C. J. Fontes

Los Alamos National Laboratory, Los Alamos, California, United States

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Publications (106)225.4 Total impact

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    ABSTRACT: We use the wave-packet propagation scheme of Goldberger and Watson to define multipole moment creation, destruction, and transfer rates for the three-body recombination (TBR) of electrons with ions. We first assume short-range interaction potentials and then consider Coulomb interactions, for which we use Dollards theory of multichannel scattering. We present the multipole moment rate coefficients in terms of the TBR amplitudes. Finally, we discuss time-reversal invariance and the reciprocity relations, both for the short-range case and for the Coulomb-interaction case, and show that the multipole moment rate coefficients can be expressed in terms of electron-impact ionization amplitudes.
    Journal of Physics B Atomic Molecular and Optical Physics 02/2015; 48(3). · 1.92 Impact Factor
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    ABSTRACT: The full text of this article is available in the PDF provided.
    Journal of Physics B Atomic Molecular and Optical Physics 03/2014; 47(8). · 1.92 Impact Factor
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    ABSTRACT: We present the main results of the 7th Non-Local Thermodynamic Equilibrium Code Comparison Workshop held in December 2011 in Vienna, Austria. More than twenty researchers from nine countries, who actively work on development of collisional-radiative codes for plasma kinetics modeling, attended the meeting and submitted their results for a number of comparison cases. The cases included free-electron-laser-inspired time-dependent relaxation of photoexcited Ne-like Ar, ionization balance and spectra for highly charged tungsten, spectroscopic diagnostics of krypton L-shell spectra, and an investigation of Ne model convergence with principal quantum number.
    High Energy Density Physics 12/2013; 9(4):645–652. · 1.52 Impact Factor
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    ABSTRACT: Expanding on previous works that involved elastic and inelastic scattering of electrons by atoms and ions, we use the wave-packet propagation scheme of Dollard to define multipole moment creation, destruction and transfer cross sections for electron- and proton-impact ionization of atoms and ions. The electron-impact cross sections can then be used by defining appropriate rate coefficients for use in Fujimoto’s population-alignment collisional-radiative model for cylindrically symmetric plasmas. Our result for the alignment creation cross section is in agreement with those formulae that were obtained earlier intuitively or by semi-classical collisional methods. The multipole cross sections obtained here can be used also for modelling the relaxation behaviour of laser-excited plasmas under cylindrical symmetry conditions. We have also derived the electron- and proton-impact ionization multipole cross sections in terms of Liouville-space quantities, which then enabled us by using group theoretical methods to obtain the azimuthal-angle dependence of the multipole cross sections and symmetry properties that are results of reflection across a plane over the collisional axis.
    Journal of Physics B Atomic Molecular and Optical Physics 12/2013; 46(24):5202-. · 1.92 Impact Factor
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    ABSTRACT: The international OPAC consortium consists of astrophysicists, plasma physicists and experimentalists who examine opacity calculations used in stellar physics that appear questionable and perform new calculations and laser experiments to understand the differences and improve the calculations. We report on iron and nickel opacities for envelopes of stars from 2 to 14M⊙ and deliver our first conclusions concerning the reliability of the used calculations by illustrating the importance of the configuration interaction and of the completeness of the calculations for temperatures around 15–27 eV.
    High Energy Density Physics 09/2013; 9(3):473–479. · 1.52 Impact Factor
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    ABSTRACT: Collisional-radiative atomic models are widely used to help diagnose experimental plasma conditions through fitting and interpreting measured spectra. Here we present the results of a code comparison in which a variety of models determined plasma temperatures and densities by finding the best fit to an experimental L-shell Kr spectrum from a well characterized, but not benchmarked, laser plasma. While variations in diagnostic strategies and qualities of fit were significant, the results generally confirmed the typically quoted uncertainties for such diagnostics of ±20% in electron temperature and factors of about two in density. The comparison also highlighted some model features important for spectroscopic diagnostics: fine structure was required to match line positions and relative intensities within each charge state and for density diagnostics based on emission from metastable states; an extensive configuration set was required to fit the wings of satellite features and to reliably diagnose the temperature through the inferred charge state distribution; and the inclusion of self-consistent opacity effects was an important factor in the quality of the fit.
    High Energy Density Physics 09/2013; 9(3):523-527. · 1.52 Impact Factor
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    ABSTRACT: We present new calculations of local-thermodynamic-equilibrium (LTE) light element opacities from the Los Alamos ATOMIC code [1] for systems of astrophysical interest. ATOMIC is a multi-purpose code that can generate LTE or non-LTE quantities of interest at various levels of approximation. Our calculations, which include fine-structure detail, represent a systematic improvement over previous Los Alamos opacity calculations using the LEDCOP legacy code [2]. The ATOMIC code uses ab-initio atomic structure data computed from the CATS code, which is based on Cowan's atomic structure codes [3], and photoionization cross section data computed from the Los Alamos ionization code GIPPER [4]. ATOMIC also incorporates a new equation-of-state (EOS) model based on the chemical picture [5]. ATOMIC incorporates some physics packages from LEDCOP and also includes additional physical processes, such as improved free-free cross sections and additional scattering mechanisms. Our new calculations are made for elements of astrophysical interest and for a wide range of temperatures and densities.
    07/2013;
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    ABSTRACT: Exact relativistic plane-wave Born (RPWB) matrix elements of the Møller interaction are incorporated in the “analytic Born subtraction technique” and employed in the relativistic convergent close-coupling method. Application to the calculation of high-energy electron-impact-excitation cross sections of highly charged hydrogenlike ions demonstrates the “Bethe rise,” an effect that is manifest in Bethe's original 1932 work on relativistic high-energy, electron-impact excitation. The result represents an improvement over Bethe's relativistic high-energy theory developed in the 1930s in that (i) both target and projectile electrons are represented relativistically with Dirac spinor wave functions and (ii) the dipole approximation plus additional assumptions are not employed in the RPWB scattering amplitude of the Møller interaction.
    Physical Review A 07/2013; 88(1):012711. · 2.99 Impact Factor
  • Hong Lin Zhang, Christopher J. Fontes
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    ABSTRACT: Relativistic distorted-wave collision strengths have been calculated for the 16 Δn=0Δn=0 optically allowed transitions with n=2n=2 in the 67 Be-like ions with nuclear charge number ZZ in the range 26≤Z≤9226≤Z≤92. The calculations were made for the four final, or scattered, electron energies E′=0.20E′=0.20, 0.42, 0.80, and 1.40, where E′E′ is in units of Zeff2 Ry with Zeff=Z−2.5. In the present calculation, an improved “top-up” method, which employs relativistic plane waves, was used to obtain the high partial-wave contribution for each transition, in contrast to the partial-relativistic Coulomb–Bethe approximation used in previous work by Zhang and Sampson [H.L. Zhang and D.H. Sampson, At. Data Nucl. Data Tables 52 (1992) 143]. In that earlier work, collision strengths were also provided for Be-like ions, but for a more comprehensive data set consisting of all 45 Δn=0Δn=0 transitions, six scattered energies, and the 85 ions with ZZ in the range 8≤Z≤928≤Z≤92. The collision strengths covered in the present work should be more accurate than the corresponding data given by Zhang and Sampson [H.L. Zhang and D.H. Sampson, At. Data Nucl. Data Tables 52 (1992) 143] and are presented here to replace those earlier results.
    Atomic Data and Nuclear Data Tables 07/2013; 99(4):416–430. · 1.46 Impact Factor
  • High Energy Density Physics 06/2013; · 1.52 Impact Factor
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    ABSTRACT: We present new calculations of local-thermodynamic-equilibrium (LTE) light element opacities from the Los Alamos ATOMIC code. ATOMIC is a multi-purpose code that can generate LTE or non-LTE quantities of interest at various levels of approximation. A program of work is currently underway to compute new LTE opacity data for all elements H through Zn. New opacity tables for H through Ne are complete, and a new Fe opacity table will be available soon. Our calculations, which include fine-structure detail, represent a systematic improvement over previous Los Alamos opacity calculations using the LEDCOP legacy code. Our opacity calculations incorporate atomic structure data computed from the CATS code, which is based on Cowan's atomic structure codes, and photoionization cross section data computed from the Los Alamos ionization code GIPPER. We make use of a new equation-of-state (EOS) model based on the chemical picture. ATOMIC incorporates some physics packages from LEDCOP and also includes additional physical processes, such as improved free–free cross sections and additional scattering mechanisms. In this report, we briefly discuss the physics improvements included in our new opacity calculations and present comparisons of our new opacities with other work for C, O, and Fe at selected conditions.
    High Energy Density Physics 06/2013; 9(2):369–374. · 1.52 Impact Factor
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    ABSTRACT: The K shell excitation of H-like uranium (U^{91+}) in relativistic collisions with different gaseous targets has been studied at the experimental storage ring at GSI Darmstadt. By performing measurements with different targets as well as with different collision energies, we were able to observe for the first time the effect of electron-impact excitation (EIE) process in the heaviest hydrogenlike ion. The large fine-structure splitting in H-like uranium allowed us to unambiguously resolve excitation into different L shell levels. State-of-the-art calculations performed within the relativistic framework which include excitation mechanisms due to both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the EIE process.
    Physical Review Letters 05/2013; 110(21):213201. · 7.73 Impact Factor
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    ABSTRACT: In previous works, expressions for the atomic multipole moment cross sections were derived from a traditional collision approach. In the present work, we have derived the fundamental formula (see equation (35)) from which all of the atomic multipole moment cross sections can be obtained by using Liouville-space methods introduced by Fano (1963 Phys. Rev. 131 259). This simple, elegant formula is an expression for the multipole cross sections in terms of the Liouville-space transition operator (sometimes referred to as the tetradic transition matrix or the transition superoperator). The transition superoperator, in turn, can be expressed in terms of the traditional quantum mechanical transition operators via a formula which is sometimes referred to as 'Fano's convolution formula'. Upon application of this formula to our Liouville-space expression for the multipole cross sections, the resulting cross section formulae are identical to those obtained in previous works. Establishing this connection with the Liouville-space formalism allows us to apply powerful group theoretical techniques in order to obtain expressions of practical interest. As a specific example, we consider the transition rate for the final-state multipole moment which can be obtained via the use of a 'connecting factor' from the initial values of the multipole moments. The 'connecting factor', in turn, is expressed in this work as a Liouville-space matrix element of the tetradic transition matrix. Based on this expression and the symmetry properties of the electron–atom collisional system, certain symmetry relations are obtained for the 'connecting factors'. Since these factors are proportional to the multipole cross sections, corresponding relations are also obtained for those cross sections, which results in a reduction in the number of values that needs to be calculated for plasma modelling applications. An additional corollary of practical importance is that, in the case of cylindrically symmetric plasmas, the same symmetry relations also hold for the multipole rate coefficients. We provide an explicit derivation of this new, important result.
    Journal of Physics B Atomic Molecular and Optical Physics 03/2013; 46(8):085202. · 1.92 Impact Factor
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    ABSTRACT: We report on recent efforts to generate high quality, self-consistent atomic physics models for L-shell ion stages for iron and the use of these data in collisional-radiative modeling of X-ray spectra of supernova remnants. As a specific example, we present comparisons between observed and theoretical X-ray spectra produced by Tycho's supernova remnant.
    High Energy Density Physics 01/2013; · 1.52 Impact Factor
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    ABSTRACT: We report on preliminary efforts to apply non-local thermodynamic equilibrium (NLTE) effects in the modeling of light curves for Type Ia supernovae. Significant differences are obtained for the opacity of iron at relevant conditions when account is taken of non-thermal electrons resulting from the down-scattering of gamma rays that arise from the radioactive decay of nickel and cobalt. These results provide evidence that a more precise, detailed investigation is warranted.
    Journal of Physics Conference Series 11/2012; 388(1):2022-.
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    ABSTRACT: Expanding on previous works that involved the scattering of electrons by atoms, we use the wave-packet propagation scheme of Dollard to define multipole moment creation, destruction and transfer cross sections for elastic and inelastic scattering of electrons by ions. We find that our cross section formulae for inelastic scattering agree with those obtained by Fujimoto and coworkers, who used semi-classical collision theory and the impact approximation, but differ from the expressions obtained by them for elastic scattering cross sections. This latter result is due to the fact that Dollard’s theory takes into account the concept that, in the case of electron–ion scattering, the incident and scattered electrons are asymptotically not free. In addition, we apply the Gell-Mann–Goldberger (‘two-potential’) formula in order to provide an unambiguous definition of elastic cross sections, as well as to provide a convenient way of obtaining practical expressions associated with well-established methods, such as the Coulomb–Born and distorted-wave approximations. The present theoretical framework will be the basis for numerical calculations considering specific examples in a future work.
    Journal of Physics B Atomic Molecular and Optical Physics 05/2012; 45(10). · 1.92 Impact Factor
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    ABSTRACT: The Breit interaction and its generalizations have been used for many decades to produce improved results in the calculation of atomic structure and fundamental processes involving continuum electrons. While the importance of these interactions has been experimentally verified for a number of processes, explicit verification is yet to be provided in the case of electron-impact excitation (EIE). In this work, we provide a brief history of the Breit interaction and its computational development. Specific attention is paid to past and present theoretical predictions of the effect of the Breit interaction on EIE in highly charged ions and recent attempts to measure this effect.
    05/2012;
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    ABSTRACT: We have entered the era of explosive transient astronomy, in which upcoming real-time surveys like the Large Synoptic Survey Telescope (LSST), the Palomar Transient Factory (PTF) and Panoramic Survey Telescope and Rapid Response System (Pan-STARRS) will detect supernovae in unprecedented numbers. Future telescopes such as the James Webb Space Telescope may discover supernovae from the earliest stars in the universe and reveal their masses. The observational signatures of these astrophysical transients are the key to unveiling their central engines, the environments in which they occur, and to what precision they will pinpoint cosmic acceleration and the nature of dark energy. We present a new method for modeling supernova light curves and spectra with the radiation hydrodynamics code RAGE coupled with detailed monochromatic opacities in the SPECTRUM code. We include a suite of tests that demonstrate how the improved physics is indispensable to modeling shock breakout and light curves.
    The Astrophysical Journal Supplement Series 03/2012; 204(2). · 14.14 Impact Factor
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    ABSTRACT: Explosive transient astronomy is entering an era where supernovae (SNe) and gamma-ray bursts will be observed in real time with surveys like the LSST and Pan-STARRS, probing the universe back to very early times. The discovery of Pop III SNe could reveal many details about the formation and evolution of the first stars. Observations of shock breakout in SNe will provide new information about the engines powering these explosions. Shock breakout occurs when the shock wave from core collapse reaches an optically thin region and radiation can stream out. This first burst of radiation interacts with the star's immediate surroundings, showing the effects of the surrounding environment on emission and evolution. This profusion of data will contain brief snapshots from a wide range of progenitor systems which simulations can help interpret and explain. We present a new pipeline for creating model supernova spectra and lightcurves using radiation-hydrodynamic simulations and a new Spectrum code. Spectrum maps 1-D or 2-D data onto a two dimensional grid and assumes rotational symmetry, using monochromatic opacities to calculate emission and absorption as a function of radius and angle. We use these spectra to create lightcurves in any band from infrared to x-ray. This pipeline is being used to study the effects of stellar environment on core-collapse and Type Ia SNe, as well as several types of Pop III SNe.
    01/2012;
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    ABSTRACT: For the non-equilibrium, ionizing (NEI) conditions present in the X-ray gas of young supernova remnants, the iron peak elements are often observed to be in intermediate charge states whose emission lines are not adequately represented in the current generation of plasma codes. We report on our progress in generating with modern atomic physics codes the data necessary to model accurately the NEI X-ray spectra of these ions. We will compare newly calculated spectra against deep Chandra and Suzaku observations of Tycho's supernova remnant, allowing for the first time a reliable measurement of the X-ray emitting mass of iron, as well as other less abundant iron peak elements (Mn, Cr, and Ni).
    01/2012;

Publication Stats

981 Citations
225.40 Total Impact Points

Institutions

  • 1994–2014
    • Los Alamos National Laboratory
      • • Theoretical Division
      • • Applied Physics Division
      Los Alamos, California, United States
  • 2011
    • University of Texas at San Antonio
      • Department of Physics and Astronomy
      San Antonio, Texas, United States
  • 2000
    • Drake University
      • Physics and Astronomy
      Des Moines, Iowa, United States
  • 1996–1998
    • California State University, Fullerton
      • Department of Physics
      Fullerton, CA, United States
  • 1990–1991
    • Pennsylvania State University
      • Department of Astronomy and Astrophysics
      University Park, MD, United States