Interatomic potentials for atomistic simulations of the Ti-Al system

Semi-empirical interatomic potentials have been developed for Al, alpha-Ti, and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large database of experimental as well as ab-initio data. The ab-initio calculations were performed by the linear augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide reasonable description of the lattice thermal expansion, demonstrating their usefulness in the molecular dynamics or Monte Carlo studies at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in gamma-TiAl calculated with the EAM potential is in a fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in gamma-TiAl are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that gamm-TiAl is an antisite disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press

DSMC modeling of the differentially pumped Magnum-PSI vacuum system

The FOM Institute for Plasma Physics Rijnhuizen (FOM = Foundation for Fundamental Research on Matter) has started a new line of research to study the interaction of intense plasma fluxes with a material surface. An important experimental tool for this programme will be the Magnum-PSI high-flux linear plasma generator operating in the ITER- relevant regime of plasma surface interaction (PSI). ITER is the next step in fusion reactor research (ITER is Latin for 'the way'). In Magnum-PSI a plasma beam is guided from the source to a target by a strong axial magnetic field. Besides ionized particles, the plasma source produces hot neutral gas which has to be prevented from reaching the target region. In this paper it is investigated to what extent a differential pumping scheme can prevent the influx of neutral gas from the source in the target region. The results of neutral gas simulations using the Direct Simulation Monte Carlo (DSMC) method will be discussed. We will focus on the supersonic expansion as a function of background pressure and determine the influence of the skimmers on the expansion. We will demonstrate that differential pumping can be used in Magnum-PSI to reach low enough pressures in the target region. Finally, we give the optimum position of the skimmer

DSMC modeling of the differentially pumped Magnum-PSI vacuum system

The FOM Institute for Plasma Physics Rijnhuizen (FOM = Foundation for Fundamental Research on Matter) has started a new line of research to study the interaction of intense plasma fluxes with a material surface. An important experimental tool for this programme will be the Magnum-PSI high-flux linear plasma generator operating in the ITER- relevant regime of plasma surface interaction (PSI). ITER is the next step in fusion reactor research (ITER is Latin for 'the way'). In Magnum-PSI a plasma beam is guided from the source to a target by a strong axial magnetic field. Besides ionized particles, the plasma source produces hot neutral gas which has to be prevented from reaching the target region. In this paper it is investigated to what extent a differential pumping scheme can prevent the influx of neutral gas from the source in the target region. The results of neutral gas simulations using the Direct Simulation Monte Carlo (DSMC) method will be discussed. We will focus on the supersonic expansion as a function of background pressure and determine the influence of the skimmers on the expansion. We will demonstrate that differential pumping can be used in Magnum-PSI to reach low enough pressures in the target region. Finally, we give the optimum position of the skimmer