LTE Plasmas Research Articles

Full non–LTE spectral line formation

Radiative transfer out of local thermodynamic equilibrium (NLTE) has been increasingly adressed, mostly numerically, for about six decades now. However, the standard NLTE problem most often refers to the only deviation of the distribution of photons from their equilibrium, that is to say a Planckian distribution. Hereafter we revisit after Oxenius (1986, Kinetic theory of particles and Photons – Theoretical Foundations of non–LTE Plasma Spectroscopy, Springer) the so-called full NLTE problem, which considers coupling and therefore solving self–consistently for deviations from equilibrium distributions of photons as well as for massive particles constituting the atmospheric plasma.

Calculation of two-temperature plasma composition: I. Mass action law methods and extremum searching methods

Composition of species is one of the key parameters in calculating the plasma properties and assessing the plasma chemical processes. Unlike the calculation of LTE plasma composition, the calculation of 2T plasma composition still remains in dispute. Different researchers have chosen different methods which lead to different results. In this work, we compare two kinds of methods for calculating 2T plasma composition: the mass action law methods and extremum searching methods. The former methods include the two described by Potapov and van de Sanden et al mass action laws, respectively. The latter methods include those of searching minimum Gibbs free energy and maximum entropy of a plasma system respectively. The entropy maximization method is first reported in this work and has the same power as the commonly used Gibbs free energy minimization method. We demonstrate both mathematically and numerically that the method of 2T Gibbs free minimization is completely the same as the 2T Potapov mass action law, and the method of 2T entropy maximization is exactly consistent with the 2T van de Sanden et al mass action law if we assume that the term μi/Ti is independent of the number density ni. It is also found that the assumption of number density-independent μi/Ti mainly affects the density of charged particles. When the plasma reaches the LTE state, the composition obtained by the methods of entropy maximization with and without this assumption will be exactly the same.

Influence of Resonance Radiation Transport on Chemical Equilibrium in an Argon Arc

Deviations from chemical equilibrium in argon arc plasma are analysed by means of collisional-radiative model. Corresponding comprehensive kinetic scheme has been developed and applied form study of free-burning arc at the conditions typical for welding applications. While the natural lifetime have been used for radiation emitted from highly excited argon states, the resonance radiation was described taking into account the radiation transport effects. Resulting spatial distributions of excited argon atoms are compared for the cases of LTE and two-temperature plasma using different approaches for the description of the resonance radiation transport.

NLTE opacity calculations: C-Si and C-Ge mixtures

The opacity is an important issue in the knowledge of the radiative properties of ICF and astrophysical plasmas. We present the opacity of dopants (silicon, germanium) embedded in the ablator of some ICF capsules. In recent works, Hill and Rose calculated the opacity of silicon in LTE and non-LTE plasmas, while Minguez and co-workers focused on the opacity of carbon.We have used the Cowan code to calculate the atomic structure of carbon, silicon and germanium in various ionic stages. The cross-sections of atomic processes (collisional excitation, collisional ionization) are obtained by fitting the values given by the code FAC to the Van Regemorter-like formulas of Sampson and Zhang. A corrected Gaunt factor is then obtained. A collisional-radiative code was developed in order to obtain the ionic populations, the level populations and the opacity. Line broadening and line shift are taken into account. The ionization potential depression is included in our calculations. The effect of a radiation field on the opacity is examined.

The effect of first order superconfiguration energies on the opacity of hot dense matter

We have recently completed developing a new super transition array (STA) code for calculating absorption and emission spectra of LTE plasmas. The code follows the theory of Bar-Shalom et al. with various improvements. In this work we focus on the first order correction for the Boltzmann populations for which traditional calculations can be very costly. We present here a method faster by an order of magnitude than the traditional method. We then investigate the effect of this correction on the opacity spectra of several elements. Finally we interpret results of a recent opacity experiment on gold plasma. A good agreement is reached.

The DAVROS opacity code: Detailed term accounting calculations for LTE plasmas

The radiative opacity of a plasma is a key parameter in understanding a diverse range of high energy density systems including inertial confinement fusion and astrophysics. The accurate calculation of opacity is hampered by the potentially enormous number of ionic configurations, the detailed internal structure of each giving rise to the term structure, and the line broadening models which must typically be applicable across densities from 10−6 g/cc to several times solid.The DAVROS opacity code (Detailed Accounting of Various configurations for Radiative Opacity Spectra) has been developed at AWE over recent years, and by making use of the large scale High Performance Computing (HPC) systems, implements a number of models and algorithms aimed at a more direct calculation of opacities than has traditionally been feasible. The results are both more physically based and spectrally accurate than codes based upon statistical accounting approximations. In particular, the bound–bound line spectrum can be explicitly calculated using the Detailed Term Accounting (DTA) method, which, although computationally expensive, is necessary to understand the true frequency dependent structure of the opacity spectrum. Additionally, M. Baranger (1958) [1] quantum mechanical formulism of pressure (or electron impact) broadening is implemented, thereby representing a significant improvement upon alternative approximations.We present a summary of some of the key issues, models and algorithms in the code, and show some representative results, including comparisons with opacity measurements made at AWE.

The role of visible and resonance radiation in the energy balance of LTE plasma in argon

Energy losses due to radiation in an LTE arc plasma in argon are investigated in the temperature range from 5000 to 15 000 K. Calculations of the radiation transport require us to know the absorption spectra; for that purpose, free–free, bound–free and bound–bound transitions are taken into account. The energy losses due to resonance transitions with large absorption coefficients are analyzed. The transport of visible radiation with small absorption coefficients is calculated by accurate integration over a whole spectrum. The resonance radiation transport in LTE plasma is described by the Biberman–Holstein equation, which is usually used in non-equilibrium plasma. As the temperature grows, radiation processes become the main channel of energy losses. It is shown that, despite the resonance radiation being trapped, the energy losses due to resonance radiation escape can achieve 10% of the total radiation losses.

Identification of X-ray spectra in the Na-like to O-like rubidium ions in the range of 3.8–7.3 Å

The X-rays emitted by a rubidium plasma source created by the PHELIX laser at an intensity of about 6×1014W/cm2 were studied. The lines have been measured using Bragg crystals in the wavelength range between 3.8 and 7.3Å and identified by means of a numerical method developed to describe highly charged rubidium ions in LTE plasma. The experimental plasma temperature, density and charge state distributions have been estimated using non-LTE codes such as CHIVAS and AVERROES. The LTE plasma temperature and density used in the calculations are those allowing to reproduce the calculated NLTE charge state distribution. In order to optimize the use of computational resources, a criterion is established to select the configurations contributing most to the spectra among all those obtained in detailed level accounting based on the MCDF code. Seventy Rb X-rays have been identified among which forty-nine are reported for the first time. The capabilities of our method are demonstrated by the good agreement of our identifications with previously published data when available.

LTE plasma reactors for materials conversion

Plasma aided materials conversion is the use of charged particles for producing new materials with unusual and superior properties or to convert unwanted materials such as waste into environmentally friendly products. The near LTE plasma (in atmospheric or reduced pressure) is a nice tool for engineering of materials which in consequence upgrades the performance of most of the consumer goods. The paper presents a few samples of the own research results related to the design and use of some plasma reactors for synthesis of nanomaterials and hazardous materials destruction with their conversion into valuable products.

The ultracool dwarf DENIS-P J104814.7-395606

We have obtained an XMM-Newton observation and a broad-band spectrum from the ultraviolet to the near infrared with X-Shooter for one of the nearest M9 dwarfs, DENIS-P J1048-3956 (4pc). We integrate these data by a compilation of activity parameters for ultracool dwarfs from the literature with the aim to advance our understanding of these objects by comparing them to early-M type dwarf stars and the Sun. Our deep XMM-Newton observation has led to the first X-ray detection of DENIS-P J1048-3956 (log Lx = 25.1) as well as the first measurement of its V band brightness (V = 17.35mag). Flux-flux relations between X-ray and chromospheric activity indicators are here for the first time extended into the regime of the ultracool dwarfs. The approximate agreement of DENIS-P J1048-3956 and other ultracool dwarfs with flux-flux relations for early-M dwarfs suggests that the same heating mechanisms work in the atmospheres of ultracool dwarfs, albeit weaker as judged from their lower fluxes. The observed Balmer decrements of DENIS-P J1048-3956 are compatible with optically thick plasma in LTE at low, nearly photospheric temperature or optically thin LTE plasma at 20000K. Describing the decrements with CaseB recombination requires different emitting regions for Halpha and the higher Balmer lines. The high observed Halpha/Hbeta flux ratio is also poorly fitted by the optically thin models. We derive a similarly high value for the Halpha/Hbeta ratio of vB10 and LHS2065 and conclude that this may be a characteristic of ultracool dwarfs. We add DENIS-P J1048-3956 to the list of ultracool dwarfs detected in both the radio and the X-ray band. The Benz-Guedel relation between radio and X-ray luminosity of late-type stars is well-known to be violated by ultracool dwarfs. We speculate on the presence of two types of ultracool dwarfs with distinct radio and X-ray behavior.

Characterization of laser – induced plasmas by atomic emission spectroscopy

In this work, Laser-induced breakdown spectroscopy (LIBS) has been applied to characterization of plasmas generated in air at atmospheric pressure from a calcium hydroxide sample with a known concentration of Mg by using an infrared Nd:YAG laser. The influence of laser irradiance on plasma morphology and emission intensity was studied. Spatially-integrated intensities of Mg I-II lines along the line-of-sight were measured for different laser energies and delay times. The plasma temperature and the electron density were determined in each case by using and algorithm that calculates the optical thickness of the spectral lines and reproduces their experimental profiles in a framework of an homogeneous plasma in LTE that takes into account the effects of self-absorption. The results obtained showed the usefulness of this approach to provide additional information retrieved from the optical thickness of spectral lines for plasma characterization in LIBS experiments.

Computation of thermodynamic plasma properties: A simplified approach

An approximate method for calculating the electronic partition functions of atomic systems is reported. The method is based on the idea of combining a multitude of atomic energy levels into two or three grouped levels. Dimensionless formulation suitable for practical calculations is presented. Application to real systems shows that two grouped levels are enough to model hydrogen-like and noble gas atoms, whereas three grouped levels are required to describe atoms with low lying excited states. Two methods for the calculation of degeneracy and energy values of the grouped levels are investigated. Representative mono-atomic LTE plasma properties calculations are reported. The results agree with accurate computations using partition functions that include several thousands energy levels.

Resonantly laser induced plasmas in gases: The role of energy pooling and exothermic collisions in plasma breakdown and heating

The present work is a systematic experimental study of the plasma formation in cesium vapor induced by a continuous laser tuned to the resonance transition 6 S 1/2–6 P 3/2. Taking into account the measured absolute population densities of Cs ground and excited state atoms as well as the electron densities derived from Stark broadening of the Cs lines, complete local thermodynamic equilibrium in the laser-produced plasma was found for laser power densities ≈ 10 Wcm − 2 at cesium ground state number densities of about 10 17 cm − 3 . Direct conversion of the excitation energy or parts of the excitation energy in exothermic collisions of laser-excited atoms is concluded to be the major process for atomic vapor heating and subsequent formation of LTE plasmas.

Maximum material thickness for extreme ultra-violet and X-ray backlighter probing of dense plasma

Extreme ultra-violet (EUV) lasers, X-ray lasers and other backlighter sources can be used to probe high-energy density materials if their brightness can overcome self-emission from the material. We investigate the maximum plasma thickness of aluminum, silicon and iron that can be probed with EUV or X-ray photons of energy 89–1243 eV before self-emission from the plasma overwhelms the backlighter output. For a uniform plasma, backlighter transmission decreases exponentially with increasing thickness of the material following Beer's law at a rate dependent on the plasma opacity. We evaluate the plasma opacity with the Los Alamos TOPS opacity data. The self-emission is assumed to be either that of a black body to arise from a plasma in LTE or to only consist of free–free and free–bound emission. It is shown that at higher plasma temperature (⩾40 eV), EUV radiation (e.g. photon energy=89 eV) can probe a greater thickness of plasma than X-ray radiation (e.g. photon energy=1243 eV).

A statistical approach for simulating detailed-line spectra

A general statistical model for simulating electric-dipolar lines in complex spectra of LTE or NLTE plasmas is presented. In this model, each transition array is represented by a pair of Gaussian subarrays, whose characteristics (average wavenumbers, strength distributions, and numbers of lines) change in a continuous way from LS (where one subarray vanishes) to jj coupling. The transition amplitude and the two-level energies of each line are picked at random in a joint triple distribution that takes into account the correlation between the line strengths and energies. This insures that the first and the second strength-weighted moments of the line energies are equal to those obtained in the DLA approach. Examples of application to the calculation of Rosseland and Planck mean opacities are given (Fe plasma in LTE, for Te = 20 eV and ρ = 10−2, 10−3, 10−4 g cm−3) which show very good agreement with the OPAL values.

Combination of the ionic-to-atomic line intensity ratios from two test elements for the diagnostic of plasma temperature and electron number density in Inductively Coupled Plasma Atomic Emission Spectroscopy

In Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) spectrochemical analysis, the MgII(280.270 nm)/MgI(285.213 nm) ionic to atomic line intensity ratio is commonly used as a monitor of the robustness of operating conditions. This approach is based on the univocal relationship existing between intensity ratio and plasma temperature, for a pure argon atmospheric ICP in thermodynamic equilibrium. In a multi-elemental plasma in the lower temperature range, the measurement of the intensity ratio may not be sufficient to characterize temperature and electron density. In such a range, the correct relationship between intensity ratio and plasma temperature can be calculated only when the complete plasma composition is known. We propose the combination of the line intensity ratios of two test elements (double ratio) as an effective diagnostic tool for a multi-elemental low temperature LTE plasma of unknown composition. In particular, the variation of the double ratio allows us discriminating changes in the plasma temperature from changes in the electron density. Thus, the effects on plasma excitation and ionization possibly caused by introduction of different samples and matrices in non-robust conditions can be more accurately interpreted. The method is illustrated by the measurement of plasma temperature and electron density in a specific analytic case.

RCI Simulation for EUV spectra from Sn ions

Using the relativistic-configuration-interaction atomic structure code, RCI simulations for EUV spectra from Sn10+, Sn11+ and Sn12+ ions are carried out, where it is assumed that each ion is embedded in a LTE plasma with the electron temperature of 30 eV. To make clear assignment of the measured spectra, the value of the excitation energy limit, which is introduced to limit the number of excited states in the simulation, is changed to see the excitation-energy-limit dependence of the spectral shape. The simulated spectra are obtained as a superposition of line intensities due to all possible transitions between two states whose excitation energy from the ground state is lower than the excitation energy limit assumed. The RCI simulated spectra are compared to the spectra measured with the chargeexchange- collision experiment in which a rare gas such as Xe or He as a target is bombarded by a charge-selected tin ion. Applicability of the LTE model to a decay model in the charge exchange collision experiment is also discussed.

Non-equilibrium plasma kinetics: a state-to-state approach

State-to-state approaches are used to shed light on (a) thermodynamic and transport properties of LTE plasmas, (b) atomic and molecular plasmas for aerospace applications and (c) RF sustained parallel plate reactors. The efforts made by the group of Bari in the kinetics and dynamics of electrons and molecular species are discussed from the point of view of either the master equation approach or the molecular dynamics of elementary processes. Recent experimental results are finally rationalized with a state-to-state kinetics based on the coupling of vibrational kinetics with the Boltzmann equation for the electron energy distribution function.

Dissociation reactive thermal conductivity in a two-temperature plasma

Dissociation reactive thermal conductivity (DRTC) is the transfer of dissociation energy in plasma with temperature and, therefore, composition gradient. In the existing theories, calculation of the DRTC coefficient consists of the calculation of diffusion coefficients and plasma composition. The heat flux is then calculated by assigning to every molecule the dissociation energy and by the multiplication of the molecule flux density by this energy.This approach, correct for the LTE plasma, is not adequate for the non-equilibrium two-temperature plasma: it does not allow one to separate the total DRTC coefficient into two components responsible for the heat transfer by electrons and heavy particles (atoms, molecules, ions). Only at LTE, during atom reassociation, do the electrons recuperate the energy they spent during the molecule dissociation. Therefore, in order to separate these two components of DRTC, the kinetics of the dissociation–reassociation processes should be considered. This is done in this paper for nitrogen plasma at atmospheric pressure. Fe, the electron fraction of the total DRTC coefficient, was calculated for Te (electron temperature) in the range 0.4–1.0 eV and Th (heavy particles temperature) from 0.2 eV to Te. It is shown that Fe depends mostly on the electron temperature and increases with increasing electron temperature.

Modelling of a Spherical Electric Discharge at Atmospheric Pressure Under Higher Modes of Incident Microwaves

We present a spherical microwave discharge model, which takes into account the higher modes of the incident electromagnetic waves. The discharge model is constructed on the basis of the channel approximation of the LTE plasma. Numerical experiments are carried out for the discharge in an argon at atmospheric pressure. Results are presented for the characteristics of the discharge plasma against the external parameters (the size of the discharge chamber and the applied electromagnetic field mode, power and frequency).