44 research outputs found
Iron and Nickel spectral opacity calculations in conditions relevant for pulsating stellar envelopes and experiments
Seismology of stars is strongly developing. To address this question we have
formed an international collaboration OPAC to perform specific experimental
measurements, compare opacity calculations and improve the opacity calculations
in the stellar codes [1]. We consider the following opacity codes: SCO,
CASSANDRA, STA, OPAS, LEDCOP, OP, SCO-RCG. Their comparison has shown large
differences for Fe and Ni in equivalent conditions of envelopes of type II
supernova precursors, temperatures between 15 and 40 eV and densities of a few
mg/cm3 [2, 3, 4]. LEDCOP, OPAS, SCO-RCG structure codes and STA give similar
results and differ from OP ones for the lower temperatures and for spectral
interval values [3]. In this work we discuss the role of Configuration
Interaction (CI) and the influence of the number of used configurations. We
present and include in the opacity code comparisons new HULLAC-v9 calculations
[5, 6] that include full CI. To illustrate the importance of this effect we
compare different CI approximations (modes) available in HULLAC-v9 [7]. These
results are compared to previous predictions and to experimental data.
Differences with OP results are discussed.Comment: 4 pages, 3 figures, conference Inertial Fusion Sciences and
Applications, Bordeaux, 12th to 16th September 2011; EPJ web of Conferences
201
Low-frequency plasma conductivity in the average-atom approximation
Low-frequency properties of a plasma are examined within the average-atom
approximation, which presumes that scattering of a conducting electron on each
atom takes place independently of other atoms. The relaxation time tau
distinguishes a high-frequency region omega tau > 1, where the single-atom
approximation is applicable explicitly, from extreme low frequencies omega tau
< 1, where, naively, the single-atom approximation is invalid. A proposed
generalization of the formalism, which takes into account many-atom collisions,
is found to be accurate in all frequency regions, from omega =0 to omega tau
>1, reproducing the Ziman formula in the static limit, results based on the
Kubo-Greenwood formula for high frequencies, and satisfying the conductivity
sum-rule precisely. The correspondence between physical processes leading to
the conventional Ohm's law and the infrared properties of QED is discussed. The
suggested average-atom approach to frequency-dependent conductivity is
illustrated by numerical calculations for the an aluminum plasma in the
temperature range 2--10 eV.Comment: 9 pages 3 figure
Proton stopping measurements at low velocity in warm dense carbon
: Ion stopping in warm dense matter is a process of fundamental importance for the understanding of the properties of dense plasmas, the realization and the interpretation of experiments involving ion-beam-heated warm dense matter samples, and for inertial confinement fusion research. The theoretical description of the ion stopping power in warm dense matter is difficult notably due to electron coupling and degeneracy, and measurements are still largely missing. In particular, the low-velocity stopping range, that features the largest modelling uncertainties, remains virtually unexplored. Here, we report proton energy-loss measurements in warm dense plasma at unprecedented low projectile velocities. Our energy-loss data, combined with a precise target characterization based on plasma-emission measurements using two independent spectroscopy diagnostics, demonstrate a significant deviation of the stopping power from classical models in this regime. In particular, we show that our results are in closest agreement with recent first-principles simulations based on time-dependent density functional theory
Theoretical and experimental activities on opacities for a good interpretation of seismic stellar probes
Opacity calculations are basic ingredients of stellar modelling. They play a
crucial role in the interpretation of acoustic modes detected by SoHO, COROT
and KEPLER. In this review we present our activities on both theoretical and
experimental sides. We show new calculations of opacity spectra and comparisons
between eight groups who produce opacity spectra calculations in the domain
where experiments are scheduled. Real differences are noticed with real
astrophysical consequences when one extends helioseismology to cluster studies
of different compositions. Two cases are considered presently: (1) the solar
radiative zone and (2) the beta Cephei envelops. We describe how our
experiments are performed and new preliminary results on nickel obtained in the
campaign 2010 at LULI 2000 at Polytechnique.Comment: 6 pages, 4 figures, invited talk at SOHO2
Radiative properties of stellar plasmas and open challenges
The lifetime of solar-like stars, the envelope structure of more massive
stars, and stellar acoustic frequencies largely depend on the radiative
properties of the stellar plasma. Up to now, these complex quantities have been
estimated only theoretically. The development of the powerful tools of helio-
and astero- seismology has made it possible to gain insights on the interiors
of stars. Consequently, increased emphasis is now placed on knowledge of the
monochromatic opacity coefficients. Here we review how these radiative
properties play a role, and where they are most important. We then concentrate
specifically on the envelopes of Cephei variable stars. We discuss the
dispersion of eight different theoretical estimates of the monochromatic
opacity spectrum and the challenges we need to face to check these calculations
experimentally.Comment: 6 pages, 5 figures, in press (conference HEDLA 2010
Evaluation of aluminum critical point using an ab initio variational approach
We present a method to evaluate the critical point of aluminum using a variational approach based on the ab initio molecular-dynamics code CPMD. We found that the critical density, temperature, and pressure are equal to 0.44 g/cm(3), 7963 K, and 0.35 GPa, respectively. At the critical point, the system is rarefied, coupled, and degenerate. The shear-viscosity and the self-diffusion were estimated at the critical point. Using the Kubo-Greenwood formula, we obtained the electrical conductivity, the absorption coefficient, the index of refraction, and the reflectivity at the critical point. We followed Mott's ideas to study the metal-nonmetal transition related to the critical point. Our method can be useful to investigate phase transition and the critical point of metals