M. Sherrill

Comment on “Large Enhancement in High-Energy Photoionization of Fe XVII and Missing Continuum Plasma Opacity”

Recent R-matrix calculations claim to produce a significant enhancement in the opacity of Fe XVII due to atomic core excitations [S. N. Nahar & A.K. Pradhan, Phys. Rev. Letters 116, 235003 (2016), arXiv:1606.02731] and assert that this enhancement is consistent with recent measurements of higher-than-predicted iron opacities [J. E. Bailey et al., Nature 517, 56 (2015)]. This comment shows that the standard opacity models which have already been directly compared with experimental data produce photon absorption cross-sections for Fe XVII that are effectively equivalent to (and in fact larger than) the new R-matrix opacities. Thus, the new R-matrix results cannot be expected to significantly impact the existing discrepancies between theory and experiment because they produce neither a large enhancement nor account for missing continuum plasma opacity relative to standard models.

Spectroscopy diagnostics of a low temperature laser ablated LiAg plasma plume

In experiments performed at Sandia National Laboratories, laser-generated LiAg plasma plumes were produced by irradiation of solid targets using a 10 ns pulse duration, 1×108u200aW/cm2 intensity Nd YAG laser. Time- and spatially resolved (along a direction normal to the target’s surface) optical spectra were recorded with a framing spectrograph. The observed spectra consist of optical line emission in Li and Ag atoms. Evidence of ions in the plume is suggested by the presence of a forbidden line and Stark-broadened line shapes. A spectroscopic model based on time-dependent collisional-radiative atomic kinetics that self-consistently calculate the Li and Ag level populations in conjunction with detailed line shapes and radiation transport is used to interpret the data. From this analysis, temperature, density, and ionization in the plume as a function of time and position along the normal to the target surface are extracted.

Detailed atomic kinetics model for the spectroscopic analysis of laser-ablated plasma plumes

Atomic kinetics and spectral modeling have revealed that level populations of plasma atoms in laser-ablated plumes may behave in a time-dependent manner, i.e. far from Local Thermodynamic Equilibrium, and that cascading population mechanisms can lead to long-lived atomic line emission. The time-scales associated to this phenomena and the interpretation of spectral data critically depend on the details of the atomic kinetics model and the quality of the rate coefficients. In order to generate accurate atomic data for neural atoms and low-charge ions present in plasma plumes, a semi-empirical techniques has been implemented in the Los Alamos atomic structure and electron scattering codes. This procedure has allowed neutrals with complex atomic structure--such as those atoms from elements often used in industrial applications--to be calculated with spectroscopic quality. Details of the atomic kinetics model for the case of a Li-Ag plasma plume and the rates generated with this new procedure are presented and discussed.

Characterization of a laser-generated LiAg plasma plume

Time- and spatially-resolved spectroscopy in conjunction with detailed modeling constitutes a powerful technique for the understanding of plasma plume dynamics. To this end, in a series of experiments performed at Sandia National Laboratories, laser-generated LiAg plasma plumes were produced by irradiation of solid targets using a Nd laser. Time- and spatially-resolved (along a direction normal to the targets surface) optical spectra were recorded with a framing spectrograph. In order to limit gradients along a direction perpendicular to the target normal, targets with strips of LiAg coated on top of Pt were used. The PT plume collisionally confines the LiAg, thus reducing the LiAg lateral expansion. This technique allows a better characterization of the LiAg plasma. The spectra displays line transitions in Li and Ag atoms, and evidence of ions in the plume is suggested by the presence of forbidden lines and Stark-broadened line shapes. A spectroscopic model based on collisional-radiative atomic kinetics, detailed line shapes, and radiation transport calculations is used to interpret the data. From this analysis temperature, density and ionization in the plume as a function of time and position along the normal to the target surface can be extracted.