Partial Local Thermodynamic Equilibrium Research Articles

Experimental study on dynamic characteristics of charged particles in vacuum arc

The vacuum arc is essentially a process of evaporation, collision and ionization of metal materials to keep burning. Electron temperature and density are important parameters to describe vacuum arc plasma transport. This paper was based on the theory of continuous spectral radiation, combined with the principle of two-colorimetric temperature measurement, and realized 2D calculation and observation of electron temperature and density. The experimental platform for vacuum arc breaking and an optical image acquisition system were established to achieve synchronous acquisition of two wavelengths of arc by imaging separation method. On the basis of partial local thermodynamic equilibrium hypothesis, and obtained the 2D dynamic distribution of the electron temperature and density. The results showed that electron density decayed along the radial direction and the density near anode was lower in the axial direction. The 2D distribution trend of electron temperature was opposite to the density. Electron temperature had a little change and the electron density expands outward in different arcing times. With the increase of current amplitude, the peak value of electron density increased, and the electron density in the arc gap also increased. The measured results could reflect the formation and movement of microscopic particles at the initial diffusion stage.

Characteristics of plasma stream evolution in a pulsed plasma accelerator

The time-integrated spectroscopic measurements of argon plasma are carried out to characterize the transient plasma stream produced in a coaxial pulsed plasma accelerator. Transient plasmas produced in these types of devices have a temporal change in density and temperature. The temporal evolution of the plasma stream and its characteristic variation are studied from a time of 50–300 μs in steps of 50 μs. Analysis of different spectral lines shows the existence of some metastable states of Ar+ having a long lifetime. The evolution of different Ar+ transitions to metastable and non-metastable lower levels is observed for different time frames. From the Stark broadening and Boltzmann plot method assuming the partial local thermodynamic equilibrium model, the plasma densities and excitation temperatures are estimated. The temporal evolution study shows a decrease in plasma densities from 1.96 × 1021 m−3 at 50 μs to 1.23 × 1020 m−3 at 300 μs after the initiation of plasma formation. A reduction in excitation temperature is observed from 0.86 to 0.77 eV. Based on the density profile, the estimated energy density of the plasma stream shows a time-dependent decreasing trend from 0.06 to 0.004 MJ/m2 during the observed period. The results may be useful for optimization of the plasma stream in pulsed plasma accelerators concerning its different parameters.

Shunting the electric arc helical spiral turns (numerical simulation)

A numerical simulation was performed for an open direct current (dc) electric arc in a uniform external axial magnetic field. The calculation was carried out in the frameworks of a non-stationary three-dimensional mathematical model in approximation of partial local thermodynamic equilibrium of plasma. The model does not presuppose a helical shape of the arc, as is customary in theoretical studies based on linear perturbation theory. It was shown that the helical shape of an open arc is not constant in time and periodically collapses with subsequent restoration. In a certain range of external parameters of the arc (current strength, inter-electrode distance, and external axial magnetic field), a new mode of destruction of a regular structure was revealed – the arc section shunting between the spiral turns. Shunting can be single or double when the breakdown occurs simultaneously in two places. Apparently, it is caused by convective heat transfer by plasma flows as well as by the sliding of the arc thermal field. The appeared thermal field creates a zone of electric conductivity in which the breakdown occurs.

Heavy-ions induced scintillation experiments

The French Alternative Energies and Atomic Energy Commission (CEA) is developing fission chambers relying on gas luminescence capable of high gamma signal rejection and self-diagnostic capabilities allowing high dependability level. Scintillation yield of noble gases by energetic ions (average initial energy between 68 MeV to 99 MeV) is evaluated for nuclear power reactor instrumentation application. Neutron interaction with fissile material can provide energetic heavy ions at a rate proportional to the neutron flux. The understanding of fission fragments as excitation source is required for noble gas scintillation neutron detectors. This work deals with evaluation of the optical signal heavy-ions induced in cold plasma. A dedicated experimental device (PSEG) is presented. Plutonium 238 is selected as a continuous source of heavy ions in order to avoid technological constraints of neutron flux tests, as a first step in the experimental validation process. Estimation of signal power is performed as well as Optical Emission Spectrum analysis. As expected, no Partial Local Thermodynamic Equilibrium is satisfied from experiments. The effect of pressure on the visible to infrared emission spectrum is analysed.

Evolution of space open electric arc burning in the external axial magnetic field

The calculation was made for open DC electric arc burning in an external uniform axial magnetic field. It was performed within the framework of a nonstationary three-dimensional mathematical model in approximation of partial local thermodynamic equilibrium of plasma. A “schematic” analog of electron temperature fluctuations was proposed for numerical realization of the open electric arc column of a helical shape. According to calculations, it was established that the column of the open electric arc takes a helical space shape. Plasma rotates around a longitudinal axis of the arc, at that the directions of plasma rotation near the cathode and the anode are opposite. In the arc cross-sections, the velocity of plasma rotation is unequal and the deviation value of the same part of the arc from the central axis varies in time. A helical shape of the open arc is not stable and varies in time. Apparently, the open arc cannot remain stable and invariable in the time helical shape in the external axial magnetic field.

Electron Temperature Distribution Measurements in Clouds of Polystyrene Pellets Ablating in LHD Heliotron Plasma

Two methods for measuring the spatial distribution of the temperature of electrons in a cloud near a polystyrene pellet that ablates in LHD heliotron plasma are described. The first method is based on measuring the ratio of the local emission-coefficient values at wavelengths of 486 ± 5 nm (the Hβ line) and 630 ± 5 nm (continuum). The second (new) method is based on the measurement of radiation distributions of the Hβ line in the direction along the magnetic field. Both the methods use an assumption about the partial local thermodynamic equilibrium in the cloud and show close results. It is shown for the first time that the temperature of cloud electrons increases from 0.8 eV in the immediate vicinity of the pellet surface to 6.0–7.0 eV at a distance of 6–8 mm from the pellet in the direction along the magnetic field, which agrees with the experimentally observed longitudinal distribution of the Hβ radiation in the cloud.

Измерение пространственных распределений температуры электронов в облаках полистироловых пеллетов, испаряющихся в плазме гелиотрона LHD

AbstractTwo methods for measuring the spatial distribution of the temperature of electrons in a cloud near a polystyrene pellet that ablates in LHD heliotron plasma are described. The first method is based on measuring the ratio of the local emission-coefficient values at wavelengths of 486 ± 5 nm (the H _β line) and 630 ± 5 nm (continuum). The second (new) method is based on the measurement of radiation distributions of the H _β line in the direction along the magnetic field. Both the methods use an assumption about the partial local thermodynamic equilibrium in the cloud and show close results. It is shown for the first time that the temperature of cloud electrons increases from 0.8 eV in the immediate vicinity of the pellet surface to 6.0–7.0 eV at a distance of 6–8 mm from the pellet in the direction along the magnetic field, which agrees with the experimentally observed longitudinal distribution of the H _β radiation in the cloud.

A software package for rigorously calculating optical plasma spectra and automatically retrieving plasma properties

A free software package code named OPSIAL (Optical Plasma Spectral Calculation And Parameters Retrieval) for rigorously calculating optical plasma spectra and for automatically retrieving plasma parameters is presented.

Numerical simulation of the screw shape of an electric arc in an external axial magnetic field

The characteristics of an electric arc of a direct current, burning in a cylindrical channel in a uniform external axial field, are numerically computed within a nonstationary three-dimensional mathematical model at partial local thermodynamic equilibrium. A method for the numeric modeling of the screw shape of an arc in this field is suggested. This approach is in addition to the “network” analogue of fluctuations for the temperature of electrons, which increases weak numeric asymmetry of electron temperature distribution that occurs randomly during computation. This asymmetry can be “picked up” by an external magnetic field and continue to increase up to a certain value, which is enough to form an arc column screw structure. If there are no fluctuations in the computation algorithm of fluctuations, the arc column in an external axial magnetic field maintains cylindrical axial symmetry and the arc screw shape is not observed.

Dynamics of Surface Streamer Plasmas at Atmospheric Pressure: Mixtures of Argon and Methane

A nonequilibrium atmospheric streamer discharge was investigated as a means to seed a large-gap arc breakdown. The dynamics of the streamer were analyzed with high speed imaging, photodiode light intensity, and current–voltage measurements. The temporal evolution of the discharge included a localized surface corona and a positive surface streamer. With the addition of an impurity gas (methane), the ionization was suppressed, which inhibited surface streamer propagation. The electron temperature was determined from time and spatially averaged spectra, coupled with a collisional–radiative model. The electron temperature in argon was measured at 1.25 eV for an electron density range of 1019 – 1020 m –3. Partial local thermodynamic equilibrium calculations showed that the Ar II 4p states followed a Boltzmann distribution with an excitation temperature of 0.7 eV. The gas temperature was estimated at 815 K from a black-body distribution. The velocity of the surface streamer in argon was estimated at ~100 km/s with a diameter of $\sim 500~\mu \text{m}$ .

Temporal variations of electron density and temperature in Kr/Ne/H2 photoionized plasma induced by nanosecond pulses from extreme ultraviolet source

Spectral investigations of low-temperature photoionized plasmas created in a Kr/Ne/H2 gas mixture were performed. The low-temperature plasmas were generated by gas mixture irradiation using extreme ultraviolet pulses from a laser-plasma source. Emission spectra in the ultraviolet/visible range from the photoionized plasmas contained lines that mainly corresponded to neutral atoms and singly charged ions. Temporal variations in the plasma electron temperature and electron density were studied using different characteristic emission lines at various delay times. Results, based on Kr II lines, showed that the electron temperature decreased from 1.7 to 0.9 eV. The electron densities were estimated using different spectral lines at each delay time. In general, except for the Hβ line, in which the electron density decreased from 3.78 × 1016 cm−3 at 200 ns to 5.77 × 1015 cm−3 at 2000 ns, most of the electron density values measured from the different lines were of the order of 1015 cm−3 and decreased slightly while maintaining the same order when the delay time increased. The time dependences of the measured and simulated intensities of a spectral line of interest were also investigated. The validity of the partial or full local thermodynamic equilibrium (LTE) conditions in plasma was explained based on time-resolved electron density measurements. The partial LTE condition was satisfied for delay times in the 200 ns to 1500 ns range. The results are summarized, and the dominant basic atomic processes in the gas mixture photoionized plasma are discussed.

Excited atoms in the free-burning Ar arc: treatment of the resonance radiation

The collisional–radiative model with an emphasis on the accurate treatment of the resonance radiation transport is developed and applied to the free-burning Ar arc plasma. This model allows for analysis of the influence of resonance radiation on the spatial density profiles of the atoms in different excited states. The comparison of the radial density profiles obtained using an effective transition probability approximation with the results of the accurate solution demonstrates the distinct impact of transport on the profiles and absolute densities of the excited atoms, especially in the arc fringes. The departures from the Saha–Boltzmann equilibrium distributions, caused by different radiative transitions, are analyzed. For the case of the DC arc, the local thermodynamic equilibrium (LTE) state holds close to the arc axis, while strong deviations from the equilibrium state on the periphery occur. In the intermediate radial positions the conditions of partial LTE are fulfilled.

Numerical simulation of a helical shape electric arc in the external axial magnetic field

Within the frameworks of non-stationary three-dimensional mathematical model, in approximation of a partial local thermodynamic equilibrium, a numerical calculation was made of characteristics of DC electric arc burning in a cylindrical channel in the uniform external axial magnetic field. The method of numerical simulation of the arc of helical shape in a uniform external axial magnetic field was proposed. This method consists in that that in the computational algorithm, a “scheme” analog of fluctuations for electrons temperature is supplemented. The “scheme” analogue of fluctuations increases a weak numerical asymmetry of electrons temperature distribution, which occurs randomly in the course of computing. This asymmetry can be “picked up” by the external magnetic field that continues to increase up to a certain value, which is sufficient for the formation of helical structure of the arc column. In the absence of fluctuations in the computational algorithm, the arc column in the external axial magnetic field maintains cylindrical axial symmetry, and a helical form of the arc is not observed.

Kr photoionized plasma induced by intense extreme ultraviolet pulses

Irradiation of any gas with an intense EUV (extreme ultraviolet) radiation beam can result in creation of photoionized plasmas. The parameters of such plasmas can be significantly different when compared with those of the laser produced plasmas (LPP) or discharge plasmas. In this work, the photoionized plasmas were created in a krypton gas irradiated using an LPP EUV source operating at a 10 Hz repetition rate. The Kr gas was injected into the vacuum chamber synchronously with the EUV radiation pulses. The EUV beam was focused onto a Kr gas stream using an axisymmetrical ellipsoidal collector. The resulting low temperature Kr plasmas emitted electromagnetic radiation in the wide spectral range. The emission spectra were measured either in the EUV or an optical range. The EUV spectrum was dominated by emission lines originating from Kr III and Kr IV ions, and the UV/VIS spectra were composed from Kr II and Kr I lines. The spectral lines recorded in EUV, UV, and VIS ranges were used for the construction of Boltzmann plots to be used for the estimation of the electron temperature. It was shown that for the lowest Kr III and Kr IV levels, the local thermodynamic equilibrium (LTE) conditions were not fulfilled. The electron temperature was thus estimated based on Kr II and Kr I species where the partial LTE conditions could be expected.

Compositional Analysis of Aerosols Using Calibration-Free Laser-Induced Breakdown Spectroscopy.

We demonstrate that the elemental composition of aerosols can be measured using laser-induced breakdown spectroscopy (LIBS) without any preliminary calibration with standard samples. Therefore, a nanosecond Nd:YAG laser beam was focused into a flux of helium charged with alumina aerosols of a few micrometers diameter. The emission spectrum of the laser-generated breakdown plasma was recorded with an echelle spectrometer coupled to a gated detector. The spectral features including emission from both the helium carrier gas and the Al2O3 aerosols were analyzed on the base of a partial local thermodynamic equilibrium. Thus, Boltzmann equilibrium distributions of population number densities were assumed for all plasma species except of helium atoms and ions. By analyzing spectra recorded for different delays between the laser pulse and the detector gate, it is shown that accurate composition measurements are only possible for delays ≤1 μs, when the electron density is large enough to ensure collisional equilibrium for the aerosol vapor species. The results are consistent with previous studies of calibration-free LIBS measurements of solid alumina and glass and promote compositional analysis of aerosols via laser-induced breakdown in helium.

Stark broadening for diagnostics of the electron density in non-equilibrium plasma utilizing isotope hydrogen alpha lines

Two Stark broadening parameters including FWHM (full width at half maximum) and FWHA (full width at half area) of isotope hydrogen alpha lines are simultaneously introduced to determine the electron density of a pulsed vacuum arc jet. To estimate the gas temperature, the rotational temperature of the C2 Swan system is fit to 2500 ± 100 K. A modified Boltzmann-plot method with bi-factor is introduced to determine the modified electron temperature. The comparison between results of atomic and ionic lines indicates the jet is in partial local thermodynamic equilibrium and the electron temperature is close to 13 000 ± 400 K. Based on the computational results of Gig-Card calculation, a simple and precise interpolation algorithm for the discrete-points tables can be constructed to obtain the traditional ne-Te diagnostic maps of two Stark broadening parameters. The results from FWHA formula by the direct use of FWHM = FWHA and these from the diagnostic map are different. It can be attributed to the imprecise FWHA formula form and the deviation between FWHM and FWHA. The variation of the reduced mass pair due to the non-equilibrium effect contributes to the difference of the results derived from two hydrogen isotope alpha lines. Based on the Stark broadening analysis in this work, a corrected method is set up to determine ne of (1.10 ± 0.08) × 1021 m−3, the reference reduced mass μ0 pair of (3.30 ± 0.82 and 1.65 ± 0.41), and the ion kinetic temperature of 7900 ± 1800 K.

Unsteady three-dimensional model of electric arc. Part 1. Mathematical model and testing results

A non-stationary three-dimensional mathematical model of the electric arc of constant current in the approximation of a partial local thermodynamic equilibrium of plasma is presented. For the purpose of testing the model and computer code a computation of the isothermal laminar viscous fluid flow past a circular cylinder (the testing of the dynamic part of the system of equations) and a computation of the axisymmetric electric arc with a reduced anode attachment have been done. A fair agreement of numerical results with experimental data points overall to the correctness of the mathematical model and the numerical solution technique.

Study on electron temperature in an ablative pulsed plasma thruster by optical emission spectroscopy

The electron temperature obtained by optical emission spectroscopy was studied in an ablative pulsed plasma thruster. The validity of partial local thermodynamic equilibrium criteria for the correct spectroscopic determination of the electron temperature can be proved and the spectral absorption of the plasma was calculated and ignored. It is shown that the existence of two distinguishable electron groups is found. The variation trends of the part data of the experiment are in agreement with those of Gatsonis and Markusic under some conditions, respectively. Combined with the discharge current, the variation of the electron temperature and the transferring process of electron energy were discussed.

Characterization of an Atmospheric-Pressure Helium Plasma Generated by 2.45-GHz Microwave Power

An atmospheric-pressure helium plasma generated in a 2.45-GHz microwave-induced cylindrical plasma torch has been investigated. The atmospheric-pressure helium plasma can be ignited at a microwave power of less than 400 W and sustained at as low as 100 W, at a gas flow of 50 L/min. The electron temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> , electron density n <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> , and gas temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> measured by optical emission spectroscopy are 0.28-0.5 eV, 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">15</sup> -10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">16</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , and 1000-2000 K, respectively, at a power of 200-400 W and a gas flow rate of 20-50 L/min. In addition, it is noted that T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> ≈ T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</sub> (vibrational temperature) >; T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ≈ T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> (rotational temperature) for these experiment parameters. These results show that the helium plasma generated in this atmosphericpressure plasma torch is in partial local thermodynamic equilibrium (pLTE).

Reduction of matrix effects in quantitative and semi-quantitative inductively coupled plasma-optical emission spectrometry using a partial local thermodynamic equilibrium model and an internal standard

A means to reduce matrix effects by using a simple pLTE model to account for changes in sensitivity due to matrix induced changes in plasma temperature and an internal standard to account for matrix induced changes in sample transport rate is described. For quantitative analysis where standards contain all of the elements of interest the changes in sensitivity due to 1.0M Ba and Na and 1.5M Li and Ca are reduced to less than 20% for most analyte lines investigated using this method. The maximum error due to the presence of 1.5M La is reduced from more than a factor of 14 to less than 40%. Semi-quantitative concentrations from the pLTE model based single element calibration approach described previously are essentially independent of matrix composition (within the uncertainty of semi-quantitative concentrations in the absence of a high matrix concentration) for the concomitant elements tested. However, the pLTE based approach is less accurate for semi-quantitative analysis than a one-time multi-element calibration, except when the most severe matrix effects are observed (such as those due to 1.5M La).