[Show abstract][Hide abstract] ABSTRACT: We have studied the excitation of H-like and He-like uranium (U91+ and U90+) in relativistic collisions with gaseous targets by observing the subsequent x-ray emission. The experiment was conducted at the ESR storage ring of the GSI accelerator facility in Darmstadt, Germany. The measurements were performed with a newly developed multi-phase target at different collision energies. This enabled us to explore the proton (nucleus) impact excitation as well as the electron impact excitation processes in the relativistic collisions. The large fine-structure splitting in uranium allowed us to unambiguously resolve excitation to different L-shell levels. Moreover, information about the population of different magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. The experimental results are compared with calculations performed within the relativistic framework including excitation mechanisms due to both protons (nucleus) and electrons.
Journal of Physics B Atomic Molecular and Optical Physics 07/2015; 48(14). DOI:10.1088/0953-4075/48/14/144006 · 1.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The Los Alamos suite of relativistic atomic physics codes is a robust, mature platform that has been used to model highly charged ions in a variety of ways. The suite includes capabilities for calculating data related to fundamental atomic structure, as well as the processes of photoexcitation, electron-impact excitation and ionization, photoionization and autoionization within a consistent framework. These data can be of a basic nature, such as cross sections and collision strengths, which are useful in making predictions that can be compared with experiments to test fundamental theories of highly charged ions, such as quantum electrodynamics. The suite can also be used to generate detailed models of energy levels and rate coefficients, and to apply them in the collisional-radiative modeling of plasmas over a wide range of conditions. Such modeling is useful, for example, in the interpretation of spectra generated by a variety of plasmas. In this work, we provide a brief overview of the capabilities within the Los Alamos relativistic suite along with some examples of its application to the modeling of highly charged ions.
Journal of Physics B Atomic Molecular and Optical Physics 07/2015; 48(14). DOI:10.1088/0953-4075/48/14/144014 · 1.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on the use of the Los Alamos suite of relativistic atomic physics codes to generate radiative opacities for the modeling of astrophysically relevant plasmas under local thermodynamic equilibrium (LTE) conditions. The atomic structure calculations are carried out in fine-structure detail, including full configuration interaction. Three example applications are considered: iron opacities at conditions relevant to the base of the solar convection zone, nickel opacities for the modeling of stellar envelopes, and samarium opacities for the modeling of light curves produced by neutron star mergers. In the first two examples, comparisons are made between opacities that are generated with the fully and semi-relativistic capabilities in the Los Alamos suite of codes. As expected for these highly charged, iron-peak ions, the two methods produce reasonably similar results, providing confidence that the numerical methods have been correctly implemented. However, discrepancies greater than 10% are observed for nickel and investigated in detail. In the final application, the relativistic capability is used in a preliminary investigation of the complicated absorption spectrum associated with cold lanthanide elements.
High Energy Density Physics 06/2015; 16. DOI:10.1016/j.hedp.2015.06.002 · 1.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We have previously developed an equation of state (EOS) model called ChemEOS (Hakel and Kilcrease, Atomic Processes in Plasmas, Eds., J. Cohen et al., AIP, 2004) for a plasma of interacting ions, atoms and electrons. It is based on a chemical picture of the plasma and is derived from an expression for the Helmholtz free energy of the interacting species. All other equilibrium thermodynamic quantities are then obtained by minimizing this free energy subject to constraints, thus leading to a thermodynamically consistent EOS. The contribution to this free energy from the Coulomb interactions among the particles is treated using the method of Chabrier and Potekhin (Phys. Rev. E 58, 4941 (1998)) which we have adapted for partially ionized plasmas. This treatment is further examined and is found to give rise to unphysical behavior for various elements at certain values of the density and temperature where the Coulomb coupling begins to become significant and the atoms are partially ionized. We examine the source of this unphysical behavior and suggest corrections that produce acceptable results. The sensitivity of the thermodynamic properties and frequency-dependent opacity of iron is examined with and without these corrections. The corrected EOS is used to determine the fractional ion populations and level populations for a new generation of OPLIB low-Z opacity tables currently being prepared at Los Alamos National Laboratory with the ATOMIC code.
High Energy Density Physics 06/2015; 16. DOI:10.1016/j.hedp.2015.05.005 · 1.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent measurements using an X-ray Free Electron Laser (XFEL) and an Electron Beam Ion Trap at the Linac Coherent Light Source facility highlighted large discrepancies between the observed and theoretical values for the Fe XVII 3C/3D line intensity ratio. This result raised the question of whether the theoretical oscillator strengths may be significantly in error, due to insufficiencies in the atomic structure calculations. We present time-dependent spectral modeling of this experiment and show that non-equilibrium effects can dramatically reduce the predicted 3C/3D line intensity ratio, compared with that obtained by simply taking the ratio of oscillator strengths. Once these non-equilibrium effects are accounted for, the measured line intensity ratio can be used to determine a revised value for the 3C/3D oscillator strength ratio, giving a range from 3.0 to 3.5. We also provide a framework to narrow this range further, if more precise information about the pulse parameters can be determined. We discuss the implications of the new results for the use of Fe XVII spectral features as astrophysical diagnostics and investigate the importance of time-dependent effects in interpreting XFEL-excited plasmas.
[Show abstract][Hide abstract] 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). DOI:10.1088/0953-4075/48/3/035001 · 1.98 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Relativistic distorted-wave collision strengths have been calculated for the 185 δn=0 transitions with n=2 in the 67 C-like ions with nuclear charge number Z in the range 26≤Z≤92. The calculations were made for the six final, or scattered, electron energies E'=0.03,0.08,0.20,0.42,0.80, and 1.40, where E' is in units of Zeff2 Ry with Zeff=Z-4.17. In addition, electric dipole oscillator strengths are provided. In the present collision-strength calculations, 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, D.H. Sampson, At. Data Nucl. Data Tables 63 (1996) 275]. In that earlier work, collision strengths were also provided for the same 185 δn=0 transitions in C-like ions, but for the more limited list of 46 ions with Z in the range 9≤Z≤54. The collision strengths covered in the present work, particularly those for optically allowed transitions, should be more accurate than the corresponding data given by Zhang and Sampson [H.L. Zhang, D.H. Sampson, At. Data Nucl. Data Tables 63 (1996) 275] and are presented here to replace those earlier results.
Atomic Data and Nuclear Data Tables 01/2015; 101:41-142. DOI:10.1016/j.adt.2014.08.001 · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Relativistic distorted-wave collision strengths have been calculated for the 16 δn=0 optically allowed transitions with n=2 in the 67 O-like ions with nuclear charge number Z in the range 26≤Z≤92. The calculations were made for the four final, or scattered, electron energies E'=0.20, 0.42, 0.80, and 1.40, where E' is in units of Zeff2 Ry with Zeff=Z-5.83. In the present calculations, 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, D.H. Sampson, At. Data Nucl. Data Tables 82 (2002) 357]. In that earlier work, collision strengths were also provided for O-like ions, but for a more comprehensive data set consisting of all possible 45 δn=0 transitions, six scattered energies, and the 79 ions with Z in the range 14≤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, D.H. Sampson, At. Data Nucl. Data Tables 82 (2002) 357] and are presented here to replace those earlier results.
Atomic Data and Nuclear Data Tables 01/2015; 101:143-157. DOI:10.1016/j.adt.2014.08.002 · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Nearly a century ago it was recognized1 that radiation absorption by stellar matter controls the internal temperature profiles within stars. Laboratory opacity measurements, however, have never been performed at stellar interior conditions, introducing uncertainties in stellar models2-5. A particular problem arose2,3,6-8 when refined photosphere spectral analysis9,10 led to reductions of 30-50 per cent in the inferred amounts of carbon, nitrogen and oxygen in the Sun. Standard solar models11 using the revised element abundances disagree with helioseismic observations that determine the internal solar structure using acoustic oscillations. This could be resolved if the true mean opacity for the solar interior matter were roughly 15 per cent higher than predicted2,3,6-8, because increased opacity compensates for thedecreased element abundances. Iron accounts for a quarter of the total opacity2,12 at the solar radiation/convection zone boundary. Here we report measurements of wavelength-resolved iron opacity at electron temperatures of 1.9-2.3 million kelvin and electron densities of (0.7-4.0)31022 per cubic centimetre, conditions very similar to those inthe solar region that affects thediscrepancy themost: the radiation/convection zone boundary. Themeasured wavelengthdependent opacity is 30-400 per cent higher than predicted. This represents roughly half the change in the mean opacity needed to resolve the solar discrepancy, even though iron is only one of many elements that contribute to opacity.
[Show abstract][Hide abstract] ABSTRACT: Relativistic distorted-wave collision strengths have been calculated for the 49 Delta n = 0 optically allowed transitions with n = 2 in the 67 N-like ions with nuclear charge number Z in the range 26 <= Z <= 92. The calculations were made for the four final, or scattered, electron energies E' = 0.20, 0.42, 0.80, and 1.40, where E' is in units of Z(eff)(2) Ry with Z(eff) = Z - 5. In the present calculations, 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 the previous work by Zhang and Sampson [H.L. Zhang and D.H. Sampson, At. Data Nucl. Data Tables 72 (1999) 153]. In that earlier work, collision strengths were also provided for N-like ions, but for a more comprehensive data set consisting of all possible 105 Delta n = 0 transitions, six scattered energies and the 81 ions with Z in the range 12 <= 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 72 (1999) 153] and are presented here to replace those earlier results.
Atomic Data and Nuclear Data Tables 09/2014; 100(5):1292-1321. DOI:10.1016/j.adt.2014.02.002 · 2.46 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A series of experiments featuring laser-imploded plastic-shell targets filled with hydrogen or deuterium were performed on the National Ignition Facility. The shells (some deuterated) were doped in selected locations with Cu, Ga, and Ge, whose spectroscopic signals (indicative of local plasma conditions) were collected with a time-integrated, 1-D imaging, spectrally resolved, and absolute-intensity calibrated instrument. The experimental spectra compare well with radiation hydrodynamics simulations post-processed with a non-local thermal equilibrium atomic kinetics and spectroscopic-quality radiation-transport model. The obtained degree of agreement between the modeling and experimental data supports the application of spectroscopic techniques for the determination of plasma conditions, which can ultimately lead to the validation of theoretical models for thermonuclear burn in the presence of mix. Furthermore, the use of a lower-Z dopant element (e.g., Fe) is suggested for future experiments, since the ∼2 keV electron temperatures reached in mixed regions are not high enough to drive sufficient H-like Ge and Cu line emissions needed for spectroscopic plasma diagnostics.
Physics of Plasmas 06/2014; 21(6):063306. DOI:10.1063/1.4883641 · 2.14 Impact Factor
[Show abstract][Hide abstract] 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. DOI:10.1016/j.hedp.2013.06.001 · 1.23 Impact Factor
[Show abstract][Hide abstract] 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-. DOI:10.1088/0953-4075/46/24/245202 · 1.98 Impact Factor
[Show abstract][Hide abstract] 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. DOI:10.1016/j.hedp.2013.04.004 · 1.23 Impact Factor
[Show abstract][Hide abstract] 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(3-3):523-527. DOI:10.1016/j.hedp.2013.05.002 · 1.23 Impact Factor
[Show abstract][Hide abstract] 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. DOI:10.1016/j.adt.2012.04.004 · 2.46 Impact Factor