Heavy Particle Temperature Research Articles

Analyses of nonequilibrium transport in atmospheric-pressure direct-current argon discharge under different modes

Abstract The key plasma parameters under different discharge modes, such as heavy-particle and electron temperatures, electron number density, and nonequilibrium volume of plasmas, play important roles in various applications of gas discharge plasmas. In this study, a self-consistent two-dimensional nonequilibrium fluid model coupled with an external circuit model is established to reveal the mechanisms related to the discharge modes, including the normal glow, abnormal glow, arc, and glow-to-arc transition modes, with an atmospheric-pressure direct-current (DC) argon discharge as a model plasma system. The modeling results show that, under different discharge modes, the most significant difference between the preceding four discharge modes lies in the current and energy transfer processes on the cathode side. On one hand, the current to the cathode surface is mainly delivered by the ions coming from the plasma column under the glow discharge mode due to the low temperature of the solid cathode, whereas the thermionic and secondary electrons emitted from the hot cathode surface play a very important role under the arc mode with a higher cathode surface temperature and higher ion flux toward the cathode. On the other hand, the energy transfer channel on the cathode side changes from mainly heating the solid cathode under the glow mode to simultaneously heating both the solid cathode and plasma column under the arc mode with an increase in the discharge current. Consequently, the power density in the cathode sheath (P c) was used as a key parameter for judging different discharge modes, and the range of (0.28–1.2) × 1012 W m−3 was determined as a critical window of P c corresponding to the glow-to-arc-mode transition for the atmospheric-pressure DC argon discharge, which was also verified by comparison with the experimental results in this study and the data in the previous literature.

Reversal of the electric field and the anode fall in DC arcs in air during contact opening

A unified one-dimensional model of an arc plasma in air, between copper electrodes, that includes the change of the gap distance is presented. The occurrence of multiple reversals of the electric field and the anode voltage drop is observed. The evolution of the spatial distribution of the electron and heavy particle temperatures with the gap distance and the opening speed is also studied. The model quantitatively predicts a number of plasma properties under conditions that are relevant to the contact separation in low-voltage switching devices.

Two-temperature modeling of lamellar cathode arc

A three-dimensional, two-temperature (2T) model of a lamellar cathode arc is constructed, drawing upon the conservation equations for mass, momentum, electron energy, and heavy particle energy, in addition to Maxwell’s equations. The model aims to elucidate how the physical properties of electrons and heavy particles affect heat transfer and fluid flow in a lamellar cathode arc. This is achieved by solving and comparing the fields of electron temperature, heavy particle temperature, fluid flow, current density, and Lorentz force distribution under varying welding currents. The results show that the guiding effect of the lamellar cathode on current density, the inertial drag effect of moving arc, and the attraction effect of Lorentz force at the lamellar cathode tip primarily govern the distribution of the arc’s physical fields. The guiding effect localizes the current density to the front end of the lamellar cathode, particularly where the discharge gap is minimal. Both the inertial drag effect and the attraction effect of Lorentz force direct arc flow toward its periphery. Under the influence of the aforementioned factors, the physical fields of the lamellar cathode arc undergo expansion and shift counter to the arc’s direction of motion. A reduction in welding current substantially weakens the guiding effect, causing the arc’s physical fields to deviate further in the direction opposite to the arc motion. In comparison with a cylindrical cathode arc, the physical fields of the lamellar cathode arc are markedly expanded, leading to a reduction in current density, electron temperature, heavy particle temperature, cathode jet flow velocity, and Lorentz force.

Measurement of medium-voltage AC air arc temperature and particle number density based on dual-wavelength Moiré deflection technology

AC air arcs are generated in medium-voltage (MV) power systems under the effect of harsh weather conditions, equipment aging, and high penetration of distributed generation, threatening equipment and public safety. The arc current and temperature are low due to the wide application of arc suppression devices. In this scenario, the MV AC air arc does not satisfy the local thermodynamic equilibrium (LTE) condition. In addition, the repeated arcing and extinguishing processes further complicate the arc discharge mechanism, which bring challenges in the modeling and detection of MV AC air arcs. Experimental methods are a direct and efficient approach to determine the properties of arc plasmas. In this study, a dual-wavelength Moiré deflection diagnostic system was established to determine the time evolution of the particle density and radial distribution of the temperature in an MV AC air arc without relying on the LTE assumption. The electron number density and heavy particle number density change transiently during the arc discharge process and change gradient along the radial direction. The heavy particle temperature and electron temperature were then calculated based on the measured particle number density. During the arcing stage, the temperature of the electrons exceeded that of the heavy particles significantly, and the arc deviated from LTE. Finally, the limitations of the traditional single-wavelength Moiré deflection method are analyzed. The classic single-wavelength Moiré deflection method, while capable of estimating heavy particle temperature in plasma, exhibits a significant error in electron density estimation compared to the dual-wavelength Moiré deflection method.

Numerical analysis of low-current arc characteristics in micro-TIG welding

A semi-empirical two-temperature (2-T) arc model is developed to study the near-electrode regions and thermal non-equilibrium characteristics of low-current arcs that employ pin-to-pin electrodes in micro‑tungsten inert gas (TIG) welding. Special emphasis is given to accurately depict the electrodes-arc attachment area and the spatial distributions of plasma temperature in 1–4 A argon arcs. The conditions of thermal non-equilibrium, as well as the coupling of the arc plasma and the cathode, are considered. The simulated electron temperature distributions are in good agreement with the arc appearances captured by the high-speed camera, and the predicted arc voltage is only 1.8 V lower than the experimental data. In contrast to TIG welding arcs that operate at higher currents, the temperature contours of the low-current arc column are parallel but slightly expanded in the vicinity of the electrodes. As the arc current decreases, the near-cathode voltage drop increases substantially. The electron temperature in the center of the arc column is almost constant, while the heavy particle temperature drops significantly, causing the arc column to contract. The heat flux at the center of the anode surface does not change as the arc current increases, but it increases significantly outside the 0.25 mm radius.

Characteristics of atmospheric pressure Ar-plasma around a spherical particle: Numerical study

The characteristics of atmospheric pressure Ar-plasma such as the spatial distributions of number densities, fluxes, and temperatures of electrons and heavy plasma particles, as well as the spatial distribution of electric potential around an individual spherical particle, were studied numerically on the basis of the hydrodynamic (diffusion) equations for plasma components. The governing equations were formulated for plasma that is not in thermal and ionization equilibrium. The boundary conditions near the particle were set on the edge of the space charge layer (sheath) adjacent to the particle surface. The nonlinear problem was solved taking into account the temperature dependencies of transport and kinetic coefficients. The heat flux introduced by the plasma into the particle was calculated and compared with the results of the simple heat conduction model. The range 10 − 5 – 10 − 4 m of particle radius and the range 6–18 kK of unperturbed plasma temperature were considered.

Production of a large volume non-equilibrium region in an atmospheric argon arc plasma with a counter injection of cold gas from an annular anode

In this letter, an annular anode is designed for producing arc plasmas with a large non-equilibrium region by using a counterflow cold gas through the annular anode. The coupled mass-momentum-energy exchange processes in an argon arc plasma are studied numerically and experimentally. The counter-injection of the cold argon gas from the center of the anode leads to a steep gradient of the heavy-particle temperature due to the formation of a thin stagnation layer resulting from the interaction of the high temperature plasma with the cold gas; and in particular, a large volume non-equilibrium ‘dark’ plasma region is obtained above the anode surface. The results show that, with the enhancement of the convective heat transfer process in the plasma core region, the fraction of the non-equilibrium region to the whole arc plasma region reaches 92.2% where the heavy-particle temperature can be reduced significantly, e.g. ∼2300 K, while simultaneously, the electron temperature and number density are remained at high levels greater than 8000 K and 2.4 × 1020 m−3, respectively, under the operating condition studied in this letter. This research not only deepens the understanding to the non-equilibrium synergistic transport mechanisms of arc plasmas, but also provides a method for producing a large volume non-equilibrium plasma region so as to promote various existing applications, or even creating new applications in the future.

A Simple and Compact Laser Scattering Setup for Characterization of a Pulsed Low-Current Discharge

Recent research trends show an increasing interest in non-equilibrium plasmas operated at atmospheric pressure, which are often used to tackle several environmental and health issues. Nevertheless, due to the complexity of the applications, these trends also show the need for a comprehensive characterization of such plasmas for a deeper understanding of the observed effects. One of the diagnostic methods for experimental determination of key parameters which affect the reactivity of a plasma, i.e., electron temperature, electron density and heavy particle temperature, is laser scattering. In this work, an approach based on a simple and compact laser scattering setup is proposed, which allows an estimation of the above parameters without any additional changes in the acquisition settings. Thus, the experimental effort and possible sources of error can be reduced. The proposed setup is tested experimentally with a commercially available pulsed plasma system, and the results are compared to available data. From this comparison, it is found that the plasma parameters estimated with the proposed scattering setup are plausible.

Influence of Pulse Amplitude and Frequency on Plasma Properties of a Pulsed Low-Current High-Voltage Discharge Operated at Atmospheric Pressure

Non-equilibrium conditions in plasma are often achieved by pulsed power delivery, where the pulse shape and repetition rate determine the properties of the plasma constituents and thus its chemical reactivity. The evaluation of the latter is becoming increasingly important to understand the observed effects, especially when new application fields are targeted. The composition of the plasma and the occurring chemical reactions can be calculated using various models. Thereby, the temperature of the electrons, the electron number density, as well as the heavy particle temperature are usually required as the basis of such calculations. In this work, the influence of pulse amplitude and repetition rate on these plasma parameters is determined by laser scattering for a low-current, high-voltage discharge operated with nitrogen at atmospheric pressure. In particular, the characteristic parameters regarding the plasma free electrons in such discharges have not yet been experimentally determined to this extent. The results are validated by spectroscopic measurements, i.e., the electron density is estimated from the Stark broadening of the hydrogen beta line and the heavy particle temperature is estimated by fitting the spectrum of nitrogen molecular transitions. Depending on the operating frequency, a pure nitrogen discharge with an input power of about 650 W displays an electron density between 1.7×1021m−3 and 2.0×1021m−3 with electron temperatures in the range of 40,000 K and heavy particle temperatures of about 6000 K in the core of the discharge channel. Furthermore, a relatively slow electron recombination rate in the range of 20 µs is observed.

Spectroscopic Characterization of a Pulsed Low-Current High-Voltage Discharge Operated at Atmospheric Pressure

The advantages of pulsed low-current high-voltage discharges operated at atmospheric pressure and the ease with which such discharges can be implemented industrially contributed to their popularity. However, the high reactivity of a pulsed plasma implies that thorough diagnostics are needed to fully understand the interactions inside these plasmas. Some of the key parameters determining plasma properties of low-current discharges are the electron number density and the temperature of heavy particles. Both parameters can be determined experimentally with spectroscopic techniques, for example by investigating the broadening of spectral lines due to the Stark effect and by fitting synthetic spectra to molecular transitions. To the authors’ knowledge, experimentally determined electron densities for pulsed low-current discharges operated in a power range between 300 W and 1000 W have not been performed in previous works. Thus, in this work, the electron number density and temperature of heavy particles of one of several commercially available plasma systems are determined by means of emission spectroscopy.

Analysis of Arc Electron and Heavy Particle Temperature Distribution with Lateral Flow Velocity for Elucidation of Re-strike

Analysis of Arc Electron and Heavy Particle Temperature Distribution with Lateral Flow Velocity for Elucidation of Re-strike

Development of a numerical model for metal vapor dynamics in low pressure arcs based on moving particle semi‐implicit method and influence of shield wall radius on vapor behavior

AbstractNumerical simulation was conducted for dynamic behavior of copper vapor in arcs under reduced pressure condition using the developed model. The developed model uses MPS method for atoms and ions, while it uses finite volume method for electron energy and electro‐magnetic fields. In this paper, the influence of shield wall inner radius for vapor dynamics was studied on the heavy particle temperature, electron temperature and ionization degree. The particle in MPS method was assumed to be neutralized and mirror‐reflected on the shield wall. As a result, the shield wall with a larger radius causes lower density of metal vapor, lower ionization degree between the electrodes.

Three-dimensional non-equilibrium modeling of a DC multi-cathode arc plasma torch

In this paper, a three-dimensional non-equilibrium steady arc model is used to investigate the temperature, velocity and electromagnetic field in multi-cathode arc torch, and the formation mechanism of a large-area, uniform and diffused arc plasma is analyzed. The numerical simulation results show that a large volume plasma region can be formed in the central region of the generator during discharge. During this process, the maximum electron temperature appears near the cathode and in the central convergence region, while the maximum heavy particle temperature only appears in the central convergence region. This phenomenon is consistent with the experimental arc images. Near the cathode tip, the arc column is in a contraction state. In the area slightly away from the cathode, the six arc columns begin to join together. In the plasma generator, there is a large-scale current distribution in all directions of X, Y and Z, forming a stable arc plasma with a wide range of diffusion. The calculated electron temperature distribution is in good agreement with the measured electron temperature. The results suggest that the large-area diffused arc plasma in the multi-cathode arc torch is the combined effect of current distribution, convection heat transfer and heat conduction.

Calculation of two-temperature thermodynamic and transport properties of argon-nitrogen plasma

The thermodynamic and transport properties of plasmas over a wide range of temperature and pressure are necessary to model the heat transfer and flow processes in plasma. In this study, the plasma composition is solved by simultaneous Saha equation, Dalton's partial pressure law and charge quasi-neutral equation. The thermodynamic properties of plasma computation are based on the kinetic theory for ideal gas. While the calculation of transport properties is based on the solution of Boltzmann’s equation by the Chapman-Enskog method. The thermodynamic and transport properties of argon-nitrogen plasma at pressures of 0.1, 1.0 and 10.0 atm, electron temperatures ranging from 300 to 30000 K, and non-local thermodynamic equilibrium (NLTE), where the electron temperature is not equal to the temperature of heavy particles,, are investigated by using the above method. The results show that the value of non-equilibrium parameter has a great influence on the properties of the argon-nitrogen mixture. With the increase of non-equilibrium parameter, the dissociation reaction requires a higher electron temperature, which leads the dissociation peak to shift to a higher electron temperature. The ionization and dissociation reaction will enter into the high temperature region due to the increase in pressure. This change will affect the peak position and value of the specific heat, viscosity, thermal conductivity and electrical conductivity of plasma. In addition, since the electronic translational thermal conductivity and electrical conductivity mainly depend on the electron number density, when non-equilibrium parameter and pressure increase, the electron number density will increase at high electron temperature, thus improving the electronic translational thermal conductivity and electrical conductivity. Under the condition of local thermodynamic equilibrium, the transport properties of argon-nitrogen plasma obtained by calculation are in good agreement with previously reported data.

Three-Dimensional Two-Temperature Modeling of Ar Loop-Type Inductively Coupled Thermal Plasma for Surface Modification

In this paper, numerical calculations were made for Ar loop-type inductively coupled thermal plasma (loop-ICTP). The loop-ICTP was developed originally by the authors’ group for rapid surface modification of large areas. Loop-ICTP is sustained with a unique three-dimensional (3D) configuration inside a circular loop quartz tube and on the substrate. A 3D and two-temperature thermofluid thermal plasma model was adopted for this calculation. Mass, momentum, and energy conservation equations were solved using a Maxwell equation for vector potential, an electron energy transport equation, and Saha’s equation in the 3D space. Results indicate that Ar loop-ICTP can be sustained and formed in the loop tube and also on the substrate. Moreover, the heavy particle temperatures reaches 1800–2000 K, whereas the electron temperature is about 10,000 K. Loop size effects on the gas temperature and gas flow field were also investigated using the developed model. Results show that adoption of a larger loop tube can be expected to improve the plasma uniformity on the substrate when applied to rapid surface modification.

Influence of hydrogen addition on methane coupling in a moderate pressure microwave plasma

In this paper, the effect of hydrogen addition on methane coupling in a microwave moderate pressure (55 mbar and 110 mbar) plasma reactor has been studied. The use of optical emission spectroscopy allowed the determination of the rotational temperature of heavy particles and showed it to be in the range of 3000–4000 K. Due to the high temperature in the discharge the dominant product was acetylene and it was concluded that the methane coupling process is mainly through thermal decomposition with a key role of H radicals. It was revealed that the addition of hydrogen can increase both methane conversion and acetylene and ethylene yield and selectivity. With the CH4:H2 ratio of 1:1, the methane conversion increased from 31.0% to 42.1% (55 mbar) and from 34.0% to 48.6% (110 mbar), when compared to pure methane plasma. Respectively, the yield of acetylene increased from 14.4% to 25.3% (55 mbar) and from 20.1% to 34.0% (110 mbar). Moreover, the addition of hydrogen decreased the output of the problematic soot-like product. These results indicate that hydrogen addition can be a simple yet effective method of increasing selectivity to desirable products in plasma reforming of CH4.

Plasma Disposal in Problems of Ecology (Review)

The current state and issues of plasma disposal are discussed. Various types of plasma torches used for disposal, the technological schemes of existing plasma systems, a plasma torch with an expanding output electrode, equipment characteristics, diagnostic methods for the design of an effective low-temperature plasma generator, and the studying of the effects of a high-enthalpy plasma jet generated by a direct-current plasma torch on a solid target material are described. The results obtained via high-speed imaging, micropyrometry, laser profilometry, spectroscopy, and stereoscopy methods are presented. The data on spatial and temporal changes of the temperature field on the sample surface, the dynamics of material loss, and changes in the parameters (temperature and electron density, temperature of heavy particles) of the plasma-jet incident on the sample are analyzed. The collected theoretical and experimental results and engineering solutions can be used to design various low-temperature plasma generators with a power of up to 1 MW and higher.

Selective treatments of prostate tumor cells with a cold atmospheric plasma jet

Background: Low temperature plasma is an ionised gas generated at atmospheric pressure with the temperature of heavy particles (ions, molecules, atoms) close to room temperature. Recent progress highlighted the efficiency of plasma to induce cell death in many cancer cell lines. In the current context of the emergence of the use of cold plasma for medical purposes, this article aims to study the effect of a cold plasma of helium on ex-vivo prostate cells.Methods: A cold atmospheric plasma torch was developed to treat different tumor cells. Parameters like electric voltage, gas flow, cell/torch distance, and exposure time were studied to improve the effects of plasma onto two human prostate tumor cells lines, with different metastatic potential, LNCaP and PC3 and then, compared to human prostate epithelial cells P69. The plasma torch operated by a voltage up to 10 kV, without any risk of arcing, exhibited a visible plasma plume up to about 70 mm, with a temperature close to room temperature. The induced temperature rise due to the plasma treatment is measured to ensure that observed results are not thermal effects. After plasma treatment, microscopy and MTT analyses are achieved to measure cells viability. An ordinary one-way ANOVA test were used between two groups with p-value less than 0.05.Results: An optimized protocol leads to the tumor cells death with regard to cell viability. Cold plasma treatment exhibits differential effect, regarding tumor status of cells without any significant influence of thermal effect (temperature increases of only few degrees Celcius above room temperature). The results show that non-tumor cells P69 are less sensitive to cold plasma exposure with a cell viability of 67% after 120 s of treatment, compared to tumor cell lines LNCaP and PC3, respectively at cell viability of 29% and 23% for the same treatment time.Conclusion: The cold helium plasma treatment showed a selective effect on prostate cells, with an enhanced apoptosis way for tumor cells, whereas the healthy cells are more resistant to the treatment. Moreover, during the treatment, the surface temperature rises only few degrees Celcius. This result is very promising for future in-vivo treatments.

Consideration of optical time of flight measurement in laser induced plasmas

We considered some specifics of Optical Time Of Flight (OTOF) in Laser Induced Plasma if the OTOF signal is recorded at a distance of a few millimeters from the target. We demonstrated that the shape of OTOF signal is influenced by process of excitation transfer among the excited states. In case when the excitation transfer is dominant analogy with Time Of Flight (TOF) technique, developed for pulsed desorption flows into vacuum, is not meaningful. It is shown that at least two OTOF signals, for two different spectral lines, are necessary to record in order to estimate whether the excitation transfer is an issue or not. Assuming that expansion of the plasma is isotropic we derived a simple expression that takes into account geometric details concomitant for OTOF signal. It is shown that appropriate alignment of the spectrograph in respect to the laser beam position is essential for correct interpretation of experimental data and that asymptotic behavior of OTOF signal depends on a vertical opening of the slit and on the lens system. We demonstrated that proposed expression for OTOF signal provides consistent data for temperature T of heavy particles and center of mass velocity vc if OTOF signals are recorded at different distances from the target. For the same OTOF signals Shifted Maxwell-Boltzmann (SMB) function failed to correctly reproduce the both T and vc. We also discussed impact of inaccurate estimation of zero position in time and length scale on evaluated temperature of heavy particles, T, and center of mass velocity, vc. Importance of the geometric details is demonstrated considering two OTOF signals recorded for different vertical openings of the entrance slit while the rest of parameters were exactly the same. Temperature of the heavy particles T and the center of mass velocity vc, evaluated relying on the proposed expression, had a consistent values no matter if they were inferred from OTOF signal for large or small vertical slit opening. For the same OTOF signals Shifted Maxwell-Boltzmann function failed to correctly reproduce the both - center of mass velocity and temperature T of heavy particles. Also, T and vc estimated for OTOF signals recorded at different positions from the target, applying proposed expression, are consistent supporting assumption that expansion is dominantly isotropic and collisionless at chosen experimental conditions. We also discussed impact of inaccurate estimation of zero position in time and length scale on evaluated temperature of heavy particles, T, and center of mass velocity, vc.

Steady-state simulation of different discharge modes of argon arc under external magnetic field

In this study we performed a numerical investigation on different discharge modes of the magnetically rotating arc at atmospheric pressure. By using a three-dimensional simulation model taking into account of the arc-cathode interaction, it is able to predict the three different kinds of cathode arc attachments. Those modes of arc discharge have been observed in previous experiments. Arc attachments in constricted (i.e. spot) and diffuse mode are almost axisymmetric, however the edge mode offers a non-axisymmetric result. The corresponding arc plasma properties operating in different modes differ greatly from each other. Driven by the non-axisymmetric Lorentz force, a ‘twisted’ arc column emerges between the electrodes in edge mode, while the cathode arc attachment focuses to a corner of the edge. The heavy particle and electron particle temperatures are much lower in diffuse mode than the other two modes. Those plasma features of multi-modes should be considered in the applications as materials treatment.