuzz, which allows for a sufficient He flux through it to grow the fuzz further. He kinetic energy falls off exponentially with penetration depth. A BCA code was used to carry out the same ion bombardment on the same fuzz structures as in MD simulations, but with simpler, 10 million times faster calculations. Despite the poor theoretical basis of the BCA at low ion energies, and the use of somewhat different potentials in MD and BCA calculations, the ion penetration depths predicted by BCA are only similar to 12% less than those predicted by MD. The MD-BCA differences are highly systematic and trends in the results of the two methods are very similar. We have carried out more than 200 BCA calculation runs of ion bombardment of fuzz, in which parameters in the ion bombardment process were varied. For most parameters, the results show that the ion bombardment process is quite generic. The ion species (He or H), ion mass, fuzz element (W, Ta, Mo, Fe) and fuzz element lattice parameter turned out to have a modest influence on ion penetration depths at most. An off-normal angle of incidence strongly reduces the ion penetration depth. Increasing the ion energy increases the ion penetration, but the rate by which ion energy drops off at high ion energies follows the same exponential pattern as at lower energies. (C) 2017 Elsevier B.V. All rights reserved.
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  • Subcascade formation and defect cluster size scaling in high-energy collision events in metals

    De Backer, A.   Sand, A. E.   Nordlund, K.   Luneville, L.   Simeone, D.   Dudarev, S. L.  

    It has been recently established that the size of the defects created under ion irradiation follows a scaling law (Sand A. E. et al., EPL, 103 (2013) 46003; Yi X. et al., EPL, 110 (2015) 36001). A critical constraint associated with its application to phenomena occurring over a broad range of irradiation conditions is the limitation on the energy of incident particles. Incident neutrons or ions, with energies exceeding a certain energy threshold, produce a complex hierarchy of collision subcascade events, which impedes the use of the defect cluster size scaling law derived for an individual low-energy cascade. By analyzing the statistics of subcascade sizes and energies, we show that defect clustering above threshold energies can be described by a product of two scaling laws, one for the sizes of subcascades and the other for the sizes of defect clusters formed in subcascades. The statistics of subcascade sizes exhibits a transition at a threshold energy, where the subcascade morphology changes from a single domain below the energy threshold, to several or many sub-domains above the threshold. The number of sub-domains then increases in proportion to the primary knock-on atom energy. The model has been validated against direct molecular-dynamics simulations and applied to W, Fe, Be, Zr and sixteen other metals, enabling the prediction of full statistics of defect cluster sizes with no limitation on the energy of cascade events. We find that populations of defect clusters produced by the fragmented high-energy cascades are dominated by individual Frenkel pairs and relatively small defect clusters, whereas the lower-energy non-fragmented cascades produce a greater proportion of large defect clusters. Copyright (C) EPLA, 2016
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  • Production of defects in hexagonal boron nitride monolayer under ion irradiation

    Lehtinen, O.   Dumur, E.   Kotakoski, J.   Krasheninnikov, A. V.   Nordlund, K.   Keinonen, J.  

    Atomistic computer simulations based on analytical potentials are employed to investigate the response of a hexagonal boron nitride monolayer to irradiation with noble gas ions having energies from 35 eV up to 10 MeV. Probabilities for creating different types of defects are calculated as functions of ion energy and incidence angle, along with sputtering yields of boron and nitrogen atoms. The presented results can be used for the optimization of ion processing of single-layer and bulk hexagonal boron nitride samples and for predicting the evolution of the material in radiation hostile environments. (C) 2010 Elsevier B.V. All rights reserved.
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  • Cooperative effect of electronic and nuclear stopping on ion irradiation damage in silica

    Backman, M.   Djurabekova, F.   Pakarinen, O. H.   Nordlund, K.   Zhang, Y.   Toulemonde, M.   Weber, W. J.  

    Radiation damage by ions is conventionally believed to be produced either by displacement cascades or electronic energy deposition acting separately. There is, however, a range of ion energies where both processes are significant and can contribute to irradiation damage. The combination of two computational methods, namely binary collision approximation and molecular dynamics, the latter with input from the inelastic thermal spike model, makes it possible to examine the simultaneous contribution of both energy deposition mechanisms on the structural damage in the irradiated structure. We study the effect in amorphous SiO2 irradiated by Au ions with energies ranging between 0.6 and 76.5 MeV. We find that in the intermediate energy regime, the local heating due to electronic excitations gives a significant contribution to the displacement cascade damage.
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  • Light and heavy ion effects on damage clustering in GaAs quantum wells

    Bjorkas, C.   Nordlund, K.   Arstila, K.   Keinonen, J.   Dhaka, V.D.S.   Pessa, M.  

    Recent experiments have shown that ion irradiation can strongly affect charge carrier dynamics in GaAs quantum wells. The irradiation conditions are such that the ions penetrate deep beyond the active layer, showing that the effect is due to damage in the lattice. Moreover, ions of different mass and energy can lead to clearly different effects even after normalization with the nuclear deposited energy. Using molecular dynamics simulation of the damage production, we show that the experimentally observed effects on charge carrier lifetime correlate well with the production of large (more than 100 disordered atoms) damage clusters. Moreover, we show that 400keV Ne and 10MeV Ni produce very similar damage in the near-surface active regions, indicating that 500kV ion implanters are sufficient to achieve the desired modification effects. [All rights reserved Elsevier].
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  • Interatomic potentials for the Be-C-H system RID B-1414-2009

    Bjorkas, C.   Juslin, N.   Timko, H.   Vortler, K.   Nordlund, K.   Henriksson, K.   Erhart, P.  

    Analytical bond-order potentials for beryllium, beryllium carbide and beryllium hydride are presented. The reactive nature of the formalism makes the potentials suitable for simulations of non-equilibrium processes such as plasma-wall interactions in fusion reactors. The Be and Be-C potentials were fitted to ab initio calculations as well as to experimental data of several different atomic configurations and Be-H molecule and defect data were used in determining the Be-H parameter set. Among other tests, sputtering, melting and quenching simulations were performed in order to check the transferability of the potentials. The antifluorite Be(2)C structure is well described by the Be-C potential and the hydrocarbon interactions are modelled by the established Brenner potentials.
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  • Atomistic simulations of irradiation effects in carbon nanotubes: an overview RID A-3473-2009

    Kotakoski, J.   Krasheninnikov, A. V.   Nordlund, K.  

    In this article, we first review recent work on the basic science of the mechanisms of irradiation-induced defect production and migration in carbon nanotubes. We then discuss a few ways in which understanding of the irradiation effects can be used to modify the properties of single nanotubes as well as macroscopic materials based on them. The results illustrate that there indeed exist several ways in which defects can have potentially useful effects on nanotube-based materials.
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  • Electronic processes in molecular dynamics simulations of nanoscale metal tips under electric fields

    Parviainen, S.   Djurabekova, F.   Timko, H.   Nordlund, K.  

    Electronic effects play a crucial role in the temperature evolution of metal parts which have electric currents running through them. The increase in temperature due to resistive heating can cause the melting of metal nanoscale wires creating damage in electric circuits. Likewise, electric currents are also present in sharp features on metal surfaces exposed to high electric fields. The destruction of such tips can lead to vacuum arcs, supplying the neutral species to build up plasma over the surface. To follow the temperature evolution caused by electric currents in such a tip, we developed a new model, based on an existing molecular dynamics code, to include resistive heating and electronic thermal conduction. The results given by the new simulation model are in good agreement with analytical predictions. (C) 2011 Published by Elsevier B.V.
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  • Molecular dynamics simulations of nanoscale metal tips under electric fields

    Parviainen, S.   Djurabekova, F.   Pohjonen, A.   Nordlund, K.  

    Vacuum arcing is a plasma discharge over a metal surface under high electric fields. Plasma formation requires the supply of neutral atoms, which under high vacuum condition can only come from the surface itself. Nevertheless, the mechanisms by which the atoms are supplied are not known. In the present work, we propose a model for the onset of surface roughness and field-enhanced atom evaporation. Specifically, we describe a dislocation mechanism of tip growth from near-surface voids. We also simulate surface charging and resistive heating using a hybrid electrodynamics and molecular dynamics (ED&MD) code for dynamic simulations of electronic effects. We study the morphological evolution of the nanoscale protrusion under the electronic effects, such as the stretching of the tip by the stress induced by the electric field. (C) 2010 Elsevier B.V. All rights reserved.
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  • #DOESNTSHOWDOESNTEXIST / #SYNSINTEFINNSINTE - A PHOTO CAMPAIGN BY UNGA REUMATIKER

    Nordlund, K.   Beermann, M.  

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  • Reflection of hydrogen and deuterium atoms from the beryllium, carbon, tungsten surfaces

    Meluzova, D.S.   Babenko, P.Yu.   Shergin, A.P.   Nordlund, K.   Zinoviev, A.N.  

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  • Enhancement of irradiation-induced defect production in Si nanowires RID B-3127-2012

    Hoilijoki, S.   Holmstrom, E.   Nordlund, K.  

    We performed classical molecular dynamics simulations of defect production in small-diameter hexagonal Si nanowires under Ar ion irradiation. Using irradiation energies of 30 eV to 10 keV, we find that for low energies the defect production in the nanowires may be enhanced by as much as a factor of 3 in comparison to bulk Si due to the large surface-to-volume ratio of the systems. Conversely, at higher energies the increased transmission of ions causes a significant decrease in defect production. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3627234]
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  • Chemical sputtering of Be due to D bombardment

    Bjorkas, C.   Vortler, K.   Nordlund, K.   Nishijima, D.   Doerner, R.  

    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.
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