Opacity calculations: including more and more states

One of the main challenges of opacity calculations is the balance between accuracy and completeness, i.e. the search for the best compromise between the use of elaborate and precise physical models, and the number of states (levels, configurations, superconfigurations) included in the computation. Statistical methods have the advantage of representing a large number of states, but the corresponding spectra are often too crude for some applications. In this work, we discuss the strengths and drawbacks of different approaches aiming at reaching the best compromise between completeness and accuracy.

Firstly, we propose to start from global approaches and refine them, either by combining statistical approaches such as Unresolved Transition Arrays, Spin-Orbit Split Arrays or Super Transition Arrays (STA), with Detailed Line Accouting (DLA) calculations [1], or by revealing the underlying structure of the spectra as in the Configurationally-Resolved Super Transition Array (CRSTA) [2] or Partially-Resolved (PR)-CRSTA [3] approaches.

In a second step, we try to go the other way, starting with detailed calculations and including (even in an approximate manner) more spectral features.  In multiply-charged ion plasmas, a significant number of electrons may occupy high-energy orbitals. These Rydberg electrons, when they act as spectators, are responsible for a huge number of satellites of X-ray absorption or emission lines, yielding a broadening of the red wing of the resonance lines. We suggest to model the perturbation induced by the Rydberg spectators as a partial DLA calculation, their effect being included through a shift and width, expressed in terms of the canonical partition functions, which are key-ingredients of the STA model [4].

 

[1] G. Hazak and Y. Kurzweil, High Energy Density Phys. 8, 280 (2012).

[2] Y. Kurzweil and G. Hazak, Phys. Rev. E 94, 053210 (2016).

[3] J.-C. Pain, F. Gilleron and T. Blenski, Laser Part. Beams 33, 201 (2015).

[4] J.-C. Pain and F. Gilleron, High Energy Density Phys. 15, 30 (2015).