J.K. Nash

Review of the NLTE kinetics code workshop

Abstract The first workshop to compare the output from Non-LTE kinetics codes using a standardized set of problems is reviewed. The background for the workshop is discussed and the planning is briefly outlined. The participation and general constraints for the workshop are given. Next the defined cases and a motivation for each case is presented. Some results from the workshop are shown which indicate both the utility of the workshop and some of the difficulties faced by those involved in NLTE kinetics modeling. Plans for the next workshop are discussed in the conclusion.

ESTIMATING PLASMA TEMPERATURES FROM TRANSMISSION SPECTRA

Abstract Recent laser-produced plasma experiments have relied on spectroscopic comparisons with models to infer plasma temperatures. The models use an experimentally determined value for the matter density as input and treat the temperature as an adjustable parameter to obtain a best fit to the experimental absorption spectrum. However, uncertainties in the ionization balance theories lead to inferred temperatures that are model dependent. We report results of a new approach which combines high-quality atomic data with an ionization balance obtained from systematic expansions of the grand canonical ensemble. The latter avoids the ad hoc cutoffs required in free energy minimization schemes and includes Coulomb corrections usually neglected in other models. Comparisons to experimental spectra show excellent agreement.

Spectroscopic measurements of Rosseland mean opacity

Abstract The first quantitative measurement of photoabsorption in the region determining Rosseland and Planck mean opacity, is obtained for an x-ray heated iron plasma, using novel techniques and instrumentation. The plasma density of 0.0113 ± 0.0013 g/cm3 and temperature of 59 ± 3 eV are accurately constrained experimentally by imaging plasma expansion and observing and modeling absorption in sodium dopant ions. The measured iron absorption spectrum is compared with several newly developed opacity models. The data constrains Rosseland and Planck group means with of order 15% precision. This is the first quantitative experimental certification of opacity models germane to radiative transfer in LTE plasmas.

THE DEVELOPMENT OF A DISTRIBUTED COMPUTING ENVIRONMENT FOR THE DESIGN AND MODELING OF PLASMA SPECTROSCOPY EXPERIMENTS

The design and analysis of plasma spectroscopy experiments can be significantly complicated by relatively routine computational tasks arising from the massive amount of data encountered in the experimental design and analysis stages of the work. Difficulties in obtaining, computing, manipulating and visualizing the information represent not simply an issue of convenience -- they have a very real limiting effect on the final quality of the data and on the potential for arriving at meaningful conclusions regarding an experiment. We describe ongoing work in developing a portable UNIX environment shell with the goal of simplifying and enabling these activities for the plasma-modeling community. Applications to the construction of atomic kinetics models and to the analysis of x-ray transmission spectroscopy will be shown.

Time-resolved temperature measurements of radiatively heated tamped MgO foils by Kα absorption spectroscopy

Abstract In this paper we present time-resolved temperature measurements of a radiatively heated, thin tamped foil via Kα absorption spectroscopy. The sample foil was 1500 A MgO, tamped on both sides with 1000 A paralyene, and was heated by soft x-rays transmitted through the back side of a separate laser-irradiated 1500 A Au foil. The resulting plasma was then backlit by a separate laser-irradiated front-surface U foil, providing a time-resolved K -shell absorption spectrum over the duration of the heating phase. We compare the resulting data to synthetic spectra obtained from detailed simulations of the heating and expansion of the tamped foils, and find poor agreement. Much better agreement is obtained when we determine the temperatures through best-fit absorption spectra which assume temperatures ∗> 35% higher than predicted; the reason for this discrepancy is not yet understood.

Spectroscopic temperature measurements of non‐equilibrium plasmas

The characterization of laser‐produced plasmas has required the application of spectroscopic techniques to non‐standard conditions where kinetics models have not been extensively tested. The plasmas are produced by the Nova laser for the study of inertial confinement fusion, can be mm in size, and evolve on sub‐nanosecond time scales. These targets typically achieve electron temperatures from 2–4 keV and electron densities of 1020–1022 cm−3. We have measured the electron temperature of two types of targets: bags of gas and hohlraums, Au cylinders with laser entrance holes in the flat ends. By comparing data from different targets, we examine the time‐dependence of spectroscopic plasma diagnostics.