Session K11: Focus Session: Atomic Physics in Plasmas

A Platform for Opacity Experiments on the NIF

Radiation-hydrodynamic simulations of the sun, stars, and other high-energy-density plasma require knowing the x-ray opacities of the constituent materials. These opacities are generally calculated from theory. There is little spectrally resolved, experimental opacity data of plasma at the temperatures and densities found in the interior of stars. Recent opacity experiments on the Sandia National Laboratories Z machine have shown up to factors of two discrepancies between theory and experiment. To help resolve this discrepancy an experimental platform for doing opacity experiments has been developed on the National Ignition Facility (NIF). The effort has been led by scientists from both Los Alamos and Lawrence Livermore National Laboratories with support from Sandia, the Nevada National Security Site, General Atomics, the Laboratory for Laser Energetics, and the University of Texas at Austin. Initial opacity data has been taken on both iron and oxygen at temperatures around 150 eV and densities around 10²² electrons/cc. This data will be compared with theory and with similar data obtained at the Z facility. This data may have implications for modeling the structure of the sun and the age of white dwarf stars. The current status and future plans for the experiments will be presented.    

Transient spatial localization in dense plasmas

Dense plasmas are prevalent throughout the universe, being present in all types of stars, the giant planets and in the laboratory [1]. In dense plasmas, a variety of anomalous phenomena have been experimentally observed on the electron-ion collisional ionization cross section [2] and rate [3] and opacity [4]. Yet the physics behind these anomalous phenomena is not clear. In addition to the plasma screening, we proposed a concept of transient spatial localization [5] to understand the physics and explain the experiments. In dense plasmas, the continuum electrons tend to be localized in finite space due to the frequent collision with particles and therefore they are not completely free in the environment. We developed a theoretical framework to deal with the effect of localization on the microscopic atomic processes of photoionization and electron impact ionization and macroscopic properties of opacity. Our findings show that the proposed concept is helpful to understand the physics behind the experiments.

[1] S. M. Vinko et al., Nature 482, 59 (2012).

[2] S. M. Vinko et al., Nat. Commun. 6, 6397 (2015).

[3] Q. Y. van den Berg et al., Phys. Rev. Lett. 120, 055002 (2018).

[4] J. E. Bailey et al., Nature 517, 56 (2015).

[5] P. F. Liu et al., Commun. Phys. 1, 95 (2018).   

The Opacity Project: R-Matrix Calculations of Opacities (RMOP)

The R-matrix methodology is extended to complete calculations of astrophysical and laboratory plasma opacities. The aim is to solve outstanding problems such as the solar interior opacity problem to reconcile new chemical abundances of common volatile elements C, N, O and Ne that are up to 50% lower than previously determined. In addition to complex atomic radiative processes using the Breit-Pauli R-Matrix (BPRM) method, a new theoretical and computational method has been developed for the heretofore unsolved problem of plasma broadening of intrinsic autoionizing resonances computed in ab initio close coupling calculations. Extrinsic broadending mechanisms such as collisional, thermal (Doppler), and electric microfields (Stark) are included, as well as due to core-excitations and free-free transitions. Resutls are presented for Fe XVII, an important contributor to opacity at the base of the solar convection zone. Details of RMOP calculations for other iron ions are presented the poster on the Iron Project by Nahar et al. at this meeting.   

Time dependent diagnostics of laser ablated silicon plasma using fully relativistic electron excitation cross-section of Si+2

Laser ablated silicon plasma has significant application in pulse laser deposition, etching processes etc. [1,2]. In view of these, we have developed a time dependent collisional radiative (CR) model for the diagnostics of laser ablated Silicon plasma. In such plasmas, the electron and gas temperatures are low and electron impact excitation (EIE) and de-excitation processes are dominant. For a reliable CR model, the EIE cross-sections for a large number of transitions of Si⁺² are required [3] which we have calculated for the first time using Relativistic Distorted Wave (RDW) approach[4]. The transitions considered are from ground state 3s² (J = 0) to the fine structure levels of the 3s3p, 3p², 3s3d, 3s4s, 3s4p, 3s5s, 3s4d , 3s4f, 3s5p, 3s5d and 3s5f configurations as well as among the upper levels. We also include radiative decay, ionization, and recombination processes in the CR model. We have validated our model with the optical emission spectroscopic (OES) measurement of Wang et al.[5] on laser induced silicon plasma. The obtained plasma parameters viz. electron temperature, electron density etc. are compared with the corresponding experimental values. The details of our EIE cross-section calculations, full description of the CR model along with the obtained results will be presented in the conference.   

[1]      Schwan J  et al., 2022 J. Phys. D. Appl. Phys. 55 094002

[2]      Traversari S et al., 2021 Plant Physiol. Biochem. 166 1014

[3]      Boffard J B et al., 2018  Advances in Atomic, Molecular and Optical Physics  67 1

[4]      Baghel S S, Gupta S, Gangwar R K and Srivastava R 2019 Plasma Sources Sci. Technol. 28 115010

[5]      Wang K et al., 2020 Phys. Plasmas 27 063513   

Ion-atom-atom three-body recombination in cold hydrogen and deuterium plasmas

We present a detailed study about three-body recombination in cold hydrogen and deuterium plasmas based on classical trajectory calculations in hyperspherical coordinates [1,2]. In particular, we study A + A + A⁺→ and A₂⁺ + A  and A + A + A⁺→ and A₂  + A⁺ reactions, where A=H or D. Our results show that the A + A + A⁺→ and A₂⁺ + A reaction is the dominant channel in energies ranging from 100 to 10⁵K. However, when the collision energy is of the same order of magnitude as the dissociation energy of the neutral molecule, A₂, the three-body recombination rate shows a drastic reduction, and a new trend appears. These results are relevant for tokamak physics because they may affect the ultimate divertor detachment and recycling stages.   

Observation and Identification of Extreme Ultraviolet Spectra from Ca-like Nd XLI to Na-like Nd L

Extreme-ultraviolet spectra of M-shell transitions in highly charged Ca-like Nd⁴⁰⁺ to Na-like Nd⁴⁹⁺ has been measured at the electron beam ion trap (EBIT) facility at the National Institute of Standards and Technology ^[1]. To create the corresponding charge states,  the electron beam energy was varied between 3.60 keV and 12.02 keV. The spectra were observed between 2.67 nm and 17.30 nm using a flat-field grazing incidence spectrometer ^[2]. The spectra calibrations were based on well-known lines from neon, xenon, iron, oxygen, and argon produced in the EBIT. Simulated spectra generated with a detailed collisional-radiative (CR) model ^[3] of the non-Maxwellian EBIT plasma were used to classify the observed lines. Forty-seven spectral lines were identified for the first time. The total uncertainties included contributions from estimated systematic uncertainties, statistical uncertainties from fitting, and calibration uncertainties. The measured Nd spectra and the tabulated spectral lines with the associated line identifications will be presented and discussed.

1. J. D. Gillaspy, Phys. Scr. T71, 99 (1997). 2. B. Blagojevic et al., Rev. Sci. Instrum.76, 083102 (2005). 3. Yu. Ralchenko and Y. Maron, J. Quant. Spectr. Rad. Transfer 71, 609 (2001).   

Toward a frame-work for calculating comprehensive electron collision data sets for low-temperature plasma modeling

Modeling low-temperature non-equilibrium plasmas with particle-in-cell Monte Carlo and Boltzmann solver codes requires comprehensive sets of collision cross sections. However, these data sets are particularly difficult to calculate for low-temperature plasmas, where near-neutral atoms and molecules, and excited-state species are abundant. As a result, while some data reside in specialized databases, in general, comprehensive sets of cross sections do not exist in the literature.

 

Recently we have developed a universal frame-work for quickly calculating electron-impact excitation and ionization cross sections of ground and excited state species of atoms, molecules and ions. Here we apply the approach to a range of air diatomic molecule species (such as N₂ and O₂), and present comparisons of the new results with benchmark theory and experiment in order to quantify the uncertainties.