Electron Kinetic Temperature Research Articles

Two-temperature transport coefficients in argon–helium thermal plasmas

The knowledge of two-temperature transport coefficients is of interest in the modelling of flow in plasma processes and heat transfer. The transport coefficients in argon–helium plasmas at atmospheric pressure are calculated assuming that the kinetic electron temperature, Te, is different from that of the heavy species, Th. The electrical conductivity, the viscosity, the total thermal conductivity and the combined diffusion coefficients are calculated up to 30 000 K. The influence of the molar percentage of argon as well as that of the non-equilibrium parameter θ = Te/Th are investigated. The plasma composition is calculated using the modified Saha equation of van den Sanden et al. The most recent data to obtain collision integrals are also presented. It is shown that the viscosity and the combined diffusion coefficients strongly depend on θ, through the plasma composition and the collision integrals. The ion-dominated regime occurs all the more quickly as θ is high, resulting in a regime of interactions between charged species which induces a decrease of the viscosity and the combined diffusion coefficients. The electrical conductivity, which is directly linked to the electron number density, and the thermal conductivity increase as θ increases.

Transport coefficients including diffusionin a two-temperature argon plasma

This paper presents the first application to an argon atmospheric plasma of a very recent derivation of a two-temperature (2T) transport properties theory, based on the Chapman-Enskog method expanded up to the fourth approximation, where only elastic processes are considered. The kinetic electron temperature Te is assumed to be different from that of heavy species Th, chemical equilibrium being achieved. This new theory, where electrons and heavy species are coupled, allows one to determine 2T diffusion coefficients which was not the case of the previous ones.First, basic definitions of transport fluxes are recalled and a binary diffusion coefficient approximation is defined which involves an asymmetric relationship between these coefficients. Second, a particular care is taken in choosing the most recent data of potential interactions or elastic differential cross sections in order to determine the collision integrals. Third, a convergence study of transport coefficients is led to evaluate the influence of the non-equilibrium parameter θ = Te/Th on this convergence. It is shown that changing θ does not modify the convergence of transport coefficients. Moreover, ordinary and thermal diffusion coefficients, electrical and electron translational thermal conductivities as well as viscosity are displayed as functions of the electron temperature for different values of θ = Te/Th. It is pointed out that the non-equilibrium parameter θ has a non-negligible influence on transport coefficients. Besides, recently, it has been shown that the 2T simplified theory of transport properties, very often used in modelling, does not allow one to achieve mass conservation. Consequently, a comparison is presented between the 2T simplified theory and this new approach. Significant differences are found in the electrical conductivity and the electron translational thermal conductivity.

Atomic Model Calculations of Gain Saturation in the 46.9 nm Line of Ne-like Ar

The gain saturation in the 46.9 nm line of the Ar+8 laser is analyzed using an atomic kinetics code. The dependence of the gain (G) on the electron kinetic temperature (Te) in the region (50 -150 eV) is calculated in the quasi steady-state approximation for the different values of the electron density (Ne) and the plasma radius (rpl). The influence of radiat on trapping, ion random and mean velocities, Stark line broadening and refraction losses on the gain saturation is taken into consideration. For rpl = 150-600 μm, the amplplication (G > 0 cm-1) exists in the large temperature/density domain (Te = 60-150 eV, Ne = 0.5-10 × 1018 cm-3). However, the value Gs ∼ 1.4 cm-1 required for the gain saturation at the typical plasma length Lpl ∼ 15 cm is reached in the extremely narrow density regions at the high temperatures. The saturation is reached for rpl = 600 μm at Tse = 150 eV in the region Nse = 1.8-2 × 1018 cm -3, for rpl = 300 μm at Tse = 125 eV and Nse = 2.5-3 × 1018 cm-3, and for rpl = 150 μm at Tse = 110 eV and Nse = 3-4 × 1018 cm-3. The broadest density region (Nse = 2 -8 × 1018 cm-3) is predicted for the narrowest column (rpl = 150 μm) at the highest temperature (Tse = 150 eV). The operation in the broadest density region Nse, should make easier achievement of the gain saturation in the experiments.

Atomic Model Calculations of Gain Saturation in the 46.9 nm Line of Ne-like Ar

The gain saturation in the 46.9 nm line of the Ar+8 laser is analyzed using an atomic kinetics code. The dependence of the gain (G) on the electron kinetic temperature (Te) in the region (50 -150 eV) is calculated in the quasi steady-state approximation for the different values of the electron density (Ne) and the plasma radius (rpl). The influence of radiat on trapping, ion random and mean velocities, Stark line broadening and refraction losses on the gain saturation is taken into consideration. For rpl = 150-600 μm, the amplplication (G > 0 cm-1) exists in the large temperature/density domain (Te = 60-150 eV, Ne = 0.5-10 × 1018 cm-3). However, the value Gs ∼ 1.4 cm-1 required for the gain saturation at the typical plasma length Lpl ∼ 15 cm is reached in the extremely narrow density regions at the high temperatures. The saturation is reached for rpl = 600 μm at Tse = 150 eV in the region Nse = 1.8-2 × 1018 cm -3, for rpl = 300 μm at Tse = 125 eV and Nse = 2.5-3 × 1018 cm-3, and for rpl = 150 μm at Tse = 110 eV and Nse = 3-4 × 1018 cm-3. The broadest density region (Nse = 2 -8 × 1018 cm-3) is predicted for the narrowest column (rpl = 150 μm) at the highest temperature (Tse = 150 eV). The operation in the broadest density region Nse, should make easier achievement of the gain saturation in the experiments.

Transport properties in a two-temperature plasma: theory and application.

An alternate derivation of transport properties in a two-temperature plasma has been performed. Indeed, recent works have shown that the simplified theory of transport properties out of thermal equilibrium introduced by Devoto and then Bonnefoi, very often used in two-temperature modeling, is questionable and particularly does not work when calculating the combined diffusion coefficients of Murphy. Thus, in this paper, transport properties are derived without Bonnefoi's assumptions in a nonreactive two-temperature plasma, assuming chemical equilibrium is achieved. The electron kinetic temperature T(e) is supposed to be different from that of heavy species T(h). Only elastic processes are considered in a collision-dominated plasma. The resolution of Boltzmann's equation, thanks to the Chapman-Enskog method, is used to calculate transport coefficients from sets of linear equations. The solution of these systems allows transport coefficients to be written as linear combinations of collision integrals, which take into account the interaction potential for a collision between two particles. These linear combinations are derived by extending the definition and the calculation of bracket integrals introduced by Chapman et al. to the thermal nonequilibrium case. The obtained results are rigorously the same as those of Hirschfelder et al. at thermal equilibrium. The derivation of diffusion velocity and heat flux shows the contribution of a new gradient, that of the temperature ratio straight theta=T(e)/T(h). An application is presented for a two-temperature argon plasma. First, it is shown that the two-temperature linear combinations of collision integrals are drastically modified with respect to equilibrium. Secondly, the two-temperature simplified theory of transport coefficients of Devoto and Bonnefoi underestimates the electron thermal conductivity with respect to the accurate value at T(e)=20 000 K. Lastly, contrary to the simplified theory of transport coefficients, the diffusion coefficients satisfy the symmetry conditions. An example is given at T(e)=6000 K for different values of straight theta for the diffusion coefficient between electrons and heavy species D(e-Ar) as well as for that between argon atoms and argon ions D(Ar-Ar+).

Measurement of Excitation Equilibrium Departure Evolution in an Argon Pulsed Plasma

This work reports information about the temporal evolution of the excitation equilibrium departure in a high current pulsed arc. An argon plasma has been created in a linear lamp by discharging a capacitor bank on its electrodes. Spatially and temporally-resolved measurements have allowed us to determine electron density N e and excitation temperature T with great accuracy. Single-wavelength interferometry and H α -Stark broadening have been used to measure N e (0.2–0.98× 10 23 m -3 ). Excitation temperature has been obtained from Boltzmann-plot of ArII lines and ranges in this experiment from 13500 to 26500 K. By assuming this temperature is equal to kinetic electron temperature and by comparing the theoretically predicted and the experimental ArII/ArI intensities ratios at each instant of the plasma life, it is possible to measure the departure of the excitation processe from local them dynamic equilibrium (LTE) conditions and to show the ionizing characteristics of this close-to-LTE plasma. Different t...

The Effect of Mass Motions Inside the Coronal Loops on Emission Line Profiles

The observed green coronal emission line profiles have been often found to have multi-components. Further examinations reveal that the occurrence of multi-components in line profiles is related to the solar cycle variations as well as the activity of the coronal region. The spatial correspondence between the intense loops in active regions and strong multi-components in line profiles suggests that the presence of loops affects the line shapes. The emission line profiles have been found to be fitted well with single or multi-Gaussians with line-of-sight velocities up to 70 km s−1. A simple radiative transfer model of coronal emission line profiles is developed which shows that coronal loops with mass motions inside may give rise to multi-components in line profiles. The effects of loop parameters such as electron density, flow velocity and kinetic temperature and the line-of-sight variations are studied. It is found that line profiles strongly reflect the physical conditions inside the loop.

Nonlocal thermodynamic equilibrium in laser-sustained plasmas

An argon laser sustained plasma (LSP) at atmospheric pressure has been studied spectroscopically, and the existence of a nonlocal thermodynamic equilibrium state has been determined. The spectroscopic data consist of argon neutral and ion line emissions used to spatially resolve electronic energy level population densities in each plasma species. A hydrogen seed is added to the argon flow for the purpose of determining the electron number density by Stark broadening analysis of the Balmer series alpha line. Electron and heavy particle kinetic temperatures are calculated through the use of an appropriate nonequilibrium model. The dominant nonequilibrium effect in this plasma is kinetic nonequilibrium where the electron kinetic temperature can be more than twice the heavy particle kinetic temperature in high laser power flux regions. Typical electron and heavy particle kinetic temperatures are 14,000 K and 8000 K, respectively. Electron number density ranges from 6 x 10 exp 16/cu cm to 2.1 x 10 exp 17/cu cm. 31 refs.

Semiconductor laser absorption diagnostics of atomic oxygen in an atmospheric-pressure plasma

Narrow-bandwidth semiconductor (GaAlAs) lasers have been used to record spectrally resolved atomic oxygen (7772 Å) and argon (8425 Å) lineshapes, corresponding to the 3 s 5 S 0 2→3 p 5 P 3 and 4 s 3 P 1→4 p 3 D 2 transitions, in an atmospheric pressure, 1.4- kW, 27-MHz inductively coupled argon-oxygen (12% O 2/argon) plasma. Electron number density and kinetic temperature values were inferred from the Stark- and Doppler-broadening components of the absorption lineshapes, respectively. The ionization temperature was calculated from the measured electron number density assuming ionization (Saha) equilibrium. Values of excited- state species number density and population temperature were determined from the frequency- integrated absorption coefficient for each transition. The difference between the ionization and population temperatures reflects the presence of a suprathermal electron number density in the flowfield. In addition, the excellent agreement between the kinetic and population temperatures suggests that the population in the oxygen and argon lowest excited states may be described by a Boltzmann distribution at the kinetic temperature. The methods presented extend effectively the range of semiconductor-laser diagnostics to mixed-gas plasmas and flowfields containing atomic oxygen.

Electron energy distributions and kinetic temperatures in high-frequency argon discharges

The electron energy distribution functions and kinetic temperatures in argon discharges at low pressures under the action of a uniform high-frequency electric field are calculated by numerically solving the homogeneous Boltzmann equation. The calculation covers the whole range from low electron densities to sufficiently high electron densities. The influence of electron-electron collisions on the electron energy distribution functions and kinetic temperatures for various amplitudes and frequencies of the HF electric field and degree of ionization is investigated.

A collisional-radiative model applicable to argon discharge over a wide range of conditions. IV. Application to inductively coupled plasmas

For pt.III see ibid., vol.23, p.526 (1990). The extensive collisional-radiative model is applied to plasmas generated by a RF coil in an atmospheric argon flow in order to investigate excitation mechanisms and departures from local thermodynamic equilibrium in several spatial positions in these discharges. Computations are carried out for various sets of input parameters, such as the electron kinetic temperature Te, the atom temperature Ta, the electron number density ne, the ground-state atom population n1, the plasma column radius R and the escape factors Lambda mn and Lambda m, characterizing the non-equilibrium plasmas under consideration. The numerical modelling presented yields reliable information on the populations in the excited levels and on the validity of the so-called 'close to LTE concept' and the occurrence of an ionizing or a recombining regime in the locations investigated. The effect of the changes in the discharge parameters Te, Ta, ne, R and Lambda 1n on the excitation mechanisms, together with the role played by the inelastic atom-atom collisions and by the recombination flow of electrons from a continuum, are also shown.

A collisional-radiative model applicable to argon discharges over a wide range of conditions. III. Application to atmospheric and subatmospheric pressure arcs

For pt.II see ibid., vol.22, p.632 (1989). The extensive collisional-radiative model for an argon atom plasma is applied to atmospheric and subatmospheric pressure wall-stabilised arcs in order to clarify the mechanisms by which the excited levels are populated in the axial region of these discharges. Computations are carried out for various sets of input parameters, such as the electron kinetic temperature Te, the atom temperature Ta, the ion temperature Ti, the electron number density ne, the ground-state atom population n1, the plasma column radius R and the escape factors Lambda mn and Lambda m, characterising the non-equilibrium plasmas under consideration. The predicted values of the populations in the excited levels and the qualities derived from them, as well as the values of the critical electron densities for establishing the local thermodynamic equilibrium in the axial region of the arcs investigated, are in good agreement with the corresponding experimental results. The effect of the changes in the discharge parameters Te, Ta, ne, R and Lambda 1n on the population mechanism, together with the role played by the recombination flow of electrons from a continuum, the deviations of the actual electron distribution function from the corresponding Maxwellian form and by the atom-atom inelastic collisions, are also shown.

Density Irregularity of the Inner Corona determined from Simultaneous Measurements of the XUV and the K Coronal Brightness

In this paper we report preliminary results from a study of the inner corona based on the direct comparison of XUV resonance emission line λ174.53 FeX with that of the white-light emission from the K corona. The data sets were obtained 17718th of March, 1988, during a total solar eclipse of the sun and consists of co-spatial and co-temporal measurements of these two quantities as a function of position angle and height above the solar limb. The local emission of a coronal resonance line is proportional to the electron density squared, the chemical abundance, and the electron kinetic temperature, while the white-light emission (arising from Thomson scattering by electrons) depends directly on the electron density and the local radiation field. Taken together these measurements yield an estimate of the quantity where n is the electron density. This quantity, called “X the coronal irregularity factor” by C.W. Allen, is found to be >1.

Comparison of ionospheric electron temperature rocket measurements over Natal, Brazil, with the IRI model

A SONDA III rocket was launched from Natal on 11 December 1985, at 2030 LT, carrying a Langmuir probe that measured the kinetic electron temperature in the E and F regions of the equatorial ionosphere over Natal, Brazil. The rocket attained an altitude of 524 km. The electron temperature was determined through the derivative of the current versus voltage characteristics of the Langmuir probe and the detection of the floating potential. Results are compared with the IRI model, and the divergence of the results with the model is discussed.

A collisional-radiative model applicable to argon discharges over a wide range of conditions. II. Application to low-pressure, hollow-cathode arc and low-pressure glow discharges

For pt.I see ibid., vol.22, p.623 (1989). The extensive collisional-radiative model for an argon atom plasma is applied to a low-pressure, hollow-cathode arc discharge and to the positive column of a low-pressure glow discharge in order to clarify the mechanisms by which the excited levels in these discharges are populated, the results being compared with experimental investigations in the literature. Computations are carried out for various sets of input parameters, such as the electron kinetic temperature Te, the atom temperature Ta, the ion temperature Ti, the electron number density ne, the ground state atom population n1, the plasma column radius R and the escape factors lambda mn and lambda m, characterising the non-equilibrium plasmas under consideration. The predictions of the authors' model. i.e the populations in the excited levels as a function of the electron number density, the effective principal quantum number and the discharge current, are compared with the experimental results and in two cases also with the theoretical results of other authors. It is shown that all calculated dependences are fairly close to the corresponding experimental curves referring to both discharges. The results presented confirm the applicability of the so-called 'analytical top model' of van der Mullen et al. (1978, 1980) and Walsh's formula for Lambda 1n interpreted according to Mills and Hieftje (1984).

A collisional-radiative model applicable to argon discharges over a wide range of conditions. I. Formulation and basic data

A collisional-radiative model with an extended region of applicability is developed for an argon atom plasma. Atom-atom inelastic collisions and diffusion losses of the metastable states along with the electron-atom inelastic collisions and radiative processes are considered in this model, taking into account 65 effective levels. Among the analytical expressions used for the corresponding cross sections, special attention is paid to those determining the set of cross sections for excitation by electrons from the ground state, owing to the possibility of utilising the formulae recommended in kinetic modelling studies of discharges in argon or in mixtures including argon atoms. The numerical method developed makes it possible to investigate the mechanisms by which the excited levels are populated in a non-equilibrium argon plasma characterised (even in the case of a non-Maxwellian electron distribution) by a set of parameters, such as the electron kinetic temperature Te, the atom temperature Ta, the ion temperature Ti, the electron number density ne, the ground state atom population n1, the discharge tube (or the plasma column) radius R and the optical escape factors Lambda mn and Lambda m, which are dependent only on the quantities Ta, n1 and R in many cases of practical interest.

Radiative and diffusional effects to the population densities of the excited-state atoms in hydrogen plasma

Collisional–radiative (C–R) calculations based on the generalized multithermal equilibrium (GMTE) model are used to find the effects of radiative transfer and diffusion on excited-state populations in a hydrogen plasma. Radiation absorption factors are introduced to study the nonlocal aspects of radiative transfer. Unlike escape factors, the absorption factor can represent the absorption of more radiation than that emitted, and hence provides C–R analysis in strong gradients such as the outer edge of arcs or plasma jets. The Boltzmann temperatures of excited atoms are found not necessarily equal to the electron kinetic temperature, contrary to the usual assumption.

Ion‐neutral coupling in the high‐latitude F region: Evaluation of ion heating terms from Dynamics Explorer 2

We study a set of south (summer) polar passes from the Dynamics Explorer 2 data base that provide measurements of ion and neutral velocities along the track of the spacecraft for various geomagnetic conditions. The ion velocities are obtained from the ion drift meter and the retarding potential analyzer instruments; the neutral velocities are obtained from the Fabry‐Perot interferometer and the wind and temperature spectrometer instruments. These data enable a direct determination of the in situ ion‐neutral difference velocity. Simultaneous measurements of atmospheric composition using the neutral atmosphere composition spectrometer and of electron temperatures and densities using the Langmuir probe enable the ion heating and cooling rates to be determined. The rates are discussed in relationship to values for the electron, ion, and neutral kinetic temperatures. The comprehensive nature of the data set allows for a quantitative study of ion‐neutral coupling and, in particular, affords a more rigorous test of the common assumptions used in the ion energy equation than has been previously possible. From our study we find the following: (1) The ion‐neutral difference velocities, which drive the Joule heating, have a complex morphology that depends on the stability of the high‐latitude ion convection and the inertia of the neutral gas. (2) Ionospheric “hot spots” are observed to be related to large perpendicular electric fields that give rise to frictional (Joule) heating. (3) These events are often seen to occur in conjunction with troughs in the electron density and with decreases in the ratio of atomic oxygen to molecular nitrogen. In such cases, momentum transfer to the neutral gas is reduced, and the ion‐neutral velocity difference and associated Joule heating can be maintained over longer time periods. (4) An approximate balance between local frictional heating of the ions and local cooling of the ions to the neutrals is seen in regions of strong ion convection. The commonly used approximate form of the ion energy equation involving only local processes appears at times, however, to be inadequate for large regions of the summer polar cap. The ion temperatures observed in these regions are higher than expected from simple local heating/local cooling arguments. Additional source(s) of ion heating would be required to provide energy balance.

Non‐Maxwellian Electron Velocity Distribution Resulting from Electron‐SF6 Attachment Collisions

AbstractA model reaction system is considered consisting of a heat bath of argon atoms (c) and small concentrations of SF6‐molecules (m) and electrons (e) (number densities: ne ≪ nm ≪ nc). Within the model the time behaviour of the electron velocity distribution function is studied under the influence of elastic electron‐argon collisions and electron SF6 attachment collisions. On the basis of the distribution function time‐dependent macroscopic quantities (kinetic electron temperature. nonequilibrium rate constant of the attachment reaction) are calculated.

Galactic Radio Emission Below 16·5 MHz and the Galactic Emission Measure

New maps of the distribution of the galactic background radio emission are given for wave frequencies of 2�1,3�7,4�7,5�5,8�3,13 and 16�5 MHz. The angular resolutions of the observations were 7.5�, 6.8�, 3 � � 10�, 4.5�, 3.0�, 1.9� and 1.5� respectively. A map of the quantity ?? in galactic coordinates is obtained from an analysis of the changes in the radio brightness distributions with frequency. For an assumed electron kinetic temperature of 104 K, the emission measure is found to vary from 3�9 cm -6 pc near the south galactic pole to 140 cm -6 pc near the equator.