M. Foster

Los Alamos National Laboratory, Los Alamos, NM, United States

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Publications (44)83.91 Total impact

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    ABSTRACT: We present calculations of ionization balance and radiative power losses for tungsten in magnetic fusion plasmas. The simulations were performed within the framework of Non‐Local Thermodynamic Equilibrium (NLTE) Code Comparison Workshops utilizing several independent collisional‐radiative models. The calculations generally agree with each other; however, a clear disagreement with experimental ionization distributions at low temperatures 2 keV<Te<3 keV is observed. Comparison is made with other calculations. The obtained results can be used in planning new experiments in magnetic confinement fusion.
    AIP Conference Proceedings. 09/2009; 1161(1):242-250.
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    ABSTRACT: We examine the time-dependent close-coupling (TDCC) approach to electron-impact single ionization of helium and study the convergence properties of our method. As an example, we compare our calculations to recent measurements of the triple differential cross sections from He after 102 eV electron impact, made for asymmetric electron energies and a variety of electron geometries. We find that our calculations compare well to the measurements and to convergent close-coupling calculations.
    Journal of Physics B Atomic Molecular and Optical Physics 06/2009; 42(14):145002. · 2.03 Impact Factor
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    ABSTRACT: Please see PDF file for details.
    Journal of Physics B Atomic Molecular and Optical Physics 01/2009; 42. · 2.03 Impact Factor
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    ABSTRACT: A comprehensive set of calculations of the radiative losses of solar coronal plasmas is presented. The Los Alamos suite of atomic structure and collision codes is used to generate collisional data for 15 coronal elements. These data are used in the Los Alamos plasma kinetics code ATOMIC to compute the radiative power loss as a function of electron temperature. We investigate the sensitivity of the loss curves to the quality of the atomic data and changes in the coronal elemental abundances, and we compare our results with previous work.
    The Astrophysical Journal 12/2008; 689(1):585. · 6.73 Impact Factor
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    ABSTRACT: A comprehensive study is made of the energy relaxation rates between ions and electrons in a dense hydrogen plasma. Results of classical molecular dynamics (MD) simulations are compared with quantal calculations using the Fermi golden rule and using dimensional continuation. The rates from the molecular dynamics simulations employing a screened potential are found to be in reasonable agreement with the Landau-Spitzer relaxation rates, and are around 30% higher than the Fermi golden rule rates. By inverting the classical MD relaxation rate vs the quantal result, a semiclassical value for the screening length is suggested. We present energy relaxation rates relevant for radiation-hydrodynamic simulations of inertial confinement fusion devices.
    Physical Review E 10/2008; 78(3 Pt 2):036403. · 2.31 Impact Factor
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    ABSTRACT: The importance of projectile interactions in triple differential cross sections (TDCS) is explored for the problem of simultaneous excitation–ionization of helium by electron impact using a new approach that we call the four-body distorted wave model (4DW). The 4DW model includes all projectile interactions, namely initial- and final-state projectile–target interactions, and the post-collision interaction between the two continuum electrons. Results are presented for an incident electron energy of 500 eV and are compared to experimental data, as well as a second-order R-matrix theory and the first Born approximation. Results for absolute TDCS ratios of ionization without excitation to excitation–ionization are also presented.
    Journal of Physics B Atomic Molecular and Optical Physics 06/2008; 41(13):135203. · 2.03 Impact Factor
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    M Foster, J Colgan, M S Pindzola
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    ABSTRACT: A time-dependent close-coupling approach is used to investigate 6 MeV proton-impact double ionization of helium. We extract doubly differential (in the emission angles of both outgoing electrons) cross sections for comparison with recent experimental measurements. Good qualitative agreement is found between our calculations and the measurements. We also present differential cross sections for cases where the energy available to the outgoing electrons is unrestricted, which allows further information about the double escape process to be inferred.
    Journal of Physics B Atomic Molecular and Optical Physics 05/2008; 41(11):111002. · 2.03 Impact Factor
  • Matthew Foster, J. Abdallah Jr, J. Colgan
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    ABSTRACT: Controlled fusion tokamaks such as ITER present challenging theoretical plasma modeling issues. Since the divertor region of ITER will be coated with tungsten, accurate collisional-radiative (CR) models are required to understand the high radiative power losses associated with tungsten. The energy loss due to radiative processes for high-Z ions can be critical in understanding the ionization balance of the plasma. We present non-local thermodynamic equilibrium (non-LTE) calculations for a tungsten plasma using the Los Alamos National Laboratory suite of atomic codes. We examine the radiated power losses and ion balance distributions for a variety of electron temperatures ranging from 2 keV to 30 keV and densities associated with conditions found in ITER.
    03/2008;
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    M Foster, J Colgan, M S Pindzola
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    ABSTRACT: We present total cross sections for single and double ionization of helium by antiproton impact over a wide range of impact energies from 10 keV/amu to 1 MeV/amu. A nonperturbative time-dependent close-coupling method is applied to fully treat the correlated dynamics of the ionized electrons. Excellent agreement is obtained between our calculations and experimental measurements of total single and double ionization cross sections at high impact energies, whereas for lower impact energies, some discrepancies with experiment are found. At an impact energy of 1 MeV we also find that the double-to-single ionization ratio is twice as large for antiproton impact as for proton impact, confirming a long-standing unexpected experimental measurement.
    Physical Review Letters 02/2008; 100(3):033201. · 7.94 Impact Factor
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    ABSTRACT: We present calculations of ion-atom collisions using a partial-wave expansion of the projectile wavefunction. Most calculations of ion-atom collisions have typically used classical or plane-wave approximations for the projectile wavefunction, since partial-wave expansions are expected to require prohibitively large numbers of terms to converge scattering quantities. Here we show that such calculations are possible using modern high-performance computing. We demonstrate the utility of our method by examining elastic scattering of protons by hydrogen and helium atoms, problems familiar to undergraduate students of atomic scattering. Application to ionization of helium using partial-wave expansions of the projectile wavefunction, which has long been desirable in heavy-ion collision physics, is thus quite feasible.
    European Journal of Physics 01/2008; 30(3). · 0.64 Impact Factor
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    ABSTRACT: The importance of projectile interactions in fully differential cross sections (FDCS) will be explored for the problem of simultaneous excitation-ionization by electron impact. We will compare the results of two theories- the first Born approximation (FBA) and the four-body distorted wave model (4DW). In the first Born approximation (FBA), the projectile electron is treated as a plane wave, the ejected electron is treated as a Hartree Fock distorted wave, and the final state Coulomb interaction between the two continuum electrons is ignored. In the 4DW model, all continuum electrons are treated as Hartree Fock distorted waves, and a Coulomb distortion factor is included in the final state to account for the interaction between the two outgoing electrons. Results will be presented for an incident electron energy of 500 eV and will be compared to experimental data.
    10/2007;
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    ABSTRACT: We present total cross sections for single and double ionization of helium by various charged ion impact. A non-perturbative time-dependent close-coupling method (TDCC) has been developed to treat the correlated dynamics of ionized electrons by bare-ion impact [Journal of Physics B (accepted)]. The two-electron helium wavefunction is subject to a time-dependent projectile interaction. The projectile-atom interaction is constructed as a multipole expansion that includes monopole, dipole, quadrupole, and octopole terms. For proton, antiproton, and alpha particle impact, good agreement is obtained between our calculations and experimental measurements of total single and double ionization cross sections. We will also report on our progress in using the TDCC method to extract differential cross sections for double ionization by fast protons [Physical Review Letters 90, 243201 (2003)].
    06/2007;
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    ABSTRACT: We present both non-perturbative and perturbative calculations for photoionization of H^+2. For the perturbative approach, we have investigated two different final state wavefunctions for the ionized electron. The first wavefunction is a product of two Coulomb functions (2C) where each Coulomb function represents the two-body interaction between the ionized electron and one of the residual protons in the nucleus. The second final state wavefunction, we investigated was a distorted wave for the ionized electron calculated using a spherically symmetric potential for the two residual protons. These methods are compared to the results computed using the non-perturbative time-dependent method. The time-dependent method solves the time-dependent Schrodinger equation for H^+2 using the variational principle in spherical coordinates centered on the center of mass of the H^+2 system.
    06/2007;
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    ABSTRACT: The importance of projectile interactions in fully differential cross sections (FDCS) is explored for the problem of simultaneous excitation-ionization by electron impact. We will compare the results of two theories- the first Born approximation-Hartree Fock (FBA-HF) and the four-body distorted wave model (4DW). In the first Born approximation-Hartree Fock (FBA-HF), the projectile electron is treated as a plane wave, the ejected electron is treated as a Hartree Fock distorted wave, and the final state Coulomb interaction between the two continuum electrons is ignored. In the 4DW model, all continuum electrons are treated as Hartree Fock distorted waves, and a Coulomb distortion factor is included in the final state to account for the interaction between the two outgoing electrons. Results are presented for an incident electron energy of 500 eV and will be compared to experimental data.
    06/2007;
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    ABSTRACT: We review the development of the time-dependent close-coupling method to study atomic and molecular few body dynamics. Applications include electron and photon collisions with atoms, molecules, and their ions.
    Journal of Physics B Atomic Molecular and Optical Physics 03/2007; 40(7):R39. · 2.03 Impact Factor
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    ABSTRACT: Three-dimensional fully differential cross sections for heavy-particle-impact ionization of helium are examined. Previously, the three-body distorted-wave (3DW) model has achieved good agreement with experiment in the scattering plane for small momentum transfers, but poor agreement for large momentum transfers. Poor agreement was also observed outside the scattering plane for all momentum transfers. In particular, the 3DW calculations predicted cross sections that were too small both perpendicular to the scattering plane and for large momentum transfers. The important unanswered question concerns the physical effects that cause the significant disagreement between experiment and theory. In previous works, the role of the projectile-ion interaction has been examined. Although the importance of exchange between the ejected electron and the residual bound electrons has been well established, and frequently studied, for electron-impact ionization, the importance of this effect has not been examined for heavy-particle scattering. In this paper we examine the role of this effect for heavy-particle scattering.
    Physical Review A 01/2007; · 3.04 Impact Factor
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    ABSTRACT: We report on our progress in understanding the dynamics of atomic and molecular collision processes using time-dependent close-coupling methods
    Journal of Physics Conference Series 01/2007; 88(1).
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    ABSTRACT: We present experimental and theoretical results for the electron-impact-induced ionization of ground-state helium atoms. Using a high-sensitivity toroidal electron spectrometer, we measured cross-section ratios for transitions leading to the first three excited states of the residual helium ion relative to the transition leaving the ion in the ground state. Measurements were performed for both symmetric- and asymmetric-energy-sharing kinematics. By presenting results as a ratio, a direct comparison can be made between theoretical and experimental predictions without recourse to normalization. The experimental data are compared to theoretical predictions employing various first-order models and a second-order hybrid distorted-wave + convergent R matrix with pseudostates (close-coupling) approach. All the first-order models fail in predicting even the approximate size of the cross-section ratios. The second-order calculations are found to describe the experimental data for asymmetric-energy-sharing with reasonable fidelity, although significant disparities are evident for the symmetric-energy-sharing cases. These comparisons demonstrate the need for further theoretical developments, in which all four charged particles are treated on an equal footing.
    Physical Review A - PHYS REV A. 01/2007; 75(4).
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    ABSTRACT: A study is made of the differential cross sections arising from the photoionization of H2+ . Previous studies indicated surprising differences in the shapes of the angular distributions calculated from exterior complex scaling and 2C methods. To further explore these differences, we have calculated the angular distributions from the photoionization of H2+ using an independent two-body Coulomb function (2C) method and a distorted wave approach. As a final test, we also present calculations using a time-dependent technique. Our results confirm the discrepancies found previously and we present possible reasons for these differences.
    Physical Review A 01/2007; 75(6). · 3.04 Impact Factor
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    ABSTRACT: We use the time-dependent close-coupling method to explore the triple differential cross sections produced by double photoionization of He and H2. Recent extensions of our technique have allowed extraction of the triple differential cross section as the double photoionization process evolves, allowing further exploration of the underlying mechanisms. We demonstrate that the angular distributions of the two outgoing electrons typically reach a converged shape after 5 or 6 field periods. We also further compare our method with recent experimental measurements, as well as demonstrating some similarities in the triple differential cross sections for He and for H2 at certain molecular orientations.
    Journal of Physics B Atomic Molecular and Optical Physics 01/2007; 40(22):4391-4402. · 2.03 Impact Factor