Axial Electron Temperature Research Articles

Influence of electron temperature anisotropy on the <i>m</i> = 1 helicon mode power deposition characteristic

As a core phenomenon in helicon discharge, the plasma temperature anisotropy may play a crucial role in helicon wave power deposition. Under radially inhomogeneous plasma circumstances, by employing the warm plasma dielectric tensor model and considering the finite Larmor radius (FLR) effect and plasma temperature anisotropy effect, under the typical helicon discharge parameter conditions, the helicon wave and Trivelpiece-Gould (TG) wave mode coupling characteristic and influence of electron temperature anisotropy on the helicon wave power deposition induced by collisional and Landau damping mechanism are theoretically investigated. Detailed analysis shows that for typical helicon plasma electron temperature <i>T</i><sub>e </sub>= 3 eV and low magnetic field <i>B</i><sub>0</sub> = 48 G, the electron FLR effect should be considered, while the ion FLR effect can be ignored due to its large inertia effect; compared with the <inline-formula><tex-math id="Z-20240326171950">\begin{document}$| n | < 2 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231759_Z-20240326171950.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231759_Z-20240326171950.png"/></alternatives></inline-formula> cyclotron harmonics, the contribution of the <inline-formula><tex-math id="Z-20240326171926">\begin{document}$| n | > 1 $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231759_Z-20240326171926.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20231759_Z-20240326171926.png"/></alternatives></inline-formula> harmonics in the calculation of plasma dielectric tensor elements can be ignored due to low magnetic field conditions. For the propagation constant, detailed investigation indicates that the phase constant has a maximum value at a certain radial position, near the same position mode coupling between helicon wave and TG wave happens. Full analysis shows that the power deposition of the <i>m</i> = 1 helicon mode peaks at a certain radial position and increases gradually with the increase of the axial electron temperature. Besides, compared with the Landau damping, the collisional damping plays a dominant role in the power deposition under current parameter conditions; importantly, the electron temperature anisotropy exerts a significant influence on the power deposition characteristic, both the increase and decrease of electron temperature anisotropy factor (<i>χ</i> = <i>T</i><sub>e<i>,</i>⊥</sub>/<i>T</i><sub>e,<i>z</i></sub>) can lead the power deposition intensity to change drastically. All these conclusions are very important for us to understand the discharge mechanism of helicon plasma.

A 2D steady state analytical model for atmospheric pressure RF plasma jet

This paper presents a 2-dimensional analytical model for plasma inside the discharge tube of an RF atmospheric pressure plasma jet (APPJ) in coaxial geometry. Since the model is a steady state one, it requires that the frequency of the applied RF be high enough so that the formation of streamers and plasma bullets is suppressed and that continuous plasma is present throughout the RF cycle. It is also assumed that power absorption is due to electron-atom collisions in the bulk plasma. The model takes into account gas flow, yielding a flow-diffusion axial flux that aids the exit of the plasma into the atmosphere. The inputs to the model are the reactor dimensions, average power fed (P av ) and the flow velocity of the gas (ug) using which the model yields the plasma density along with its radial and axial profiles, electron temperature, axial and radial particle fluxes, gas heating, flux, and energy of the plasma particles escaping from the discharge tube into the ambient air. Experiments were undertaken on an RF-powered jet at 13.56 MHz. Substituting the average RF power, measured using voltage and current probes, into the model gives the plasma density n ≈ 3.9 × 1017(≅2.8 × 1019)m−3 at for He (Ar). The corresponding electron temperature given by the model is for He (Ar). It is hoped that the developed model would be able to provide a comprehensive methodology for analyzing APPJs.

Obtaining plasma parameters by Langmuir probes and optical emission spectroscopy in low-pressure DC plasma

The plasma environment in low-pressure systems allows coating different materials such as glasses, textiles, metals, etc. at the atomic level. Although vacuum coating is one of the best methods for thin film coating, the coating quality is closely affected by the spatial and temporal non-uniformities inside the reactor. Therefore it is quite important to determine the plasma properties within the chamber. The determination of plasma parameters such as electron temperature, electron excitation temperature, electron and ion densities can be realized using Langmuir probe systems, and by Optical Emission Spectroscopy (OES) by using Boltzmann plot method. Knowing such parameters can provide important information about specific reaction mechanisms, as well as it provides invaluable information that can increase reproducibility in various discharge process control in thin film coating. In this study, a homemade single Langmuir probe (s-LP) and a multiple Langmuir probe system (m-LP) were used to calculate electron temperature, electron density, saturation current, and plasma potential of the plasma at different pressures and at different voltages. One of the objectives of this work was to evaluate the electron temperature and ion density inside the plasma during a coating process. The second objective was to find the spatial change of electron temperature in the axial direction between cathode and anode by m-LP system including 8 independent tips. The last objective was to measure the axial plasma electron temperature distribution inside the vacuum chamber by using OES.

Simulating glow discharge positive column parameters in neon

The nonlocal self-consistent model for the low temperature plasma parameters in a glow discharge positive column is presented. The model implies relaxation method for the solving of the nonlocal Boltzmann equation for electron energy distribution function, continuity equation for ions distribution and Poisson equation for radial electric field. Calculations are provided for a cylindrical discharge tube used in the Plasma Kristall-4 experiments for neon as a plasma forming gas for the gas pressures of 30-70 Pa and discharge currents of 0.5-2.5 mA. Axial electric field strength, electron temperature and electron density are calculated as functions of the discharge current and the gas pressure. The results are in good agreement with the data provided by other authors.

Combined influence of axial electron temperature and exponential plasma density ramp on the self-focusing of a chirped laser in plasma

Abstract An analysis of the self-focusing of highly intense chirped pulse laser under exponential plasma density ramp with higher order value of axial electron temperature has been done. Beam width parameter is derived by using paraxial ray approximation and then solved numerically. It is seen that self-focusing of chirped pulse laser is intensely affected by the higher order values of axial electron temperature. Further, influence of exponential plasma density ramp is studied and it is concluded that self-focusing of laser enhances and occurs earlier. On the other hand defocusing of beam reduces to the great extent. It is noticed that the laser spot size reduces significantly under joint influence of the density ramp and the axial electron temperature. Present analysis may be useful for the analysis of quantum dots, the laser induced fusion and etc.

Simulations of the glow discharge positive column parameters in helium

The self-consistent model for the positive column of a glow discharge is presented. The model is based on the simultaneous solution of the nonlocal Boltzmann equation for electron energy distribution function, continuity equation for ions and Poisson equation for electric field. Calculations were performed for a discharge tube similar to the Plasma Kristall-4 experiments for plasma forming gas helium for the range of pressures of 20-70 Pa and the range of discharge currents of 0.5-2.5 mA. Axial electric field strength, electron temperature and electron density dependences on the discharge current and the gas pressure are obtained. The results show a good correlation with the data obtained by the other authors.

2D axial-azimuthal particle-in-cell benchmark for low-temperature partially magnetized plasmas

The increasing need to demonstrate the correctness of computer simulations has highlighted the importance of benchmarks. We define in this paper a representative simulation case to study low-temperature partially-magnetized plasmas. Seven independently developed particle-in-cell codes have simulated this benchmark case, with the same specified conditions. The characteristics of the codes used, such as implementation details or computing times and resources, are given. First, we compare at steady-state the time-averaged axial profiles of three main discharge parameters (axial electric field, ion density and electron temperature). We show that the results obtained exhibit a very good agreement within 5% between all the codes. As discharges are known to cause instabilities propagating in the direction of electron drift, an analysis of these instabilities is then performed and a similar behaviour is retrieved between all the codes. A particular attention has been paid to the numerical convergence by varying the number of macroparticles per cell and we show that the chosen benchmark case displays a good convergence. Detailed outputs are given in the supplementary data, to be used by other similar codes in the perspective of code verification.

Effect of axial electron temperature and plasma density ramp on self-focusing / defocusing of a laser beam in plasma

In the present study, we analyze the self-focusing / defocusing of an intense laser beam in the plasma under combined effect of density ramp and higher order axial electron temperature. The differential equation for beam width parameter is obtained and then solved numerically under paraxial ray approximation. The behavior of beam width parameter with propagation distance is extensively investigated for different values of optimized parameters and is strongly influenced by the higher order axial electron temperature, Tp4. The results indicate that the electron temperature component Tp4 is not of little significance than Tp0 and Tp2. Under the influence of plasma density ramp and higher order axial electron temperature Tp4 the defocusing is reduced and self-focusing becomes stronger, especially for large propagation distance. Furthermore, increase in the density ramp results more reduction in the laser beam spot size and hence, increase in the intensity of the laser which may be useful for various applications including laser-driven fusion.

Experimental and numerical investigations of electron characteristics in 2 MHz and 13.56 MHz inductively coupled hydrogen plasmas with an expansion region

The electron characteristics are investigated in 2 MHz and 13.56 MHz inductively coupled hydrogen discharges with an expansion region. The influence of the gas pressure and radio-frequency of the power source on the electron energy probability functions (EEPFs), electron density, and electron temperature at high deposition power has been presented. The measured EEPFs in the driver region of the discharge evolve from a three-temperature Maxwellian distribution to a Maxwellian distribution as the pressure increases. Different characteristic frequencies calculated based on the measured plasma parameters show that stochastic heating of electrons dominates at pressures lower than 0.3 Pa and it has to be considered for pressures lower than 1.0 Pa, while Ohmic heating dominates at higher pressure. Furthermore, the EEPFs as a function of the total energy evolve from the identical shape to discrete shapes with axial position and pressure, indicating a transition of electron kinetics from nonlocal to local regimes. This can be explained by the calculated electron energy relaxation length. In order to verify the experimental results, COMSOL Multiphysics is used to calculate the electron density and electron temperature at different pressures and frequencies of power source. The simulated axial distributions of the plasma parameters agree well with the measured results at 5.0 Pa, while the calculated electron density is lower and the calculated electron temperature is higher at 1.0 Pa. In addition, there is no frequency dependence of axial resolved EEPFs, electron density, and electron temperature in high power deposition discharges (1.5 kW).

Plasma temperature rise toward the plasma-facing surface

Detailed measurements of axial electron temperature, Te, profiles in the presheath region were carried out using a Langmuir probe and the line intensity ratio technique for both He I (728.1nm/706.5nm) and Be II (467.3nm/313.1nm). The results show that Te increases toward the material surface, which contradicts the standard picture that Te is constant along the magnetic field in the sheath-limited regime. While no target bias voltage, Vb, dependence is seen, the Te rise becomes more prominent with decreasing neutral pressure. Similarly, the ion temperature, Ti, evaluated from Doppler broadening of a He II line emission at 468.6nm is found to increase toward the surface, but also does not depend on Vb. Possible mechanisms of the Te and Ti rise as well as validity of the line intensity ratio technique near the material surface are discussed.

Experimental study of the near-electrode plasma-tungsten cathode system in high-current atmospheric-pressure nitrogen arcs

Results of spectroscopic and pyrometric studies of the near-electrode plasma-tungsten cathode system are presented. The 2D-spectra of radiation of the multiply ionized nitrogen plasma of electric-arc discharge under atmospheric pressure are obtained at different current values. The axial electron temperature in this plasma is determined depending on the distance from the cathode tip, and the temperature distribution over the current-collecting surface of the cathode is found. The high-speed visualization of the cathode-near-cathode plasma region made it possible to observe phase transitions on the cathode surface and processes of crack formation and detachment of its fragments. The power released at the cathode is analyzed including the contribution of the arc column resonance emission, and the peculiarities of phase transitions on the cathode surface are revealed. The densities of the electron emission current at the cathode are estimated theoretically and compared with the data obtained in the experiment. Possible causes of the difference between the theoretical values of the emission current density and those found experimentally are discussed.

Effect of Higher Order Axial Electron Temperature on Self-Focusing of Electromagnetic Pulsed Beam in Collisional Plasma

Effect of higher order axial electron temperature on self-focusing of electromagnetic pulsed beam in collisional plasma is investigated. It is shown that higher order axial electron temperature Tp4 is not trivial than Tp0 and Tp2, which can modify slightly radial redistribution of electron density and increases effective dielectric constant. As a result, on one hand, slightly reduce electromagnetic beam self-focusing in the course of oscillatory convergence, on the other hand, quicken beam divergence in the course of steady divergence, i.e., higher order axial electron temperature Tp4 can decrease the influence of collisional nonlinearity in collisional plasma.

Measurements of velocity shear and ion viscosity profile in a magnetohydrodynamic plasma jet

Time-dependent, two-dimensional profiles of the axial flow velocity, density, electron temperature, and magnetic field components are measured at two axial locations in a screw pinch plasma column of the reconnection scaling experiment. The results show that the ion momentum flux for a given column radius is dissipated by the ion-ion Coulomb scattering viscosity due to a significant radial shear of the axial velocity. By comparing the terms of the magnetohydrodynamic momentum balance equation, radial profile of ion viscosity is determined. Chord-integrated ion temperature measurements performed at several radial locations using Doppler broadening spectroscopy show ion temperature of about 1 eV. Measured ion viscosity agrees within a factor of 2 with the classical Braginskii expectations.

Focusing of electromagnetic beams in ionosphere with finite thermal conduction

In this communication the focusing/defocusing of (1) a single Gaussian electromagnetic beam, (2) a number of coaxial Gaussian electromagnetic beams, and (3) a Gaussian ripple on an electromagnetic beam of uniform irradiance in the ionosphere has been investigated in the paraxial approximation. The nonlinearity in the dielectric function, responsible for the focusing/defocusing arises from the redistribution of the electron density, caused by the nonuniform distribution of electron temperature, determined by the Ohmic heating of the electrons and the energy loss of electrons on account of collisions and thermal conduction. The wave frequencies have been assumed to be much larger than the electron collision frequency and the gyrofrequency. A self‐consistent solution of the electromagnetic wave equation, energy balance equation (considering the solar radiation), and Fourier's equation of heat conduction has been obtained in the paraxial approximation; the available database for the midlatitude daytime ionosphere has been used for numerical computation. It is seen that as the thermal conductivity increases, the axial electron temperature and its radial gradient decrease, with the net effect that the nonlinearity in the dielectric function increases. This causes the critical curves to rise up and the two humps to get more pronounced as the thermal conduction becomes more important.

A sensing torch for on-line monitoring of the gas tungsten arc welding process of steel pipes

Non-intrusive and real-time monitoring techniques are increasingly required by manufacturing industry in order to detect flaws in arc welding processes. In this work the development of an optical inspection system, for monitoring the manual gas tungsten arc welding (GTAW) process of steel pipes, is described. The arc plasma visible emission produced during the process was acquired and spectroscopically analysed. Measuring the intensities of selected argon emission lines allowed real time calculation and recording of the axial electron temperature of the plasma. Experimental results showed that the temperature signal varies greatly in the case of instabilities of the weld pool that cause weld defects. A suitable algorithm, based on a statistical analysis of the signal, was developed in order to real time flag defective joints. It is shown that several weld defects such as porosity, dropout, lack of fusion, solid inclusions and craters were successfully detected in a production environment. The performances of the optical sensor were compared with the results of state-of-the-art post-weld controls such as x-rays and penetrating dyes, showing good agreement and thus demonstrating the validity of this quality monitoring system.

Temperature gradient in Hall thrusters

Plasma potentials and electron temperatures were deduced from emissive and cold floating probe measurements in a 2 kW Hall thruster, operated in the discharge voltage range of 200–400 V. An almost linear dependence of the electron temperature on the plasma potential was observed in the acceleration region of the thruster both inside and outside the thruster. This result calls into question whether secondary electron emission from the ceramic channel walls plays a significant role in electron energy balance. The proportionality factor between the axial electron temperature gradient and the electric field is also significantly smaller than might be expected by fluid models.

Spectroscopic validation of the supersonic plasma jet model

Optical emission spectroscopy is applied to validate numerical simulations of supersonic plasma flow generated by induction torch with a convergent-divergent nozzle. The plasmas exhausting from the discharge tube with the pressure 0.4-1.4 atm. through two nozzle configurations (the outlet Mach number equals 1.5 and 3) into low-pressure (1.8 kPa) chamber are compared. Both modelling and experiments show that the effect of the nozzle geometry on physical properties of plasma jet is significant. The profiles of electron number density obtained from modeling and spectroscopy agree well and show the deviations from local thermodynamic equilibrium. Analysis of intercoupling between different sorts of nonequilibrium processes is performed. The results reveal that the ion recombination is more essential in the nozzle with the higher outlet number than in the nozzle with the lower outlet number. It is demonstrated that in the jets the axial electron temperature is quite low (3000-8000 K). For spectroscopic data interpretation we propose a method based on the definition of two excitation temperatures. We suppose that in mildly under expanded argon jets with frozen ion recombination the electron temperature can be defined by the electronic transitions from level 5p (the energy E = 14.5 eV) to level 4p (E = 13.116 eV). The obtained results are useful for the optimization of plasma reactors for plasma chemistry and plasma processing applications.

Plasma source for ion and electron beam lithography

A new plasma source configuration, coaxial source, has been developed at the Lawrence Berkeley National Laboratory suitable for ion and electron beam lithography applications. The axial ion energy spread and electron temperature of the multicusp ion source have been reduced considerably from 2 and 0.3 eV to a record low of 0.6 eV by employing a coaxial source arrangement. Results of ion projection lithographic exposure at the Fraunhofer Institute demonstrate that feature size less than 65 nm can be achieved by using a filter-equipped multicusp ion source. Langmuir probe measurements also show that very low energy spread electron beams can be obtained with the multicusp plasma generator.

Langmuir probe measurements in a vacuum arc plasma centrifuge

This work presents experimental data on several important parameters related to isotope enrichment and throughput in a vacuum arc plasma centrifuge. Using Langmuir probe measurements in the rotating magnetized plasma column, the axial drift velocity, electron temperature, and ion density were investigated for eight different metallic plasmas: C, Mg, Al, Ni, Cu, Zn, Cd, and Pb. For magnesium, the angular velocity of the plasma column was estimated from the axial velocity measurements using a modified slit technique, and the result was compared to values obtained from probe signals. Cathode erosion measurements were also made and checked against the plasma parameters for consistency. It was found that although the centrifuge operation at high magnetic field does not offer any obvious advantages, higher currents should increase throughput without degrading performance.

Neutral transport in a gas target divertor: experimental simulation in PISCES-A and DEGAS modeling results

The DEGAS Monte Carlo neutral transport code is applied to gas target divertor simulation experiments in the PISCEA-A linear plasma device to study the neutral gas dynamics in a well diagnosed environment. The experimental geometry is accurately represented by a novel use of the toroidal symmetry option. Axial and radial density and electron temperature profiles, experimentally determined from Langmuir probe data are used as plasma input data for the code. It is found that the molecular and atomic hydrogen density profiles obtained from DEGAS agree well with the experimentally determined neutral densities. The electron energy balance from DEGAS indicates electron power losses by ionization, radiation and dissociation in excess of 95% of the input power for the highest gas puffing rates ( S gas ≤ 3 Torr 1/s), in agreement with experimental observations.