FRVT - Artículos en Revistas Internacionales

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    Hydrodynamic Model for the Plasma-Gas Flow in a Cutting Torch Nozzle.
    (2004) Kelly, Héctor; Minotti, Fernando; Prevosto, Leandro; Mancinelli, Beatriz
    We present a simple hydrodynamic model to obtain the profiles of the relevant physical quantities along a nozzle of arbitrary cross-section in a cutting torch. The model uses a two-zone approximation (a hot central plasma carrying the discharge current wrapped by a relatively cold gas which thermally isolates the nozzle wall from the plasma). Seeking for a solution with sonic conditions at the nozzle exit, the model allows expressing all the profiles in terms of the externally controlled parameters of the torch (geometry of the torch, discharge current, mass flow of the gas and plenum pressure) and the values of the arc and gas temperatures at the nozzle entrance. These last two values can be estimated simply appealing to energy conservation in the cathode-nozzle region. The model contains additional features compared with previous reported models, while retaining simplicity. The detailed consideration of an arc region coupled to the surrounding gas dynamics allows determining voltage drops and consequent delivered power with less assumptions than those found in other published works, and at the same time reduces the set of parameters needed to determine the solution.
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    Experimental Characterization of a Low-Current Cutting Torch.
    (2004) Kelly, Héctor; Mancinelli, Beatriz; Prevosto, Leandro; Minotti, Fernando; Márquez, Andrés
    An experimental characterization of a low-current (30-40 A) cutting torch is presented. To avoid contamination of the plasma arc by removed anode material, a rotating steel cylinder was used as the anode and the arc was anchored onto the cylinder lateral surface. The cathode-anode and cathode-nozzle voltage drops, together with the gas pressure in the plenum chamber were registered for different values of the mass flow rate injected into the plenum chamber. By employing an optical system with a large magnification (≈ 15 X), the arc radius at the nozzle exit was also determined with a digital optical camera. The obtained experimental quantities were used to evaluate several flow properties at the nozzle exit (hot arc plasma and cold gas temperatures, arc and gas velocities, etc.) by employing a simplified theoretical model for the plasma flow in the nozzle. The obtained results are in reasonable agreement with the data reported in the literature by other authors. Explanations of the origin of the clogging effect and the nozzle voltage are also presented.
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    On the Use of Sweeping Langmuir Probes in Cutting-Arc Plasmas—Part II: Interpretation of the Results.
    (2008) Prevosto, Leandro; Kelly, Héctor; Minotti, Fernando
    A semiempirical Langmuir probe model is intro duced that is particularly adapted to high-energy-density cutting arcs, for which, as we have shown in Part I, the ion current collected by negatively biased probes shows no plateau in the ion branch of the current–voltage (I–V ) probe characteristic, and the signal amplitude is independent of the probe radius. According to the model, the ion drag due to the high-velocity plasma flow around the probe limits the effectively collecting area to a small fraction of the probe surface. If, according to the experimental evidence, this fraction is made independent of the probe radius, then its value results proportional to the probe bias, and so no plateau is found, at least as long as the collecting area is less than (half) the probe surface, which happens only at rather high probe bias. The model requires the determination of the function relating the electric field (in the region between the unperturbed plasma and the space-charge sheath close to the probe) to the parameters of the problem. Dimensional analysis together with empirical information allow to restrict the form of this function to leave only an auxiliary dimensionless function, which can be argued to be practically constant and whose value can be determined between rather tight bounds. As an example, radial profiles of plasma temperature and density are obtained by applying the proposed model to the experimental values of a I–V probe characteristic obtained in Part I. The derived temperature profile is in good agreement with a previous published numerical simulation for a similar cutting torch.
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    On the Use of Sweeping Langmuir Probes in Cutting Arc Plasmas—Part I: Experimental Results.
    (2008) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    The first study of Langmuir probes applied to cut ting arcs using a sweeping-probe system is presented. It is found that, under a relatively broad range of experimental conditions (changes in the probe material, in the probe radii, or in the sweeping frequency of the probes), no probe damage is registered, notwithstanding the large value of the power flux present with these arcs. In practice, probes with radii down to 63 µm and with sweeping rotation frequencies down to 8.7 s−1 (probe transit time of ≈140 µs through the arc) were used without noticeable alterations. In the measurements of the ion current collected by negatively biased probes, the following two unexpected features are found: the lack of a current plateau in the ion branch of the I–V probe characteristic and the independence of the signal amplitude on the probe radius. According to the experimental evidence, as well as several estimations, we have neglected electron emission of the probe surface as a relevant mechanism in modi fying the ion branch of the characteristic. On the contrary, some arguments on which a collection model will be based are presented.
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    On the use of the metallic nozzle of a cutting arc torch as a Langmuir probe.
    (2008) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    The region inside the nozzle (bore diameter ≈1 mm) of a cutting arc torch is inaccessible to most plasma diagnostics, and numerical simulations are the only means to find out the relative importance of several physical processes. In this work, a study of electrostatic (Langmuir) probes applied to the inside of a high energy density 30 A cutting arc torch nozzle is presented. The metallic nozzle was used as a Langmuir probe, so the plasma flow is not perturbed by the probe as a solid body. Biasing the nozzle through an electric circuit that employs appropriate resistors together with the arc power source, the i–V nozzle characteristic was built. It was found that under a large positively biased nozzle, the electron current drained from the arc was relatively small, ≈1 A, notwithstanding the fact that the size of the nozzle was relatively large. On the other hand, an almost linear ion current was found for the ion branch for nozzle voltages well below the floating value. Based on the magnitude of inverse slope of the ion current, an estimation of the average electron temperature of the plasma in the vicinity of the nozzle wall was estimated from an ion sheath resistance model using a non-equilibrium two-temperature Saha-equation. An average electron temperature of about 4200 K and a corresponding plasma density of 4 × 1017 m−3 were found.La región dentro de la boquilla (diámetro del orificio ≈1 mm) de una antorcha de arco de corte es inaccesible para la mayoría de los diagnósticos de plasma y las simulaciones numéricas son los únicos medios para averiguar la importancia de varios procesos físicos. En este trabajo se ha realizado un estudio de la electrostática (Langmuir) Se presentan sondas aplicadas al interior de una boquilla de soplete de arco de corte de 30 A de alta densidad de energía. La boquilla metálica se utilizó como sonda Langmuir, por lo que el flujo de plasma no se ve perturbado por el sonda como un cuerpo sólido. Sesgo la boquilla a través de un circuito eléctrico que emplea resistencias junto con la fuente de alimentación del arco, se construyó la característica de boquilla i-V. Se encontró que bajo una gran boquilla polarizada positivamente, la corriente de electrones drenada del arco era relativamente pequeño, ≈1 A, a pesar de que el tamaño de la boquilla era relativamente grande. Por otro lado, se encontró una corriente iónica casi lineal para la rama iónica de la boquilla, tensiones muy por debajo del valor flotante. Basado en la magnitud de la pendiente inversa del ion corriente, una estimación de la temperatura media de los electrones del plasma en las proximidades de la pared de la boquilla se estimó a partir de un modelo de resistencia de la vaina iónica utilizando una ecuación Saha de dos temperaturas. Se encontró una temperatura media de los electrones de unos 4200 K y una densidad de plasma correspondiente de 4 × 1017 m−3.
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    Determination of plasma velocity from light fluctuations in a cutting torch.
    (2009) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
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    An Interpretation of Langmuir Probe Floating Voltage Signals in a Cutting Arc.
    (2009) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    An experimental study of the electrostatic probe floating voltage signals in a cutting arc and its physical interpre tation in terms of the arc plasma structure is reported. Sweeping electrostatic probes have been used to register the local floating potential and ion current at 3.5 mm from the nozzle exit in a 30-A arc generated by a high energy density cutting torch with a nozzle bore radius of 0.5 mm and an oxygen mass flow rate of 0.71 g · s−1. It is found that the floating potential signal presented a central hump with duration almost similar to that corresponding to the ion current signal but having also lateral wings with much larger duration. Capacitive coupling between the probe and the conducting body of the nozzle and arc as a source for the float ing potential signal was discarded. It is assumed that the hump in these probe voltage signals results from the presence of an electrostatic field directed in the radial direction outward the arc axis that is caused by thermoelectric effects. The probe floating voltage signal is inverted using the generalized Ohm’s law together with the Saha equation, thus obtaining the radial profiles of the temperature, particle densities, radial electric field, and potential of the plasma at the studied section of the arc. The resulting temperature and density profiles derived from our interpretation are in good agreement with the data published elsewhere in this kind of high-pressure arcs. There is not a straightforward connec tion between the measured hump amplitude in the floating signal (≈4 V) and the derived increase in the plasma potential between the arc edge and the arc center (≈10 V), due to the global zero cur rent balance condition established by the finite size of the probe. It is shown, however, that the probe takes a floating potential value close to that corresponding to the plasma temperature at the probe center.
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    On the physical origin of the nozzle characteristic and its connection with the double-arcing phenomenon in a cutting torch.
    (2009) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    The nozzle current-voltage characteristic for a cutting arc is presented in this work. The measurements are reported using a high energy density cutting arc torch with a nozzle bore radius of 0.5 mm. The arc current was fixed at 30 A while the plenum pressure and the oxygen gas mass flow rate were varied in the range of 0.55– 0.65 MPa and 0.32– 0.54 g s−1, respectively. The results show a very low electron density and the lack of electron attachment at the plasma boundary layer. No ion saturation current was found. For the smallest mass flow rate value gas breakdown was found for a biasing nozzle potential close to that of the cathode, but no evidence of such breakdown was found for the larger mass flow rate values. Using an expression for the ion speed at the entry of the collisional sheath formed between the nonequilibrium plasma and the negatively biased nozzle wall together with a generalized Saha equation coupled to the ion branch of the characteristic, the radial profile of the electron temperature, the spatial distribution of the plasma density at the plasma boundary, and the sheath thickness were obtained. In particular, the obtained thickness value at the breakdown condition was in good agreement with that obtained from the oxygen Paschen’s curve. An electron temperature of about 4700– 5700 K and a corresponding plasma density of the order of 1019 – 1020 m−3 were found close to the nozzle wall. A physical interpretation on the origin of the double-arcing phenomenon is presented, that explains why the double-arcing (that it is established when the sheath breaks down) appears at low values of the gas mass flow.
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    On the influence of the nozzle length on the arc properties in a cutting torch
    (2009) Prevosto, Leandro; Kelly, Héctor; Risso, Marcelo Natalio; Infante, Damián Leandro
    In this work, an experimental study on the influence of the nozzle geometry on the physical properties of a cutting arc is reported. Ion current signals collected by an electrostatic probe sweeping across a 30 A oxygen cutting arc at 3.5 mm from the nozzle exit were registered for different nozzle lengths. The temperature and density radial profiles of the arc plasma were found in each case by an inversion procedure of these signals. A comparison between the obtained results shows that the shorter nozzle (RN = 0.50 mm, LN = 4.5 mm operated at 0.7 MPa and 35 Nl/min) produces a thinner and hotter arc than the larger nozzle (RN = 0.50 mm, LN = 9.0 mm operated at 1.1 MPa and 20 Nl/min). This behavior is attributed to the marked difference of gas flow rate due to the clogging effect. A smaller gas mass flow reduces the convective cooling at the arc border and decreases the power dissipation of the arc column, resulting in small axis temperatures.
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    On the space-charge boundary layer inside the nozzle of a cutting torch.
    (2009) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    A numerical study of the space-charge sheath adjacent to the nozzle wall of a cutting torch is presented. The hydrodynamic model corresponds to a collision-dominated sheath and does not assume cold ions, so drift-diffusion-type equations are used. Also an improved expression for the ion-neutral momentum transfer is employed rather than the usual constant ion-mean-free-path or constant ion collision frequency approximations. Assuming a constant electron temperature in the sheath and neglecting the electron inertial term, the continuity and momentum equations for ions and electrons, together with Poisson’s equation, were solved for the electric potential, ion velocities both normal and tangential components , and for the ion and electron densities. It was found that both the ion and electron densities present a sudden drop at the sheath-plasma edge. The ion density continues to decrease slowly inside the sheath, while the electron density presents a virtually zero value everywhere inside the sheath, the electron thermal conduction flux to the nozzle wall being negligible. These wall results thus become thermally isolated in spite of the high electron temperature in its adjacency. For a nozzle biasing voltage close to the gas breakdown, it was found that the electric field value is high, reaching a value of about 9 106 V m−1 at the exit of the nozzle wall. This value is higher than the average field value across the sheath and is on the order of the breakdown threshold value. This means that an undesired sheath breakdown could occur at the vicinities of the nozzle exit even if the average electric field across the sheath is not strong enough.
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    Schlieren technique applied to the arc temperature measurement in a high energy density cutting torch.
    (2010) Prevosto, Leandro; Artana, Guillermo; Kelly, Héctor; Mancinelli, Beatriz
    Plasma temperature and radial density profiles of the plasma species in a high energy density cutting arc have been obtained by using a quantitative schlieren technique. A Z-type two-mirror schlieren system was used in this research. Due to its great sensibility such technique allows measuring plasma composition and temperature from the arc axis to the surrounding medium by processing the gray-level contrast values of digital schlieren images recorded at the observation plane for a given position of a transverse knife located at the exit focal plane of the system. The technique has provided a good visualization of the plasma flow emerging from the nozzle and its interactions with the surrounding medium and the anode. The obtained temperature values are in good agreement with those values previously obtained by the authors on the same torch using Langmuir probes.
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    Departures from local thermodynamic equilibrium in cutting arc plasmas derived from electron and gas density measurements using a twowavelength quantitative Schlieren technique.
    (2011) Prevosto, Leandro; Artana, Guillermo; Kelly, Héctor; Mancinelli, Beatriz
    A two-wavelength quantitative Schlieren technique that allows inferring the electron and gas densities of axisymmetric arc plasmas without imposing any assumption regarding statistical equilibrium models is reported. This technique was applied to the study of local thermodynamic equilibrium (LTE) departures within the core of a 30 A high-energy density cutting arc. In order to derive the electron and heavy particle temperatures from the inferred density profiles, a generalized two-temperature Saha equation together with the plasma equation of state and the quasineutrality condition were employed. Factors such as arc fluctuations that influence the accuracy of the measurements and the validity of the assumptions used to derive the plasma species temperature were considered. Significant deviations from chemical equilibrium as well as kinetic equilibrium were found at elevated electron temperatures and gas densities toward the arc core edge. An electron temperature profile nearly constant through the arc core with a value of about 14 000–15 000 K, well decoupled from the heavy particle temperature of about 1500 K at the arc core edge, was inferred.
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    On the dynamics of the space-charge layer inside the nozzle of a cutting torch and its relation with the “non-destructive” double-arcing phenomenon
    (2011) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    Experimental observations on the plasma dynamics inside the nozzle of a 30 A oxygen cutting torch operated at conditions close to the double arcing are reported. It is employed a technique previously developed in our laboratory consisting in using the nozzle as a large-sized Langmuir probe. Based on the behavior of the ion current signal and simple estimations, it is concluded that (1) the non-equilibrium plasma inside the nozzle is far from the steady state in time, in contrast to what is frequently assumed. The power supply ripple was identified as the main fluctuations source and (2) large-scale plasma fluctuations inside the nozzle could cause transient (total duration of the order of 100 ls) Townsend avalanches developing in the space-charge layer located between the arc plasma and the nozzle wall. Such events trigger the so called non-destructive double-arcing phenomena without appealing to the presence of insulating films deposited inside the nozzle orifice, as was previously proposed in the literature.
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    On the use of the Prandtl mixing length model in the cutting torch modeling.
    (2011-05-25) Mancinelli, Beatriz; Minotti, Fernando Oscar; Kelly, Héctor
    The Prandtl mixing length model has been used to take into account the turbulent effects in a 30 A high-energy density cutting torch model. In particular, the model requires the introduction of only one adjustable coefficient c corresponding to the length of action of the turbulence. It is shown that the c value has little effect on the plasma temperature profiles outside the nozzle (the differences being less than 10 %), but severely affects the plasma velocity distribution, with differences reaching about 100 % at the middle of the nozzle-anode gap. Within the experimental uncertainties it was also found that the value c = 0.08 allows to reproduce both, the experimental data of velocity and temperature.
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    Correlation methods in cutting arcs
    (IOPscience, 2011-05-25) Prevosto, Leandro; Kelly, Héctor
    The present work applies similarity theory to the plasma emanating from transferred arc, gas-vortex stabilized plasma cutting torches, to analyze the existing correlation between the arc temperature and the physical parameters of such torches. It has been found that the enthalpy number significantly influence the temperature of the electric arc. The obtained correlation shows an average deviation of 3 % from the temperature data points. Such correlation can be used, for instance, to predict changes in the peak value of the arc temperature at the nozzle exit of a geometrically similar cutting torch due to changes in its operation parameters.
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    On the dynamic behavior of the anode–arc–root at the nozzle surface in a non-transferred plasma torch
    (2012) Prevosto, Leandro; Risso, Marcelo Natalio; Infante, Damián Leandro; Cejas, Ezequiel; Kelly, Héctor; Mancinelli, Beatriz
    The dynamic behavior of the anode–arc–root at the nozzle surface of a plasma torch was experimentally investigated in this work. A gas (N2) vortex–stabilized non–transferred arc torch with a thoriated tungsten rod (2wt %) cathode (3.2 mm diameter) and a coaxial anode (5 mm diameter, 30 mm length) was used in the experiment. By using a sweeping Langmuir probe in floating condition, the voltage of the plasma jet outside the nozzle was inferred. Arc voltage waveforms were also obtained. Data have been obtained for an arc current of 100 A and a gas flow rate of 30 Nl min-1. A typical sawtooth shape (i.e., restrike mode) (with a fluctuating level of º  25 %) and a dominant frequency of º 6.5 kHz was observed in the arc voltage waveforms, which is attributed to anode–arc–root movements along the anode surface followed by a restrike at a certain point close to the cathode. By performing a time correlation between the probe and arc voltage oscillograms together with simple estimations, the amplitude of the movement of the arc–root along the anode surface as well its velocity were inferred.
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    Numerical modeling of the gas breakdown development in the space–charge layer inside the nozzle of a transferred arc torch
    (2012) Mancinelli, Beatriz; Prevosto, Leandro; Minotti, Fernando Oscar
    Double–arcing is a phenomenon that occurs when a transferred arc, flowing inside an electrically insulated nozzle, breaks into two separate arcs: one that connects the cathode with the nozzle, and another that connects the nozzle with the anode. Experimental evidence suggests that the reason for double–arcing is a Townsend like breakdown occurring in the thin space–charge layer, which separates the plasma from the metallic nozzle, due to the high voltage drop across it. Breakdown phenomena in a gas between metallic electrodes have been extensively studied; however the present case involves breakdown of a high–temperature gas between one electrode (the nozzle) and a plasma boundary. A 1–D model of the gas breakdown development in the space–charge layer contiguous to the nozzle of a cutting arc torch operated with oxygen is reported. The dynamics of the discharge is analyzed. The kinetic scheme includes processes of ionization of heavy particles by electron impact, electron attachment, electron–ion recombination and ion–ion recombination.
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    On the dynamics of cutting arc plasmas: the role of the power supply ripple
    (ADVANCED ELECTROMAGNETICS, 2012-08) Prevosto, Leandro; Mancinelli, Beatriz; Kelly, Héctor
    The power sources used in cutting arc torches are usually poorly stabilized and have a large ripple factor. The strong oscillatory components in the voltage and arc current produce in turn, large fluctuations in the plasma quantities. Experimental observations on the dynamics of the nonequilibrium plasma inside the nozzle of a 30 A oxygen cutting torch with a 7 % ripple level of its power source are reported in this work. The observed electron temperature (mean value º 5400 K) shows a rms deviation º 5 % (≤ 300 K), which is of the order of the arc voltage ripple level. A considerable different situation occurs with the plasma density (mean value º 3 ´ 1019 m-3). In this case the rms deviation was º 75 %, much greater than that of the electron temperature.
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    Langmuir probe diagnostics of an atmospheric pressure, vortex–stabilized nitrogen plasma jet
    (2012-09-19) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    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 5mm diameter. The torch was operated at a nominal power level of 15kW 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 9500K and an electron density of about 4 1022m 3, were found at the jet centre at 3.5mm 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. VC 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4752886]
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    Langmuir probe measurements in a time-fluctuating-highly ionized non-equilibrium cutting arc: Analysis of the electron retarding part of the time-averaged current-voltage characteristic of the probe.
    (2013) Prevosto, Leandro; Kelly, Héctor; Mancinelli, Beatriz
    This work describes the application of Langmuir probe diagnostics to the measurement of the electron temperature in a time-fluctuating-highly ionized, non-equilibrium cutting arc. The electron retarding part of the time-averaged current-voltage characteristic of the probe was analysed, assuming that the standard exponential expression describing the electron current to the probe in collision-free plas mas can be applied under the investigated conditions. A procedure is described which allows the determination of the errors introduced in time-averaged probe data due to small-amplitude plasma fluctuations. It was found that the experimental points can be gathered into two well defined groups allowing defining two quite different averaged electron temperature values. In the low-current re gion the averaged characteristic was not significantly disturbed by the fluctuations and can reliably be used to obtain the actual value of the averaged electron temperature. In particular, an averaged electron temperature of 0.98 ± 0.07 eV (= 11400 ± 800 K) was found for the central core of the arc (30 A) at 3.5 mm downstream from the nozzle exit. This average included not only a time-average over the time fluctuations but also a spatial-average along the probe collecting length. The fitting of the high-current region of the characteristic using such electron temperature value together with the corrections given by the fluctuation analysis showed a relevant departure of local thermal equilibrium in the arc core.