Partial Local Thermal Equilibrium Research Articles

Stratigraphic characterization of ancient Roman frescos by laser induced breakdown spectroscopy and importance of a proper choice of the normalizing lines

In order to answer to some specific unsolved questions regarding ancient Roman frescoes from Villa della Piscina, already examined by other spectroscopic techniques, LIBS stratigraphic measurements were performed on the most representative sample fragments. We found that taking into account just the line intensities or applying a single normalization approach based on ratio of the element's analytical line and a selected transition from a main sample constituent, here of Ca, might produce misleading results of depth profiling by LIBS. During the laser induced ablation, parameters of the detected plasma might change both due to the crater development and different overlaid matrix materials, as in case of heterogeneous painted samples, possibly realized with different techniques. We performed the theoretical simulations to select the pair of Cu I and Ca I lines less sensitive to variations of the plasma temperature in presence of local thermal equilibrium (LTE). However, when using not gated detectors as here, the experimental results showed that only a partial LTE holds. For this reason, the choice of the optimum normalization line must take into account also the energy gap between the excited levels of the species, which energies affect the emission lifetime and consequently, the intensity captured by not gated detectors. In following, to the each element's analytical line a specific Ca I line was attributed for deducing the relative element distribution in sample. Following this approach and by detecting simultaneously up to 20 elements per laser pulse, it was possible to distinguish clearly by LIBS also thin overpainted layers of similar colour, and to recognize the painting technique used. Through detection of trace and light elements and studies of their correlations with other elements during ablation of on painted layer, in some cases it was possible also to establish the geographical provenance of the material used.

Laser-Induced Plasma Analysis for Surrogate Nuclear Debris

This work identifies analytical lines in laser-induced plasma for chemical analyses of major elements found in surrogate nuclear debris. These lines are evaluated for interferences and signal strength to insure they would be useful to measure relative concentrations. Compact, portable instruments are employed and can be included as part of a mobile nuclear forensics laboratory for field screening of nuclear debris and contamination. The average plasma temperature is measured using the well-established Boltzmann plot technique, and plasma’s average electron density is determined using empirical formulae based on Stark broadening of the H-alpha line. These measurements suggest existence of partial local thermal equilibrium.

Self-absorption influence on the optical spectroscopy of zinc oxide laser produced plasma

Optical spectroscopy is used to study the laser ablation process of ZnO targets. It is demonstrated that even if Partial Local Thermal Equilibrium is present, self absorption process leads to a decrease of recorded lines emission intensities and have to be taken into account to obtain correct values of such parameters. It is presented a method that combines results of both Langmuir probe technique and Anisimov model to obtain correct values of plasma parameters.

Comparative study of recombining He plasmas below 0.1 eV using laser Thomson scattering and spectroscopy in the divertor simulator MAP-II

The recent upgrades of the MAP-II Laser Thomson Scattering (LTS) system and the development of a new Hetero-Tandem Double Monochromator, allowed detectable electron temperatures as low as 0.05 eV and electron densities as low as 10 12 cm - 3 . This enabled the study of He Electron Ion Recombining (EIR) plasmas and the comparison with optical emission spectroscopy for He I Rydberg states in partial local thermal equilibrium (p-LTE). 2 1 P - n 1 D and 2 3 P - n 3 D series with transitions from principal quantum numbers n = 9 – 16 were used for the T e derivation. Results from both Boltzmann plot and Collisional Radiative model fitting are consistent with those from LTS. The very low temperature of 0.06 eV, usually obtained by the Boltzmann plot for He EIR states, is confirmed.

Transition probabilities for some infrared O I spectral lines—Application for determining excitation temperatures in low temperature plasmas

Intensities of selected spectral transitions of neutral oxygen emitted from a wall-stabilized arc have been measured. Applying reliable transition probability data taken from literature, Boltzmann plots have been constructed and the temperatures of the plasma have been obtained. At arc plasma conditions ensuring partial local thermal equilibrium, transition probabilities for 4 O I spectral lines from the near infrared spectral range have been obtained. These new results, together with data for two other studied O I spectral lines taken from the National Institute of Standards and Technology data base, are proposed as a suitable set of transition probabilities applicable for diagnostics of low temperature plasmas.

A Finite-Time Thermodynamics of Unsteady Fluid Flows

Turbulentfluidhasoftenbeenconceptualizedasatransientthermodynamicphase. Here, a finite-time thermodynamics (FTT) formalism is proposed to compute mean flow and fluctuation levels of unsteady incompressible flows. The proposed formalism builds upon the Galerkin model framework, which simplifies a continuum 3D fluid motion into a finite-dimensional phase-space dynamics and, subsequently, into a thermodynamics energy problem. The Galerkin model consists of a velocity field expansion in terms of flow configuration dependent modes and of a dynamical system describing the temporal evolution of the mode coefficients. Each mode is treated as one thermodynamic degree of freedom, characterized by an energy level. The dynamical system approaches local thermal equilibrium (LTE) where each mode has the same energy if it is governed only by internal (triadic) mode interactions. However, in the generic case of unsteady flows, the full system approaches only partial LTE with unequal energy levels due to strongly mode-dependent external interactions. The FTT model is

Emission spectroscopy on a supersonically expanding argon/silane plasma

Results from emission spectroscopy measurements on an Ar/SiH4 plasma jet which is used for fast deposition of amorphous hydrogenated silicon are presented. The jet is produced by allowing a thermal cascaded arc plasma in argon (I=60 A, V=80 V, Ar flow=60 scc/s and pressure 4 × 104 Pa) to expand to a low pressure (100 Pa) background. In the resulting plasma SiH4 is injected in front of the stationary shock front. Assuming a partial local thermal equilibrium situation for higher excited atomic levels, emission spectroscopy methods yield electron densities (∼ 1018 m−3), electron temperatures (∼5000 K) as well as concentrations of H+, Si+, and Ar+ particles. The emission spectrum of the SiH radical, the A 2Δ–X 2Π electronic transition, is observed. Numerical simulations of this spectrum are performed, resulting in upper limits for the rotational and vibrational temperatures of 4000 and 5600 K, respectively. The results can be understood assuming that, in the expansion, charge exchange and dissociative recombination are dominant processes in the formation of species in excited states, notably Si+.

Low-pressure plasma spectroscopic diagnostics

Diagnostic techniques have recently been developed that permit the determination of the deviation from local thermal equilibrium (LTE) in subatmospheric electric arcs and plasma jets. A review is presented of the methods that are applicable to MPD (magnetoplasmadynamic) and arcjet thruster plasmas but that have not been used in space propulsion research. Appropriate plasma diagnostics can lead to increased thrust, better nozzle design, and improved modeling capabilities. These methods include nonintrusive techniques, and can determine the electron, Te, gas, Tg, and total excitation, Texa< temperatures, as well as the electron and atom densities, without using LTE or partial LTE assumptions. General relations for analysis and experimental results for argon constricted arcs, an arc in a rotating magnetic field, and plasma torch jets are presented. The methods discussed can also be applied to plasma mixtures.

Electrical characteristics of a noble gas diagonal MHD channel

A study is made of a diagonal channel employed with MHD plasmas under the assumption of constant gas pressure, velocity, and temperature, using an electrodynamic model coupled to the conservation equations of the electrons to analyze channel performance. The reduction formulas of the plasma parameters assume small fluctuations of the parameters given by further assuming partial local thermal equilibrium. The existence of the magnetically induced nonequilibrium conductivity coincides with the existence of constricted discharges. 7 references.

Excitation and kinetic equilibria in an argon inductively coupled plasma torch—I: Coolant flow only

A relaxation method very sensitive to any deviation from excitational equilibrium and which yields accurate values of the temperature ratio T e/ T g, (where T e and T g are the electron and gas temperature respectively) is used to investigate the equilibrium state of a single flow (coolant only) argon inductively coupled plasma. The results show that for an input power of 1200 W the excited states (4p and higher) are in partial local thermal equilibrium out to a radius of 7 mm. In the central region ( r ⩽ 3 mm) the plasma is in kinetic equilibrium with the gas temperature within 3% of the electron temperature, with T e ≈ 10000K. At r = 6 mm, T e/ T g ranges from 1.3 at 15 mm below the top of the work-coil to 1.06 at 5mm above the work-coil. As the power is reduced from 1200 W to 800 W, T e/ T g changes by typically 6% illustrating that the input power level is not a critical parameter in determining the plasma equilibrium.

Hβ‐Line Profile Measurements in Optical Discharges

AbstractIn the electrical field of a focused beam of a high‐power DC discharge multikilohertz repetition rate Q‐switched CO2 laser a so called pulsed optical discharge (POD) can be sustained. Applying laser pulses with max. 104 pulses/s, 50 mJ/pulse, pulse length 0.2 μs, average power 500 W and peak power 250 kW, the charge carrier density of a hydrogen plasma at a pressure of several MPa produced during the laser pulse is enhanced up to about the cut‐off density at λL = 10.6 μm (≃ 1025 m−3) and the electron temperature is increased to about 105 K. The non‐ideality factor reaches about 0.2. The calibrated line intensity of the Balmer Hβ was monitored by means of an optical multichannel analyzer system with spatial, frequency and time resolution. The measurements yield Stark‐broadened Hβ‐line profiles from which, assuming partial local thermal equilibrium, the electron temperature and density can be derived. The measured asymmetry of the Hβ‐line is in rough agreement with theoretical predictions available in literature.

An Investigation of Non-Equilibrium Effects in Thermal Argon Plasmas

The parameters and transport properties of a wall stabilized argon arc (40-200 A) at atmospheric pressure with diameters of 5 and 8 mm are studied by spectroscopy and interferometry. The plasma is assumed to be partial local thermal equilibrium and this assumption is verified with the aid of a collisional-radiative model. The departures from Saha-equilibrium of the argon neutral ground state are found to be associated with particle diffusion and the escape of recombination radiation. The measurement of the total excitation rate, from the ground level, including direct ionization, of neutral argon is in reasonable agreement with the literature value.

Some considerations on the excitation mechanism in the inductively coupled argon plasma

The excitation conditions in the analytical observation zone of the inductively coupled argon plasma cannot be described by the model of partial local thermal equilibrium. After some general remarks on excitation models the paper analyses in tutorial fashion four alternative models proposed in the literature, featuring metastable argon atoms, radiation trapping, reaction rates and ambipolar diffusion. It is concluded that none is completely satisfactory, but that they are complementary rather than contradictory. Finally, some elements of the four models are integrated into a new model that considers the ICAP as a plasma decaying from conditions of heterogeneous disequilibrium to a situation of homogeneous thermal equilibrium.

A theoretical study and experimental investigation of non-LTE phenomena in an inductively-coupled argon plasma—I. Characterization of the discharge

In the past considerable attention has been paid to the problem or explaining analyte and argon excitation in inductively-coupled plasmas. Deviations from Local Thermal Equilibrium (LTE) have been frequently reported. In this paper a theoretical framework is constructed to explain quantitatively analyte and argon excited level densities. An essentially new description of the discharge is proposed in terms of partial LTE and the saturation regime. Models and their applications to an ICP discharge are thoroughly discussed and compared with former LTE models.

Establishment of partial local thermal equilibrium in transient and inhomogeneous plasmas

New formulas for relevant relaxation times and relaxation lengths for the establishment of partial local thermal equilibrium are presented taking into consideration the influence of the ground state on the population of the excited levels and on the energy distribution of the free electron gas.

Influence of diffusion and nonequilibrium populations on noble-gas plasmas in electric arcs

Measurements and calculations of temperatures, densities, and field-strength-current characteristics of cascade arcs burning in noble gases under atmospheric pressure are reported. The evaluation of measured arc data assuming Saha equilibrium [complete local thermal equilibrium (LTE)] is not in agreement with the detailed solution of the balance equations. The temperatures of electrons and heavy particles and the density of electrons and neutrals have to be determined from the set of rate equations, from the equation of state, connection with the electron energy balance and the equation of state, the energy balance of the electron gas, and of the total plasma. Solutions of these equations are compared with results following from measured line intensities only solving the rate equations in connection with the electron energy balance and the equation of state. For helium, both methods give results which agree within a few percent. The deviations from Saha equilibrium are caused by diffusion and the overpopulation of ground state atoms. The excited atoms, however, are nearly in equilibrium with free electrons in the range of electron densities reached in our experiment (partial LTE). Measurements of E-I characteristics agree with calculated data, if diffusion is taken into account. A simple criterion for the limit between diffusion-dominated plasma and a plasma in thermal equilibrium is derived.