Existence Of Local Thermodynamic Equilibrium Research Articles

Characterization of the fast ionization wave induced by a CO2 laser pulse in argon

Fast ionization wave (FIW), a postbreakdown phenomenon of laser-induced plasma, is observed for a laser intensity of 1011–1013 W/m2 using the CO2 laser pulse in the atmospheric pressure condition. FIW is distinguishable as “overdriven detonation” according to Raizer's Chapmann-Jouguet detonation theory because FIW is known as the type of laser-absorption wave that has a higher propagation velocity than the laser-supported detonation wave (LSDW). Some reports have described the expansion of FIW using a solid-state laser. Nevertheless, the threshold phenomena between FIW and LSDW are not fundamentally understood. This study used the high-speed visualization and optical emission spectroscopy to investigate the transition of the laser-absorption wave in argon gaseous form. To elucidate the physics of the transition threshold, a 5 J CO2 pulse laser, an Echelle spectrometer, and an intensified CCD camera are used for the quantitative investigation of the plasma temperature and density. Results demonstrate that the FIW front had an electron temperature of 0.7 eV and an electron number density of 2.5 × 1023 m−3. At the FIW–LSDW transition, the electron temperature increased by 1 eV, and the density decreased by 2.2 × 1023 m−3. Besides, the transition threshold and the existence of local-thermodynamic equilibrium were evaluated based on the electron temperature, and the density was obtained from the spectroscopic experiments.

Thermal conductivity and local thermodynamic equilibrium of stochastic energy exchange models

In this paper we study macroscopic thermodynamic properties of a stochastic microscopic heat conduction model that is reduced from deterministic problems. Our goal is to numerically check how the ‘low energy site effect’ inherited from the deterministic model would affect the macroscopic thermodynamic properties such as the thermal conductivity and the local thermodynamic equilibrium. After a series of numerical computations, our conclusion is that neither the thermal conductivity nor the existence of local thermodynamic equilibrium is qualitatively changed by this effect.

Laser induced plasma expansion and existence of local thermodynamic equilibrium

In this paper we present a simple model of the laser induced plasma (LIP) expansion in a low pressure surrounding atmosphere. The model is based on assumption that expansion process is dominantly governed by kinematics of the heavy particles. The model is accompanied with a simple, yet effective, Monte-Carlo simulation. Results of the simulation are compared with spectroscopic measurements of the laser induced copper plasma expanding in low pressure (200Pa) hydrogen atmosphere. We found that characteristic expansion time of the LIP is proportional to the linear dimension of the initial volume heated up by the laser. For sufficiently large initial volume copper plasma remains in local thermodynamic equilibrium on the submicrosecond-microsecond scale. It is shown that diagnostics based on the spectral lines of the hydrogen atmosphere is not suitable for characterization of the core of the copper plasma. We have demonstrated importance of radially resolved spectroscopic measurements as a key step for correct diagnostics and understanding of laser induced plasma.

Diagnostics of Plasma Jet Generated in Water/Argon DC Arc Torch

Thermal plasma jet generated by the torch with water/argon stabilized arc was investigated. Plasma torches of this type have been used for plasma spraying, waste treatment and gasification of organic materials. Electric probes, enthalpy probe, and schlieren photography were used for diagnostics of the jet in the region downstream of the torch exit. Information about structure and shape of plasma jet was evaluated from the measured data. Large extent of radial plasma spread and high level of turbulence were found from both the schlieren and the probe diagnostics. Plasma temperature corresponding to measured ion saturation currents was determined using calculated composition of plasma assuming ex-istence of local thermodynamic equilibrium.

Optical Emission Spectroscopic Analysis of Plasma Plume during Pulsed Laser Deposition of PZT

Spatial variation in intensity of spectral emission, electron temperature, number density, and the time of flight (TOF) of ions and neutrals at various oxygen ambiances has been investigated on ferroelectric lead zirconium titanate (PZT) plasma using optical emission spectroscopy. Plasma produced by ablating PZT ceramic target using Nd-YAG laser operating at the third harmonics (λ=355 nm, τ=10 ns, repetition frequency 10 Hz) was investigated at various oxygen partial pressures and at various distances from the target surface. Here energy density for laser fluence was fixed as 3.13 Jcm−2 and distance from the target and ambient gas pressure were varied. The electron number density Ne and electron temperature of the PZT plasma at the early stage of plume expansion were measured as 1.7×1017 Jcm−2 and 13200 K, respectively, and thus verified the existence of local thermodynamic equilibrium (LTE). Time of flight spectra (TOF) of neutral and singly ionized species in plasma were recorded. The result shows that plasma parameters and velocity of species are of same order for various oxygen partial pressures but have a decreasing tendency with distance. The energy of almost all species in the plume become more or less same at 0.1 mbar. These conditions favour the growth of perovskite PZT thin films.

Temperature evaluation by simultaneous emission and saturated fluorescence measurements: A critical theoretical and experimental appraisal of the approach

Temperature is one of the most important physical parameters of plasmas induced by a focused laser beam on solid targets, and its experimental evaluation has received considerable attention. An intriguing approach, first proposed by Kunze (H.-J. Kunze, Experimental check of local thermodynamic equilibrium in discharges, Appl. Opt., 25 (1986) 13–13.) as a check of the existence of local thermodynamic equilibrium, is based upon the simultaneous measurement of the thermal emission and the optically saturated fluorescence of the same selected atomic transition. The approach, whose appealing feature is that neither the calibration of the set-up nor the spontaneous radiative probability of the transitions is needed, has not yet been applied, to our knowledge, to analytical flames and plasmas. A critical discussion of the basic requirements for the application of the method, its advantages, and its experimental limitations, is therefore presented here. For our study, Ba+ transitions in a plasma formed by focusing a pulsed Nd:YAG laser (1064nm) on a glass sample containing BaO are selected. At various delay times from the plasma initiation, a pulsed, excimer-pumped dye laser tuned at the center of two Ba transitions (6s 2S1/2→6p 2P°3/2; 455.403nm and 6p 2P°1/2→6d 2S1/2; 452.493nm) is used to enhance the populations of the excited levels (6p 2P°3/2 and 6d 2S1/2) above their thermal values. The measured ratio of the emission and direct line fluorescence signals observed at 614.171nm (6p 2P°3/2→5d 2D5/2) and 489.997nm (6d 2S1/2→6p 2P°3/2) is then related to the excitation temperature of the plasma. Our conclusion is that the approach, despite being indeed attractive and clever, does not seem to be easily applicable to flames and plasmas, in particular to transient and inhomogeneous plasmas such as those induced by lasers on solids.

Local thermodynamic equilibrium considerations in powerchip laser-induced plasmas

Time-resolved emission experiments are reported in the fast-decaying transient plasma induced by a microchip laser on an aluminum target in three different cover gases, i.e., air, argon and helium. The laser operates at 532nm, with a repetition frequency of 1kHz and a pulse width of less than 0.5ns. The overall persistence of plasma emission is of the order of 100ns.We examine the existence of local thermodynamic equilibrium (LTE) by evaluating the temporal criteria required (in addition to the McWhirter criterion), as recommended by Cristoforetti et al. (Spectrochim. Acta Part B 65, 2010, 86–95). The temporal criteria examine the evolution of temperature and electron number density and compare their rate of change to the rate at which electron collisions can thermalize the change. These considerations are used to determine time windows in which LTE may be present. Our results suggest that calibration-free LIBS measurements with these lasers may be possible for some elements at early times, especially under argon.

Local Thermodynamic Equilibrium in Laser-Induced Breakdown Spectroscopy: Beyond the McWhirter criterion

In the Laser-Induced Breakdown Spectroscopy (LIBS) technique, the existence of Local Thermodynamic Equilibrium (LTE) is the essential requisite for meaningful application of theoretical Boltzmann–Maxwell and Saha–Eggert expressions that relate fundamental plasma parameters and concentration of analyte species. The most popular criterion reported in the literature dealing with plasma diagnostics, and usually invoked as a proof of the existence of LTE in the plasma, is the McWhirter criterion [R.W.P. McWhirter, in: Eds. R.H. Huddlestone, S.L. Leonard, Plasma Diagnostic Techniques, Academic Press, New York, 1965, pp. 201–264]. However, as pointed out in several papers, this criterion is known to be a necessary but not a sufficient condition to insure LTE. The considerations reported here are meant to briefly review the theoretical analysis underlying the concept of thermodynamic equilibrium and the derivation of the McWhirter criterion, and to critically discuss its application to a transient and non-homogeneous plasma, like that created by a laser pulse on solid targets. Specific examples are given of theoretical expressions involving relaxation times and diffusion coefficients, as well as a discussion of different experimental approaches involving space and time-resolved measurements that could be used to complement a positive result of the calculation of the minimum electron number density required for LTE using the McWhirter formula. It is argued that these approaches will allow a more complete assessment of the existence of LTE and therefore permit a better quantitative result. It is suggested that the mere use of the McWhirter criterion to assess the existence of LTE in laser-induced plasmas should be discontinued.

Heavy metal concentrations in soils as determined by laser-induced breakdown spectroscopy (LIBS), with special emphasis on chromium

Soil is unanimously considered as one of the most important sink of heavy metals released by human activities. Heavy metal analysis of natural and polluted soils is generally conducted by the use of atomic absorption spectroscopy (AAS) or inductively coupled plasma optical emission spectroscopy (ICP-OES) on adequately obtained soil extracts. Although in recent years the emergent technique of laser-induced breakdown spectroscopy (LIBS) has been applied widely and with increasing success for the qualitative and quantitative analyses of a number of heavy metals in soil matrices with relevant simplification of the conventional methodologies, the technique still requires further confirmation before it can be applied fully successfully in soil analyses. The main objective of this work was to demonstrate that new developments in LIBS technique are able to provide reliable qualitative and quantitative analytical evaluation of several heavy metals in soils, with special focus on the element chromium (Cr), and with reference to the concentrations measured by conventional ICP spectroscopy. The preliminary qualitative LIBS analysis of five soil samples and one sewage sludge sample has allowed the detection of a number of elements including Al, Ca, Cr, Cu, Fe, Mg, Mn, Pb, Si, Ti, V and Zn. Of these, a quantitative analysis was also possible for the elements Cr, Cu, Pb, V and Zn based on the obtained linearity of the calibration curves constructed for each heavy metal, i.e., the proportionality between the intensity of the LIBS emission peaks and the concentration of each heavy metal in the sample measured by ICP. In particular, a triplet of emission lines for Cr could be used for its quantitative measurement. The consistency of experiments made on various samples was supported by the same characteristics of the laser-induced plasma (LIP), i.e., the typical linear distribution confirming the existence of local thermodynamic equilibrium (LTE) condition, and similar excitation temperatures and comparable electron number density measured for all samples. An index of the anthropogenic contribution of Cr in polluted soils was calculated in comparison to a non-polluted reference soil. Thus, the intensity ratios of the emission lines of heavy metal can be used to detect in few minutes the polluted areas for which a more detailed sampling and analysis can be useful.

Characterization of DC argon plasma jet at atmospheric pressure

An original DC double anode plasma torch operating with argon at atmospheric pressure which provides a long time and highly stable plasma jet is analyzed through its electrical and optical signals. Effects of gas flow rate and current intensity on the arc dynamics behaviour are studied using standard diagnostic tools such as FFT and correlation function. An increasing current-voltage characteristic is reported for different argon flow rates. It is noted that the takeover mode is characteristic for argon plasma jet and arc fluctuations in our case are mainly induced by the undulation of torch power supply. Furthermore, the excitation temperatures and electron densities of the plasma jet inside and outside the arc chamber have been determined by means of optical emission spectroscopy (OES).The criteria for the existence of local thermodynamic equilibrium (LTE) in plasma is then discussed. The results show that argon plasma jet at atmospheric pressure under our experimental conditions is close to LTE.

Influence of the optical depth on spectral line emission from laser-induced plasmas

The emission from a laser-induced plasma of seven self-absorbed Fe(I) spectral lines has been studied to investigate the influence of the optical depth on the line intensity. The plasma was generated using an infrared Q-switched Nd:YAG laser in air at atmospheric pressure. The plasma emission was detected with temporal resolution, using a delay of 5 μs from the laser pulse and a gate width of 1 μs. Experimental curves of growth (COGs) were obtained by measuring the line intensity for Fe–Ni alloy samples with Fe concentrations in the range 0.2–95%. Using a simple model for self-absorption, based in a homogeneous plasma with the absence of matrix effects, theoretical COGs have been calculated that fit the experimental data with good correlation. The method used allows prediction of the COG that will be obtained for a given spectral line, starting from its transition parameters (oscillator strength, energy levels and degeneracy of the lower level), the plasma temperature and the damping constant of the line. The plasma temperature (8200±100 K) was determined using the Boltzmann plot method. The existence of local thermodynamic equilibrium was verified by estimating the plasma electron density (2.6×10 16 cm −3) using the Stark broadening of an emission line. The damping constant (0.9±0.2) was estimated through the determination of the Lorentzian line width from measured line profiles. The density of Fe atoms in the plasma for the sample with 100% Fe (7.3×10 15 cm −3) was estimated using all the COGs of the lines studied. The experimental results indicate that matrix effects are not present in the ablation process of the Fe–Ni samples.

Experimentally determined transition probabilities for lines of Pb I and the 2203.5 Å line of Pb II

Transition probability for the line 2203.53 A of Pb II and transition probabilities for 39 lines of Pb I have been determined from measurements of emission line intensities in an optically thin laser-produced plasma of Pb and Sn in an atmosphere of Ar. Lines for which transition probabilities have not been previously published are included. In order to establish this homogeneity and the existence of local thermodynamic equilibrium, plasma has been studied. A temperature of 11060 K and an electron density of 10 16 cm −3 were found. The results were compared with the available experimental and theoretical data given by other authors.

Simulation of the contours of spectral lines of closed-ARC radiation

Simulated contours of self-reversed lines of Tl at 535 nm (72S1/2−62P3/2) and 378 nm (72S1/2-62P1/2) in alternating-current closed-arc radiation in mercury vapor with an addition of Tll were compared with experimental contours. It is shown that while a single self-reversed contour has many different sets of model parameters that ensure coincidence of experimental and calculated profiles, in the case of operation with a large data file of contours that correspond to different directions of observation along chords perpendicular to the discharge axis, and with different lines that have a common level, such a set of parameters becomes virtually unique. Instantaneous spatial distributions of Hg and Tl atoms in the ground state and of Tl atoms in the 62P3/2 state are determined and the parameters of the van der Waals broadening of the 72S1/2 level are found. It is shown that a considerable “red” shift of absorption contours in the outer layers of the discharge is observed. The results obtained contradict the presumed existence of local thermodynamic equilibrium in the given discharge.

Influence of radiation resorption on criteria of the existence of local thermodynamic equilibrium

The emission of radiation from a plasma volume upsets the Boltzmann equilibrium. A number of authors have proposed criteria for approximate testing of the existence of local thermodynamic equilibrium on the basis of collision and radiation processes. However, those criteria give excessive values, because they do not take into account radiation resorption, which can “moderate” the test conditions. The present article is concerned with the influence of radiation resorption on the criterion of the existence of local thermodynamic equilibrium in a low-temperature plasma. The domain of existence of local thermodynamic equilibrium (in the sense of Saha-Boltzmann equilibrium) is calculated for electric arcs.

Does local thermodynamic equilibrium exist in the excitation process of sputtered atoms?

The optical line intensities from a series of ion-bombarded oxide glasses are fitted to two different equations relating the intensity and the energy of the upper level. The plots suggest that the existence of local thermodynamic equilibrium in the excitation process during sputtering is unlikely.

Measurement of the Electron-Impact Broadening of Ionized Nitrogen and Carbon Resonance Lines in a High-Pressure Electric Shock Tube

Profiles of the overlapping fine-structure components of C II and N II vacuum ultraviolet resonance lines emitted from optically thick shock-heated layers consisting mostly of helium were measured on a shot-to-shot basis, using a $T$-type electromagnetic shock tube as a source. Plasma conditions were determined from spectroscopic measurements in the visible region. The analysis was based on a radiative transfer model for the resonance multiplets, assuming superimposed Lorentzian profiles with known oscillator strengths and the existence of local thermodynamic equilibrium. The resulting damping constants are interpreted as due mainly to elastic electron collisions and agree within factors of \ensuremath{\sim} 1.0 and \ensuremath{\sim} 1.5, respectively, with predictions based on a semiempirical method.

Temperature Profiles and Energy Balances for an Inductively Coupled Plasma Torch

A mathematical model was developed to predict radial and axial temperature profiles in an induction-coupled plasma torch. The model employed temperature dependent physical properties and included radiation heat loss. Temperature fields based on the model were calculated for an atmospheric pressure argon plasma. Computed radial temperature profiles were in agreement with experimental profiles reported in the literature. The gas flow rate and flow pattern in the torch did not greatly affect the radial temperature profiles in the hottest region of the torch under ordinary operating conditions, also in agreement with experiment. A one-dimensional analysis was developed which predicted these hot-spot profiles accurately. The plasma model was also found to accurately predict distribution of energy losses from the plasma. Based on the equilibrium current density and temperature profiles provided by the model, the question of the existence of local thermodynamic equilibrium was re-examined. It was concluded that equilibrium is closely approached in the high-temperature central portion of the plasma, in agreement with previous investigators; however, nonequilibrium conditions can exist in the region of high temperature gradient at the plasma edge.

Measurement of emission and absorption coefficients of the Balmer line H

The line was studied spectroscopically using a hydrogen plasma generated in a high-current, low-voltage, free- burning arc. Absorption coefficients and emission coefficients corrected for self-absorption were measured over a temperature range of 10,000 degrees K-20,000 degrees K at 1 atm. pressure. Transition probability and line profiles were in good agreement with theory. The ratio of emission coefficient to absorption coefficient agreed with the planck function, indicating the existence of local thermodynamic equilibrium.

Validity of complete local thermodynamic equilibrium for HeI

Validity criteria for the existence of local thermodynamic equilibrium for the ground state of He I in a time-independent and homogeneous plasma are discussed.

Local thermodynamic equilibrium in an RF argon plasma

The existence of local thermodynamic equilibrium in the center half of an inductively sustained RF argon plasma was investigated using emission spectroscopy as the diagnostic tool. The experiment was conducted for a fixed oscillator frequency of 4·2 MHz, a fixed plate input power of 2·0 kW, a constant average gas velocity of 9·0 cm/sec, and for pressure levels of 0·01, 0·1, 0·5, and 1·0 atm. Four emission lines and the continuum centered at three different wavelengths were used in the investigation to show that local thermodynamic equilibrium exists at 1·0 and 0·5 atm and that within the experimental error the bound states 3 p 6 and above are populated with a Boltzmann distribution at all the pressures investigated.