Plasma In Local Thermodynamic Equilibrium Research Articles

Self-Calibrated Laser-Induced Breakdown Spectroscopy for the Quantitative Elemental Analysis of Suspended Volcanic Ash.

Real-time analysis of fine ash in volcanic plumes, which represent magma fragments expelled from the crater during explosive eruptions, is a valuable tool for volcano monitoring and hazard assessment. To obtain the chemical characterization of the juvenile pyroclastic material emitted in volcanic plumes, many analytical techniques can be used. Among them, laser-induced breakdown spectroscopy (LIBS) is the one that can most easily be adapted to advanced applications in extreme environments. In this paper, LIBS experiments based on self-calibrated approaches are used to determine the elemental composition of suspended volcanic ash. To simulate the conditions of dispersed volcanic ash in the atmosphere, different sizes of volcanic ash samples are suspended in the air by laser-induced shockwaves in a dedicated chamber, and a parametric study is carried out to establish the optimal experimental conditions for recording usable plasma emission spectra for each ash size. The quantitative analysis is performed using a self-calibrated analytical method, including calibration-free LIBS, which is based on the calculation of the spectral radiance of a uniform plasma in local thermodynamic equilibrium. The method accounts intrinsically for self-absorption since it modifies the intensity of spectral lines and thus leads to an underestimation of the elemental fraction. An intensity calibration of the spectra based on the measurements of Fe lines intensities was also used in this work to deduce the apparatus response from the spectrum itself and avoid the use of standard calibration lamps. Results demonstrate the potential of real-time measurements of elemental fractions in volcanic ash with good agreement with the literature composition.

Extension of the Boltzmann plot method for multiplet emission lines

The Boltzmann plot method is widely used to determine the temperature of laser induced plasma. It involves the use of individual lines that are not easy to find in complex spectra and/or in the spectral range available. If the number of such lines is not enough to build a reliable Boltzmann plot, overlapping lines are often used, which are separated by software. However, line separation is a rather imprecise procedure, which, in addition, requires significant computational costs. This study proposes an extension of the Boltzmann plot method that allows a specific group of unresolved lines to be included in a Boltzmann plot without the need to separate them. This group of lines are multiplets, lines of the same element with similar upper and lower transition states. The multiplet lines along with the individual lines are included in the algorithm, which also includes a correction for self-absorption and is used to determine the plasma temperature. The algorithm is tested on synthetic spectra which are consistent with the model of a homogeneous isothermal plasma in local thermodynamic equilibrium and is shown to be superior to the standard Boltzmann plot method both in more accurate determination of the plasma temperature and in a significant reduction in the computational time. The advantages and disadvantages of the method are discussed in the context of its applications in laser induced breakdown spectroscopy.

Continuous absorption and emission of light by the heteronuclear rare gas (ArXe)+ and (KrXe)+ ions

Studies of radiative transitions between ground and charge transfer excited terms of heteronuclear rare-gas (ArXe)+ and (KrXe)+ ions with small and moderate dissociation energies (D0 ArXe+ =179 and D0 KrXe+ =389 meV) are reported. Potential energy curves and dipole transition matrix elements are evaluated using ab initio calculations based on the CASSCF method followed by the NEVPT2 treatment and perturbative description of relativistic effects as implemented in the ORCA suite. We develop an efficient approach for the calculations of Boltzmann-averaged cross sections, rate constants, and absorption and emission coefficients associated with the integral contributions of bound-bound and bound-free transitions from a manifold of rovibrational states of the (RgXe)+ ion. A similar approach is used for the description of the free-free and free-bound transitions between different electronic terms of a quasimolecular (RgXe)+ ion temporarily formed during a collision of Ar or Kr atoms with Xe+ ions. Our method is based on the quantal version of the theory of non-adiabatic transitions in molecular and quasimolecular systems combined with the approximation of a quasicontinuum for rovibrational energy levels. This allowed us to obtain a semianalytic expression for the integral absorption properties particularly relevant at elevated temperatures. We perform a comprehensive analysis of the identified radiative processes in local thermodynamic equilibrium plasmas of rare gas Rg/Xe mixtures containing both atomic, Xe+, and molecular, RgXe+, ions and determine their role in the formation of the absorption and emission spectra in a wide range of wavelengths and gas temperatures. The results obtained are in good agreement with the available experimental data.

Measurement error due to self-absorption in calibration-free laser-induced breakdown spectroscopy

Self-absorption of spectral lines is known to lower the performance of analytical measurements via calibration-free laser-induced breakdown spectroscopy. However, the error growth due to this effect is not clearly assessed.Here we propose a method to quantify the measurement error due to self-absorption based on the calculation of the spectral radiance of a plasma in local thermodynamic equilibrium. Validated through spectroscopic measurements for a binary alloy thin film of compositional gradient, the method evidences that measurement performance lowering due to self-absorption depends on the spectral shape of the analytical transition and on the intensity measurement method. Thus, line-integrated intensity measurements of Stark broadened lines enable accurate analysis, even at large optical thickness, if line width and plasma size are precisely known. The error growth due to self-absorption is significantly larger for line shapes dominated by Doppler broadening and for line-center intensity measurements. The findings present a significant advance in compositional measurements via calibration-free laser-induced breakdown spectroscopy, as they enable straightforward selection of most appropriate analytical lines.

An improved equation of state for air plasma simulations

This work is concerned with the development of a novel, accurate equation of state for describing partially ionized air plasma in local thermodynamic equilibrium. One key application for this new equation of state is the simulation of lightning strike on aircraft. Due to the complexities of species ionization and interaction, although phenomenological curve fitting of thermodynamic properties is possible, these curves are intractable for practical numerical simulation. The large difference in size of the parameters (many orders of magnitude) and complexity of the equations means they are not straightforward to invert for conversion between thermodynamic variables. The approach of this paper is to take an accurate 19-species phenomenological model and use this to generate a tabulated dataset. Coupled with a suitable interpolation procedure, this offers an accurate and computationally efficient technique for simulating partially ionized air plasma. The equation of state is implemented within a multiphysics methodology which can solve for two-way coupling between a plasma arc and an elastoplastic material substrate. The implementation is validated against experimental results, both for a single material plasma and an arc coupled to a substrate. It is demonstrated that accurate, oscillation-free thermodynamic profiles can be obtained, with good results even close to material surfaces.

Quantification of surface contamination on optical glass via sensitivity-improved calibration-free laser-induced breakdown spectroscopy

We report quantitative analysis of manufacturing-induced trace contaminants of optical glass surfaces by laser-induced breakdown spectroscopy (LIBS). Therefore, spectra recorded with an echelle spectrometer coupled to a gated detector were analysed using a calibration-free LIBS approach based on the calculation of the spectral radiance of a plasma in local thermodynamic equilibrium. The measurements were carried out in experimental conditions that enable both accurate modelling of plasma emission and high sensitivity for trace element analysis. Depth-resolved measurements were performed by recording spectra for successive laser pulses applied to the same irradiation sites. Validated via inductively coupled plasma atomic emission spectroscopy for the bulk glass composition, the measurements evidence a surface contamination that originates from polishing during glass manufacturing. The measured penetration depths of the contaminants are discussed in the frame of the underlying mechanisms of surface contamination and related to the changes of the optical properties evidenced by ellipsometric measurements. Demonstrated here for optical glass, the sensitivity-improved calibration-free LIBS approach can be used to quantify contaminations of surfaces in all kind of technological applications.

Experimental and theoretical studies of Stark profiles of Ar I 696.5 nm spectral line in laser-induced plasma

We report the results of the Stark profile studies of the 696.543 nm Ar I spectral line in laser-induced plasma generated by a nanosecond Nd:YAG laser radiation at 532 nm in argon at reduced pressure. Plasma diagnostics was performed with the use of the laser Thomson scattering method, free from assumptions about the plasma thermodynamic equilibrium, its composition but also independently of plasma emission spectra. The profiles were investigated in wide range of electron densities and temperatures, from 2.81 · 1022 m−3 to 5.69 · 1023 m−3 and from 10 430 K to 73 400 K, respectively. Stark profiles were calculated using a semi-classical method as well as by N-body numerical simulations assuming a plasma in local thermodynamic equilibrium. Our studies show agreement within 20% between experimental Stark widths and those calculated by the semi-classical method while much bigger discrepancies were found with computer simulated ones. Importantly, Stark profiles of the investigated spectral line, both experimentally determined and computed, are highly sensitive to electron temperature.

Model of the cathode region of a plasma photoelectric converter of concentrated solar radiation.

A model of the cathode region of a plasma photoelectric converter of concentrated solar radiation in the regime of an open external circuit is considered. Plasma is analyzed in sodium vapor at a pressure of 104 -105 Pa. The necessary optical power is calculated for creating plasma in the state of Local Thermodynamic Equilibrium (LTE), whose temperature is in the range 4000-4500 K. This temperature provides a small internal resistance of the plasma voltage source. A large value of radiative heat conduction coefficient ensures a small temperature drop between the central part and the plasma LTE boundary near the walls of the converter. The model of a Non-LTE zone near the wall of the cathode is analyzed to specify the boundary conditions in the energy balance equation for LTE plasma. The thermal non-equilibrium plasma model resulted in the electron temperature gradient in the Non-LTE zone. The energy losses of electrons in the ionization layer near the cathode are counterbalanced by the thermal conductivity of the electron gas. The specificity of the conditions under consideration consists in the non-local nature of the balance of the electron temperature in the Non-LTE zone.

Analysis of Multi-elemental Thin Films via Calibration-Free Laser-Induced Breakdown Spectroscopy.

Elemental analyses of thin films with complex composition are challenging as the standard analytical techniques based on measurement calibration are difficult to apply. We show that calibration-free laser-induced breakdown spectroscopy (LIBS) presents a powerful solution, enabling quantitative analyses of multielemental thin films with analytical performances better than those obtained with other techniques. The demonstration is given for a nickel-chromium-molybdenum alloy film of 150 nm thickness that was produced by pulsed laser deposition. The LIBS spectra were recorded under experimental conditions that enable simple and accurate modeling of plasma emission. Thus, a calibration-free approach based on the calculation of the spectral radiance of a uniform plasma in local thermodynamic equilibrium was applied to deduce the elemental composition. Supported by analyses via Rutherford backscattering spectrometry and energy-dispersive X-ray spectroscopy, the LIBS measurements evidence nonstoichiometric mass transfer of the alloy during the thin-film deposition process. This technique could be used even for thinner films, provided that the film-composing elements are not present in the substrate.

Two-step procedure for trace element analysis in food via calibration-free laser-induced breakdown spectroscopy

Elemental analyses via calibration-free laser-induced breakdown spectroscopy (LIBS) usually suffer low sensitivity due to continuum emission generated by the plasma in local thermodynamic equilibrium. Here we propose a two-step measurement procedure that enables improved sensitivity of calibration-free LIBS. The method consists of recording two emission spectra with different delays between the laser pulse and the detector gate. The short delay is used to probe the plasma in conditions of full local thermodynamic equilibrium in order to measure major and minor element concentrations. To evaluate the concentrations of trace elements, a second measurement with improved limits of detection is performed with a larger gate delay. In that condition, the partial equilibrium state of the plasma enables the quantification of minor and trace elements. Demonstrated via analyses of lyophilized seafood samples, the presented method is suitable for all types of food, and more general for organic materials and all materials that include elements such as carbon, oxygen, nitrogen and hydrogen, for which the equilibrium state is hardly achieved.

A New Implementation of the STA Method for the Calculation of Opacities of Local Thermodynamic Equilibrium Plasmas

We present opacity calculations with the newly developed STAR code, which implements the Super-Transition-Array (STA), with various improvements. The model is used to calculate and analyze local thermodynamic equilibrium opacities of mid and high Z elements and of the solar interior plasma. We briefly review the underlying computational model and present calculations for iron and neodymium over a wide range of temperature and density.

Local thermodynamic equilibrium in a laser-induced plasma evidenced by blackbody radiation

We show that the plasma produced by laser ablation of solid materials in specific conditions has an emission spectrum that is characterized by the saturation of the most intense spectral lines at the blackbody radiance. The blackbody temperature equals the excitation temperature of atoms and ions, proving directly and unambiguously a plasma in local thermodynamic equilibrium. The present investigations take benefit from the very rich and intense emission spectrum generated by ablation of a nickel-chromium-molybdenum alloy. This alternative and direct proof of the plasma equilibrium state re-opens the perspectives of quantitative material analyses via calibration-free laser-induced breakdown spectroscopy. Moreover, the unique properties of this laser-produced plasma promote its use as radiation standard for intensity calibration of spectroscopic instruments.

Density effects on electronic configurations in dense plasmas

We present a quantum mechanical model to describe the density effects on electronic configurations inside a plasma environment. Two different approaches are given by starting from a quantum average-atom model. Illustrations are shown for an aluminum plasma in local thermodynamic equilibrium at solid density and at a temperature of 100 eV and in the thermodynamic conditions of a recent experiment designed to characterize the effects of the ionization potential depression treatment. Our approach compares well with experiment and is consistent in that case with the approach of Stewart and Pyatt to describe the ionization potential depression rather than with the method of Ecker and Kröll.

Opacity measurements of a molybdenum plasma with open M-shell configurations

The opacity of a molybdenum plasma in local thermodynamic equilibrium with open M-shell configurations was measured in the energy range from 200 eV to 800 eV, corresponding to the Δn = 1 M to N shell and Δn = 2 M to O shell transitions. The sample was heated by thermal x-ray radiation generated with a gold cavity. The electron temperature was diagnosed with the absorption spectrum of doped aluminum, giving a value of Te = (67 ± 4) eV. The density was 9.7 mg/cm3, which was deduced from the known initial area density and the plasma thickness by radiographic measurements of the expansion of the heated foil sample. The spectrally resolved transmission data were compared to a detailed-configuration-accounting model calculation and about 20% to 100% discrepancies were found.

Spectral Studies of Nanosecond Laser Interaction with Aluminum-Based Alloy Containing Traces of Magnesium in Air

Parametric study of optical emission Characterization of the plasma-assisted laser ablation of aluminium-based alloy that contains small traces of magnesium is presented. The emission of lines from the plasma of the target in the wavelength range 200-900nm has been carefully investigated at the same experimental conditions. It was found that all the emission intensities of the ionic and neutral magnesium lines are subjected to self-absorption, whereas the aluminium ionic lines were free from self-absorption. The results assure that, in case of the presence of self- absorption effect, the temperature varies from (1.7952 - 1.1433 eV) as the delay time is varied from 0 to 5 μs. Whereas, in the case of repairing against the effect, it varies from (1.3240 - 0.9447 eV) for the same delay time variation. At the same time, according to the McWhirter criterion, the electron density was high enough to establish plasma in local thermodynamic equilibrium.

Modeling of diffusive plasmas in local thermodynamic equilibrium with integral constraints: application to mercury-free high pressure discharge lamp mixtures

The mercury free lamp model previously discussed in Gnybida et al (2014 J. Phys. D: Appl. Phys. 47 125201) did not account for self-consistent diffusion and only included two molecular transitions. In this paper we apply, for the first time, a self-consistent diffusion algorithm that features (1) species/mass conservation up to machine accuracy and (2) an arbitrary mix of integral (total mass) and local (cold spot) constraints on the composition. Another advantage of this model is that the total pressure of the gas is calculated self consistently. Therefore, the usage of a predetermined pressure is no longer required.Additionally, the number of association processes has been increased from 2 to 6. The population as a function of interatomic separation determines the spectrum of the emitted continuum radiation. Previously, this population was calculated using the limit of low densities. In this work an expression is used that removes this limitation. The result of these improvements is that the agreement between the simulated and measured spectra has improved considerably.

Conceptual design of initial opacity experiments on the national ignition facility

Accurate models of X-ray absorption and re-emission in partly stripped ions are necessary to calculate the structure of stars, the performance of hohlraums for inertial confinement fusion and many other systems in high-energy-density plasma physics. Despite theoretical progress, a persistent discrepancy exists with recent experiments at the Sandia Z facility studying iron in conditions characteristic of the solar radiative–convective transition region. The increased iron opacity measured at Z could help resolve a longstanding issue with the standard solar model, but requires a radical departure for opacity theory. To replicate the Z measurements, an opacity experiment has been designed for the National Facility (NIF). The design uses established techniques scaled to NIF. A laser-heated hohlraum will produce X-ray-heated uniform iron plasmas in local thermodynamic equilibrium (LTE) at temperatures ${\geqslant}150$ eV and electron densities ${\geqslant}7\times 10^{21}~\text{cm}^{-3}$. The iron will be probed using continuum X-rays emitted in a ${\sim}200$ ps, ${\sim}200~\unicode[STIX]{x03BC}\text{m}$ diameter source from a 2 mm diameter polystyrene (CH) capsule implosion. In this design, $2/3$ of the NIF beams deliver 500 kJ to the ${\sim}6$ mm diameter hohlraum, and the remaining $1/3$ directly drive the CH capsule with 200 kJ. Calculations indicate this capsule backlighter should outshine the iron sample, delivering a point-projection transmission opacity measurement to a time-integrated X-ray spectrometer viewing down the hohlraum axis. Preliminary experiments to develop the backlighter and hohlraum are underway, informing simulated measurements to guide the final design.

Atomic Structure Calculations and Study of Plasma Parameters of Al-Like Ions

In the present paper, the spectroscopic properties and plasma characteristics of Al-like ions are investigated in an extensive and detailed manner by adopting the GRASP2K package based on fully relativistic Multi-Configuration Dirac–Hartree–Fock (MCDHF) wave-functions in the active space approximation. We have presented energy levels for Al-like ions for Valence-Valence (VV) and Core-Valence (CV) correlations under the scheme of active space. We have also provided radiative data for E1 transitions for Al-like ions and studied the variation of the transition wavelength and transition probability for electric dipole (E1) Extreme Ultraviolet (EUV) transitions with nuclear charge. Our calculated energy levels and transition wavelengths match well with available theoretical and experimental results. The discrepancies of the GRASP2K code results with CIV3 and RMPBT (Relativistic Many Body Perturbation Theory) results are also discussed. The variations of the line intensity ratio, electron density, plasma frequency and plasma skin depth with plasma temperature and nuclear charge are discussed graphically in detail for optically thin plasma in Local Thermodynamic Equilibrium (LTE). We believe that our obtained results may be beneficial for comparisons and in fusion and astrophysical plasma research.

SOLAR OPACITY CALCULATIONS USING THE SUPER-TRANSITION-ARRAY METHOD

ABSTRACT A new opacity model has been developed based on the Super-Transition-Array (STA) method for the calculation of monochromatic opacities of plasmas in local thermodynamic equilibrium. The atomic code, named STAR (STA-Revised), is described and used to calculate spectral opacities for a solar model implementing the recent AGSS09 composition. Calculations are carried out throughout the solar radiative zone. The relative contributions of different chemical elements and atomic processes to the total Rosseland mean opacity are analyzed in detail. Monochromatic opacities and charge-state distributions are compared with the widely used Opacity Project (OP) code, for several elements near the radiation–convection interface. STAR Rosseland opacities for the solar mixture show a very good agreement with OP and the OPAL opacity code throughout the radiation zone. Finally, an explicit STA calculation was performed of the full AGSS09 photospheric mixture, including all heavy metals. It was shown that, due to their extremely low abundance, and despite being very good photon absorbers, the heavy elements do not affect the Rosseland opacity.

Microwave Air Plasma Applied to Naphthalene Thermal Conversion

In this paper, a naphthalene (C10H8) thermal cracking model is presented. The model is based on a simple model that takes into account the microwave (MW) plasma thermal influence on naphthalene cracking, accompanying its steam reforming reactions. The temperature level of 1573 K was established for complete C10H8 cracking at 1.75 kW plasma power. High conversion efficiency of C10H8 is achieved varying the air flow rate in the range of 0.6–1.2 m3/h. The model approximates the characteristics of the considered MW plasma to thermal plasma in local thermodynamic equilibrium (LTE). Experimental data have good agreement with calculated data at the cited region of the air flow rate and power. Conversion efficiency up to 99.36% was obtained.