Journal of Quantitative Spectroscopy and Radiative Transfer
Comparing plasma population kinetics codes: Review of the NLTE-3 Kinetics Workshop
Introduction
During the last decade, the advances in hardware manufacture and scientific software resulted in the development of numerical simulation codes which are becoming increasingly complex. As a result, verification and validation (VV) of such codes are now considered mandatory for establishing credibility of computational results [1]. This approach is especially important for the modeling of plasma population kinetics which includes diverse physical processes and numerous atomic states, sometimes reaching tens and hundreds of thousands. In turn, lack of non-trivial analytic results and the paucity of reliable experimental data emphasize the need for well-designed computational benchmark experiments providing the necessary testbeds for comparison of different codes.
The VV procedures for plasma population kinetic codes have been developed since the mid-1990s when the first non-LTE kinetics workshop, NLTE-1 [2], was organized in 1996. NLTE-1 adopted the format used with success in the LTE Opacity Workshops [3], namely prior selection of test cases and subsequent submission of data in well-defined formats, followed by comparison and analysis during the workshop. The substantial disagreement between codes, for the mean ion charge —a coarse, averaged parameter—clearly demonstrated the need for such meetings. The workshop brought to light the role of autoionizing states in kinetics, and the importance of high- states in recombination. NLTE-2 [4], held in 2001 as a Virtual Workshop over the Internet, concentrated on emissivity comparisons. The large spread in the results, much greater than observed in the Opacity Workshops, motivated further investigation into the level populations. As a result, NLTE-3 was held during December 1–5, 2003, at the National Institute of Standards and Technology (NIST). As with NLTE-1, NIST provided a unique venue for fruitful discussions between about 20 researchers from 5 different countries.
In this review, we present the organization of the meeting, the set of cases studied, and the results. Some of the material has been presented in a more condensed form [5]. For conciseness the data appear here mostly in the form of graphs for , although additional results are shown as appropriate. One of the goals of the meeting was to develop tools for meaningful code comparisons. The final section of the paper is therefore devoted to a discussion of useful code diagnostics.
Section snippets
Organization of the workshop and contributions
A preliminary set of test cases was defined in October 2002 by the Workshop organizers. The initial announcement was sent out to potential participants in January 2003, and a Web site (http://www.plasma-gate/weizmann.ac.il/fnralch/NLTE3) was set up. The list of cases was refined as a result of feedback from participants. The defining document was mailed out in May 2003, with a deadline for submission of October 1, 2003. About two weeks before the workshop convened, the data were shared between
Case definitions and results
In this section the motivations and selected illustrative results are presented for all cases. The relatively large amount of submitted data (hundreds of megabytes for a total of 92 steady-state and 3 TD cases) does not permit us to provide a comprehensive description of the results obtained. A WWW database containing all submitted data is currently under development at NIST and should become publicly available in mid-2005.
The cases were completely defined by electron temperature and
Comprehensive and meaningful data comparisons
One of the goals of this workshop was to try and define consistent and intelligent methods for the comparison of code results. This is especially important since the plasma kinetics codes are quite diverse in physical processes and assumptions implemented and also in computational details. The global plasma parameters and ion characteristics can easily be compared directly; however, the level of generalization in description of atomic structure may be quite different from code to code. Thus, we
Conclusions
The third NLTE Kinetics workshop succeeded in bringing together a representative set of researchers and codes from the dense plasma physics community. Its goal was to develop practical and reliable tools for benchmarking plasma kinetics codes and for providing a better understanding of the underlying physical issues. A number of codes have now become sufficiently mature that useful comparisons can be effected. Fair agreement was found for L-shell and closed shell-cases. Away from these
Acknowledgements
Organizational and financial support from NIST (W.L. Wiese) and NRL (A. Schmitt) are gratefully acknowledged.
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