Heavy Particle Temperature Research Articles

Thomson scattering diagnostics of steady state and pulsed welding processes without and with metal vapor

Thomson scattering is applied to measure temperature and density of electrons in the arc plasma of the direct current gas tungsten arc welding (GTAW) process and pulsed gas metal arc welding (GMAW) process. This diagnostic technique allows to determine these plasma parameters independent from the gas composition and heavy particles temperature. The experimental setup is adapted to perform measurements on stationary as well as transient processes. Spatial and temporal electron temperature and density profiles of a pure argon arc in the case of the GTAW process and argon arc with the presence of aluminum metal vapor in the case of the GMAW process were obtained. Additionally the data is used to estimate the concentration of the metal vapor in the GMAW plasma.

Three-Dimensional Non-equilibrium Modeling on the Characteristics of the Dual-Jet Direct-Current Arc Plasmas

In this paper, the three-dimensional (3-D) two-temperature modeling on the characteristics of the dual-jet DC arc argon plasmas under different plasma working gas flow rates ranging from 0 to 15 slpm or the chamber pressures increasing from 0.4 to 1.0 atm are conducted at a fixed arc current of 80 A. The modeling results reveal a significant 3-D feature of the temperature and flow fields of the plasma arc-jet due to the declination of the electrodes to each other geometrically, and an obvious deviation from the local thermodynamic equilibrium (LTE) state resulting from the interactions between the high temperature plasma and the cold walls or cold surrounding gas. With other parameters being unchanged, the spatial distributions of the LTE plasma region changed significantly with increasing the plasma working gas flow rate; and the decrease of the chamber pressure leads to the expansion of the high-temperature region and the shrink of the LTE plasma region, which shows a more significant non-LTE feature of the plasma arc-jet. The calculated heavy-particle temperature distributions and the arc voltages are qualitatively consistent with the experimental measurements.

On the use of the double floating probe method to infer the difference between the electron and the heavy particles temperatures in an atmospheric pressure, vortex-stabilized nitrogen plasma jet.

Sweeping double probe measurements in an atmospheric pressure direct current vortex-stabilized plasma jet are reported (plasma conditions: 100 A discharge current, N2 gas flow rate of 25 Nl/min, thoriated tungsten rod-type cathode, copper anode with 5 mm inner diameter). The interpretation of the double probe characteristic was based on a generalization of the standard double floating probe formulae for non-uniform plasmas coupled to a non-equilibrium plasma composition model. Perturbations caused by the current to the probe together with collisional and thermal processes inside the probe perturbed region were taken into account. Radial values of the average electron and heavy particle temperatures as well as the electron density were obtained. The calculation of the temperature values did not require any specific assumption about a temperature relationship between different particle species. An electron temperature of 10,900 ± 900 K, a heavy particle temperature of 9300 ± 900 K, and an electron density of about 3.5 × 10(22) m(-3) were found at the jet centre at 3.5 mm downstream from the torch exit. Large deviations from kinetic equilibrium were found toward the outer border of the plasma jet. These results showed good agreement with those previously reported by the authors by using a single probe technique. The calculations have shown that this method is particularly useful for studying spraying-type plasma torches operated at power levels of about 15 kW.

Theoretical study of the formation mechanism of laser-induced aluminum plasmas using Nd:YAG fundamental, second or third harmonics

The Collisional-Radiative model CoRaM-Al is implemented in a 0D numerical code in the purpose of describing the formation of the plasma resulting from the interaction between a 4ns Nd:YAG laser pulse and an aluminum sample in vacuum. The influence of the first three harmonics at 355nm, 532nm and 1064nm is studied. In each case, the fluence value is set equal to the threshold above which a phase explosion takes place. The sample surface temperature is therefore limited to the critical temperature (6700K). The solid→gas transition is classically treated by using the Hertz–Knudsen law. The species considered in CoRaM-Al are Al, Al+, Al2+, Al3+ and free electrons. They are in thermochemical non-equilibrium at different temperatures. Each state behaves freely owing to electron impact induced excitation and ionization, elastic collisions, Multiphoton Ionization (MPI), Inverse Bremsstrahlung (IB), thermal Bremsstrahlung, and spontaneous emission.The results show that MPI and IB play a complementary role in the heating process of electrons. In the ultraviolet and visible cases, the increase in electron temperature is mainly due to multiphoton ionization before the maximum irradiance. The electron temperature does not exceed 9000K. The heavy particle temperature mainly results from the surface temperature. Conversely, in the infrared case, electron temperature strongly increases after the maximum irradiance owing to inverse Bremsstrahlung. Electrons then give a part of their energy for the benefit of heavy particles, which leads to the increase in their temperature. The electron number density reaches its maximum during the laser pulse with a value of the order of 1025m−3 and the electron temperature exceeds 13,000K. A maximum of the ionization degree close to 0.2 is thus obtained at 1064nm.

Afterglow of Argon Plasmas with H2, O2, N2, and CO2Admixtures Observed by Thomson Scattering

This study assess the decay of a power-pulsed microwave plasma in argon mixtures using time resolved Thomson scattering. In argon at intermediate pressure, i.e. tens of mbar, the electron temperature decays very fast after power off and approaches the heavy particle temperature, while the electron density decays slower, in the order of tens of microseconds. Argon with small admixtures of O2 or H2 exhibits a similar behavior. However, strikingly different is the result for small additions of N2 or CO2. For up to 100 μs after the power is switched off, the electron temperature does not decrease to the gas temperature but remains at values as high as 0.8 eV. Nitrogen and carbon dioxide exhibit relatively large cross-sections to excite vibrational states by electron collisions. Knowing that, the observed post-heating can be attributed to super-elastic energy transfer from vibrational excited N2(X) and CO2(X) molecules to electrons.

Influences of gas flowing on the features of a helium radio-frequency atmospheric-pressure glow discharge

In this paper, a zoning model is employed to investigate the influences of gas flowing on the characteristics of the high-purity helium radio-frequency atmospheric-pressure glow discharge (RF APGD) plasma. The modeling results show that the influences of the gas flowing on the plasma features in the discharge region and the jet region are different. In the discharge region, the heavy-particle temperature decreases with the increase of helium flow rate, while the variations of the electron energy and the species concentrations are less than 0.5%. In the plasma jet region, the variation of the heavy-particle temperature shows a non-monotonous form with the gas flow rate, while the species concentrations become higher within a certain distance (e.g., smaller than 2 mm away from the plasma generator exit) at a higher helium flow rate. The modeling results are also qualitatively validated by comparing with the measured gas temperatures in both the discharge region and the plasma jet region.

Approximate equations of state in two-temperature plasma mixtures

Approximate thermodynamic state relations for multicomponent atomic and molecular gas mixtures are often constructed by artificially partitioning the mixture into its constituent materials and requiring the separated materials to be in temperature and pressure equilibrium. Iterative numerical algorithms have been employed to enforce this equilibration and compute the resulting approximate state relations in single-temperature mixtures. In partially ionized gas mixtures, there is both theoretical and empirical evidence that equilibrating the chemical potentials, number densities, or partial pressures of the free electrons is likely to produce more accurate results than equilibrating the total pressures. Moreover, in many situations of practical interest the free electrons and heavy particles have different temperatures. In this paper, we present a generalized algorithm for equilibrating the heavy-particle and electron temperatures and a third user-specified independent thermodynamic variable in a two-temperature plasma mixture. Test calculations based on the equilibration of total pressure vs. electron pressure are presented for three different mixtures.

Electrical Parameters Investigation and Zero Flow Rate Effect of Nitrogen Atmospheric Nonthermal Plasma Jet

The construction and operation of atmospheric nonthermal plasma jet, ANPJ, are presented in this work as well as the experimental investigations of its electrical parameters, the configuration of plasma jet column and its temperature. The device is energized by a low-cost Neon power supply of (10 kV, 30 mA, and 20 kHz) and the discharge takes place by using N2 gas with different flow rates from 3 to 25 L/min and input voltage of 6 kV. Diagnostic techniques such as voltage divider, Lissajous figure, image processing and thermometer are used. The electrical characteristics of discharge at different flow rates of N2 gas such as discharge voltage, current, mean power, power efficiency, and mean energy have been studied. The experimental results show that the maximum plasma jet length of 14 mm is detected at flow rate of 12 L/min. The results of plasma jet (heavy particles) temperature along the jet length show that jet plasma has approximately a room temperature at the jet column end. The results of zero flow rate effect on the ANPJ operation show damage in the Teflon insulator and a corrosion in the Aluminum electrodes.

Nonuniqueness of two-temperature Guldberg-Waage and Saha equations: Influence on thermophysical properties of SF6 plasmas

This paper focuses to study how the choice of Guldberg-Waage and Saha equations affects the thermodynamic properties and transport coefficients of SF6 plasmas under both thermal equilibrium and non-equilibrium conditions. The species composition is numerically determined using two typical forms of two-temperature Saha equations and Guldberg-Waage equations that have appeared in the literature. The great influence of the choice of the excitation temperature on the plasma composition and hence the thermodynamic properties and transport coefficients is discussed as well. Transport coefficients are calculated with most recent collision interaction potentials by adopting Devoto's electron and heavy particle decoupling approach but expanded to the third-order approximation (second-order for viscosity) within the framework of Chapman-Enskog method. Furthermore, an analysis of the effect of different definitions of Debye length on the properties values was performed as well. The results are computed for various values of pressures from 0.10 atm to 10 atm and non-equilibrium parameter, i.e., ratio of the electron temperature to the heavy particle temperature from 1 to 5 with electron temperature range from 300 to 40 000 K. Both forms of Guldberg-Waage and Saha equations used here can give completely the same value when the two-temperature model reaches the special case of local thermodynamic equilibrium. It has been observed that all above mentioned factors can significantly modify the plasma species composition and consequently affect the thermodynamic and transport properties.

Investigation on critical breakdown electric field of hot sulfur hexafluoride/carbon tetrafluoride mixtures for high voltage circuit breaker applications

Sulfur hexafluoride (SF6) gas, widely used in high-voltage circuit breakers, has a high global warming potential and hence substitutes are being sought. The use of a mixture of carbon tetrafluoride (CF4) and SF6 is examined here. It is known that this reduces the breakdown voltage at room temperature. However, the electrical breakdown in a circuit breaker after arc interruption occurs in a hot gas environment, with a complicated species composition because of the occurrence of dissociation and other reactions. The likelihood of breakdown depends on the electron interactions with all these species. The critical reduced electric field strength (the field at which breakdown can occur, relative to the number density) of hot SF6/CF4 mixtures corresponding to the dielectric recovery phase of a high voltage circuit breaker is calculated in the temperature range from 300 K to 3500 K. The equilibrium compositions of hot SF6/CF4 mixtures under different mixing fractions were determined based on Gibbs free energy minimization. Full sets of improved cross sections for interactions between electrons and the species present are presented. The critical reduced electric field strength of these mixtures was obtained by balancing electron generation and loss mechanisms. These were evaluated using the electron energy distribution function derived from the Boltzmann transport equation under the two-term approximation. The result indicates that critical electric field strength decreases with increasing heavy-particle temperature from 1500 to 3500 K. Good agreement was found between calculations for pure hot SF6 and pure hot CF4 and experimental results and previous calculations. The addition of CF4 to SF6 was found to increase the critical reduced electric field strength for temperatures above 1500 K, indicating the potential of replacing SF6 by SF6/CF4 mixtures in high-voltage circuit breakers.

Plasma chemistry in the free-burning Ar arc

Recent ambitions have been aimed at modelling of a free-burning arc in argon by means of a self-consistent description of the arc plasma and the electrodes avoiding assumptions of equilibrium. As a next step, the present work deals with the impact of the reaction chemistry used in non-equilibrium modelling. A set of reactions accounts for elastic scattering, excitation and de-excitation in collisions with electrons and atoms, step-wise ionization, recombination and spontaneous emission. A simple chemistry model based on the approximation of prompt ionization, a model with an effective excited level, and a model implying higher excited levels of argon atoms are considered. The results obtained show that all three models are capable of predicting the arc characteristics. They deliver values of electric current density, electric conductivity, arc voltage, electron and heavy particle temperature which are close to each another. The simple model can serve as a fast-track evaluation of the arc characteristics, and for sensitive studies of the impact of model parameters. The extended model allows obtaining the populations of individual and effective levels giving rise to important radiative transitions.

Two-temperature modelling and optical emission spectroscopy of a constant current plasma arc welding process

In this work, a plasma arc welding process with constant current in the range 25–70 A operating in pure Ar is characterized by means of both thermo-fluid-dynamic modelling under the assumption of local thermodynamic equilibrium (LTE) and two-temperature thermal non-equilibrium modelling (2T), allowing a comparison of the LTE temperature fields with electron and heavy particle temperature fields: thermal non-equilibrium is strongest in the fringes of the arc and upstream the plasma flow even though a temperature difference between electrons and heavy particles is also found in the arc core in the nozzle orifice, due to the high velocity of the gas. Also, excitation temperature of Ar atoms is obtained from optical emission spectroscopy measurements using a new method (called hybrid method) that extends the usability of the Boltzmann plot method to spatial regions where the signal-to-noise ratio of the spectral lines adopted in the calculation is poor. Good agreement is obtained between the modelling predicted electron temperature and the measured excitation temperature in the whole investigated spatial region.

Instabilities, waves, and nonequilibrium structures in a dense low-frequency plasma (a look at the problem after 50 years publications in the journal High Temperature)

This review is devoted to one of the fundamental problems of plasma physics, namely, waves and instabilities in a dense low-temperature plasma. Instabilities are characteristic of a large number of natural phenomena. They affect the existence of nonequilibrium systems and cause a transition from one state to another. Ensuring stability of a process is the central problem in many technologies in which a low-temperature plasma is used as an working medium. This review is aimed at revealing the key mechanisms which lead to instabilities and at presenting the instability criteria in a clear physical form. A method that allows considering the modern state of the problems of instabilities and propagation of waves with a specified wave number in a dense low-temperature plasma of gases and metal vapors from a unified standpoint is discussed. Acoustic, overheating, current-convective, ionization, and ionization-field instabilities are considered in detail. This study is performed in the framework of continuum models both for a dense plasma in which the temperatures of electrons and heavy particles (atoms and ions) do not almost differ (Ta ≈ Te) and a plasma in which the electron temperature differs from the temperature of heavy particles (Ta ≠ Te). Particular attention is given to the effects of the Ampere force, Joule heating, and the processes related to changes in the plasma composition (ionization and recombination) and the energy exchange between the plasma components in the absence of temperature equilibrium. The contribution of the half-century publications in High Temperature to investigation of this problem is traced.

Simultaneous measurement of electron and heavy particle temperatures in He laser-induced plasma by Thomson and Rayleigh scattering

Thomson and Rayleigh scattering methods were applied to quantify the electron and heavy particle temperatures, as well as electron number density, in a laser spark in helium at atmospheric pressure. Plasma was created using 4.5 ns, 25 mJ pulses from Nd:YAG laser at 532 nm. Measurements, performed for the time interval between 20 ns and 800 ns after breakdown, show electron density and temperature to decrease from 7.8×1023 m−3 to 2.6×1022 m−3 and from 95 900 K to 10 350 K, respectively. At the same time, the heavy particle temperature drops from only 47 000 K down to 4100 K which indicates a two temperature plasma out of local isothermal equilibrium.

Theoretical investigation of the decay of an SF6 gas-blast arc using a two-temperature hydrodynamic model

The behaviour of a decaying SF6 arc, which is representative of the approach to the final current-zero state of switching arcs in a high-voltage circuit breaker, is theoretically investigated by a two-temperature hydrodynamic model, taking into account the possible departure of the plasma state from local thermodynamic equilibrium (LTE). The model couples the plasma flow with electromagnetic fields in a self-consistent manner. The electrons and heavy species are assumed to have different temperatures. The species composition, thermodynamic properties and transport coefficients of the plasma under non-LTE conditions are calculated from fundamental theory. The model is then applied to a two-dimensional axisymmetric SF6 arc burning in a supersonic nozzle under well-controlled conditions; for this configuration, experimental results are available for comparison. The effect of turbulence is considered using the Prandtl mixing-length model. The edge absorption of the radiation emitted by the arc core is taken into account by a modified net emission coefficient approach. The complete set of conservation equations is discretized and solved using the finite volume method. The evolution of electron and heavy-particle temperatures and the total arc resistance, along with other physical quantities, is carefully analysed and compared with those of the LTE case. It is demonstrated that the electron and heavy-particle temperature diverge at all times in the plasma–cold-flow interaction region, in which strong gas flow exists, and further in the transient current-zero period, in which case the collision energy exchange is ineffective. This study quantitatively analyses the energy exchange mechanisms between electrons and heavy particles in the high-pressure supersonic SF6 arcs and provides the foundation for further theoretical investigation of transient SF6 arc behaviour as the current ramps down to zero in gas-blast circuit breakers.

Langmuir probe diagnostics of an atmospheric pressure, vortex–stabilized nitrogen plasma jet

Langmuir probe measurements in an atmospheric pressure direct current (dc) plasma jet are reported. Sweeping probes were used. The experiment was carried out using a dc non–transferred arc torch with a rod–type cathode and an anode of 5 mm diameter. The torch was operated at a nominal power level of 15 kW with a nitrogen flow rate of 25 Nl min−1. A flat ion saturation region was found in the current–voltage curve of the probe. The ion saturation current to a cylindrical probe in a high–pressure non local thermal equilibrium (LTE) plasma was modeled. Thermal effects and ionization/recombination processes inside the probe perturbed region were taken into account. Averaged radial profiles of the electron and heavy particle temperatures as well as the electron density were obtained. An electron temperature around 11 000 K, a heavy particle temperature around 9500 K and an electron density of about 4 × 1022 m−3, were found at the jet centre at 3.5 mm downstream from the torch exit. Large deviations from kinetic equilibrium were found throughout the plasma jet. The electron and heavy particle temperature profiles showed good agreement with those reported in the literature by using spectroscopic techniques. It was also found that the temperature radial profile based on LTE was very close to that of the electrons. The calculations have shown that this method is particularly useful for studying spraying–type plasma jets characterized by electron temperatures in the range 9000–14 000 K.

Sheath and arc-column voltages in high-pressure arc discharges

Electrical characteristics of a 1 cm-long free-burning atmospheric-pressure argon arc are calculated by means of a model taking into account the existence of a near-cathode space-charge sheath and the discrepancy between the electron and heavy-particle temperatures in the arc column. The computed arc voltage exhibits a variation with the arc current I similar to the one revealed by the experiment and exceeds experimental values by no more than approximately 2 V in the current range 20–175 A. The sheath contributes about two-thirds or more of the arc voltage. The LTE model predicts a different variation of the arc voltage with I and underestimates the experimental values appreciably for low currents but by no more than approximately 2 V for I ≳ 120 A. However, the latter can hardly be considered as a proof of unimportance of the space-charge sheath at high currents: the LTE model overestimates both the resistance of the bulk of the arc column and the resistance of the part of the column that is adjacent to the cathode, and this overestimation to a certain extent compensates for the neglect of the voltage drop in the sheath. Furthermore, if the latter resistance were evaluated in the framework of the LTE model in an accurate way, then the overestimation would be still much stronger and the obtained voltage would significantly exceed those observed in the experiment.

Measurement of Plasma Parameters of Gliding Arc Driven by the Transverse Magnetic Field

The gliding arc can offer high energy efficiency and selectivity for chemical reactions and has been widely applied in material processing, environmental protection and other industrial areas. But the discharge properties, measurement of plasma parameters and related physical processes of the gliding arc discharge still need to further studied. In this study, the gliding arc was driven by the transverse magnetic field to produce the non-equilibrium plasma at high pressure. The parameters of the plasma at our observed point were measured by optical methods. The experimental result shows that the electron temperature is about 0.6 eV and the heavy particle temperature is approximately 2987±250 K.

Thermodynamic and transport properties of two-temperature SF6 plasmas

This paper deals with thermodynamic and transport properties of SF6 plasmas in a two-temperature model for both thermal equilibrium and non-equilibrium conditions. The species composition and thermodynamic properties are numerically determined using the two-temperature Saha equation and Guldberg-Waage equation according to deviation of van de Sanden et al. Transport properties including diffusion coefficient, viscosity, thermal conductivity, and electrical conductivity are calculated with most recent collision interaction potentials by adopting Devoto’s electron and heavy particle decoupling approach but expanded to the third-order approximation (second-order for viscosity) in the frame of Chapman–Enskog method. The results are computed for various values of pressures from 0.1 atm to 10 atm and ratios of the electron temperature to the heavy particle temperature from 1 to 20 with electron temperature range from 300 to 40 000 K. In the local thermodynamic equilibrium regime, results are compared with available results of previously published studies.

Rayleigh scattering on a microwave surfatron plasma to obtain axial profiles of the atom density and temperature

The axial dependency of the central-axis value of the heavy particle density and temperature of surface-wave plasmas is studied using Rayleigh scattering (RyS). The plasma is generated at a frequency of 2.45 GHz in argon by a surfatron operating under the standard settings of a power of 45 W, a flow rate of 50 sccm and a pressure of 20 mbar. To investigate the effect of the pressure on the gas temperature, we also investigated 6 and 10 mbar plasmas. By using a two-dimensional intensified CCD array we could determine and eliminate the influence of false stray light, a major disturbing factor in the determination of the Rayleigh signal. In order to trace the energy fluxes that determine the gas temperature, we performed Thomson scattering so that the properties of the electron gas are known. It is found that the gas temperature, T a , depends on the wall temperature and the product of the gas pressure and the electron pressure. The latter implies that T a follows the electron density axially, meaning that it is highest at the launcher and decreases monotonically in the wave propagation direction. The maximum gas temperature of around T a = 800 K is found close to the launcher for the highest gas pressure of 20 mbar. For lower pressures we find lower T a values. The extrapolation of T a toward the end of the plasma column leads to a temperature of about 320 K. This study reveals that, for the argon plasmas under study, the central-axis values of the gas temperature are determined by the balance between the heating of the gas by means of elastic electron collisions and the cooling due to heat conduction from the center to the wall.