Analysis of cylindrical Langmuir probe using experiment and different theories

Cylindrical probe data have been analyzed using different theories in order to determine some plasma parameters (electron temperature and electron and ion densities). Langmuir probe data are obtained in a cylindrical DC glow discharge in the positive column plasma at argon gas pressures varied from 0.5 to 6 Torr and at constant discharge current equal to 10 mA. The electron density has calculated from the electron current at the space potential and from Orbital Motion Limited (OML) collisionless theory. Ion density has obtained from the OML analysis of the ion saturation currents. In addition, the electron temperature has measured by three different methods using probe and electrons currents. The electron temperature T _e , plasma density n _e , and space potential V _s , have been obtained from the measured single cylindrical probe I–V characteristic curves. The radial distribution of the electron temperature and plasma density along the glow discharge are measured and discussed. Using the collisionless theories by Langmuir cylindrical probe and up to several Torr argon gas pressures the differences between the values of electron temperature and electron and ion densities stay within reasonable error limits.     

Electron cyclotron resonance acceleration of electrons to relativistic energies by a microwave field in a mirror trap

Results are presented from experiments on the acceleration of electrons by a 2.45-GHz microwave field in an adiabatic mirror trap under electron cyclotron resonance conditions, the electric and wave vectors of the wave being orthogonal to the trap axis. At a microwave electric field of ≥10 V/cm and air pressures of 10^?6–10^?4 Torr (the experiments were also performed with helium and argon), a self-sustained discharge was initiated in which a fraction of plasma electrons were accelerated to energies of 0.3–0.5 MeV. After the onset of instability, the acceleration terminated; the plasma decayed; and the accelerated electrons escaped toward the chamber wall, causing the generation of X-ray emission. Estimates show that electrons can be accelerated to the above energies only in the regime of self-phased interaction with the microwave field, provided that the electrons with a relativistically increased mass penetrate into the region with a higher magnetic field. It is shown that the negative-mass instability also can contribute to electron acceleration. The dynamic friction of the fast electrons by neutral particles in the drift space between the resonance zones does not suppress electron acceleration, so the electrons pass into a runaway regime. Since the air molecules excited by relativistic runaway electrons radiate primarily in the red spectral region, this experiment can be considered as a model of high-altitude atmospheric discharges, known as “red sprites.”     

Electron acceleration by Langmuir waves excited by a laser pulse in a semi-infinite plasma

Results are presented from a theoretical investigation of the acceleration of test electrons by a Langmuir wave excited by a short laser pulse at half the electron plasma frequency. Such a pulse penetrates into the plasma over a distance equal to the skin depth and efficiently excites Langmuir waves in the resonant interaction at the second harmonic of the laser frequency. It is shown that the beam of electrons accelerated by these waves is modulated into a train of electron bunches, but because of the initial thermal spread of the accelerated electrons, the bunches widen and begin to overlap, with the result that, at large distances, the electron beam becomes unmodulated.     

Analysis of the dependence of surfatron acceleration of electrons by an electromagnetic wave in space plasma on the particle momentum along the wave front

Based on the numerical solution of the nonlinear nonstationary second-order equation for the wave phase on the particle trajectory, the dynamics of surfatron acceleration of electrons by an electromagnetic wave propagating across the external magnetic field in space plasma is analyzed as a function of the electron momentum along the wave front. Numerical calculations show that, for strongly relativistic initial values of the electron momentum component along the wave front g _y (0) (the other parameters of the problem being the same), electrons are trapped into the regime of ultrarelativistic surfatron acceleration within a certain interval of the initial wave phase Ψ(0) on the particle trajectory. It is assumed in the calculations that |Ψ(0)| ≤ π. For strongly relativistic values of g _y (0), electrons are immediately trapped by the wave for 19% of the initial values of the phase Ψ(0) (favorable phases). For the rest of the values of Ψ(0), trapping does not occur even at long times. This circumstance substantially simplifies estimations of the wave damping due to particle acceleration in subsequent calculations. The dynamics of the relativistic factor and the components of the electron velocity and momentum under surfatron acceleration is also analyzed. The obtained results are of interest for the development of modern concepts of possible mechanisms of generation of ultrarelativistic particle fluxes in relatively calm space plasma, as well as for correct interpretation of observational data on the fluxes of such particles and explanation of possible reasons for the deviation of ultrarelativistic particle spectra detected in the heliosphere from the standard power-law scalings and the relation of these variations to space weather and large-scale atmospheric processes similar to tropical cyclones.     

Effect of the transverse magnetic field on turbulence and parameters of a plasma column in the L-2M stellarator

The influence of magnetic configurations with magnetic hills or wells on the parameters of a plasma column and turbulence characteristics were studied in experiments in which the plasma was created and heated by a microwave beam at the second harmonic of the electron cyclotron frequency. Calculations show that, for 〈β〉=(1.5?2)×10^?, a configuration with a magnetic well takes place and the Mercier criterion for stability of the ideal MHD modes is satisfied. It is shown that the compensation of the Shafranov shift of the plasma column by a transverse (vertical) field (B _v /B ₀ =5×10^?3) leads to a configuration with a magnetic hill in which the Mercier stability criterion is violated in the central region of the plasma column. It is experimentally shown that the stored plasma energy in the magnetic-hill configuration is reduced by one-half in comparison with the magnetic-well configuration. In the case of a magnetic hill, the energy of fluctuations increases both in the plasma core and near the separatrix, and the quasi-regular components of the wavelet spectra grow. When the Shafranov shift is compensated only partially (B _v/B ₀～3×10^?3) and the system is near the instability threshold, the stored plasma energy and the central electron temperature are somewhat higher, and the radiation power of fast electrons from non-Maxwellian tails at the second harmonic of the electron gyrofrequency decreases. It is found that the wavelet spectra of fluctuations change, the coherence coefficient for spectral components increases, and the radial electric field near the separatrix decreases.     

Double layer in a cylindrical hollow-cathode discharge

A dc cylindrical coaxial glow discharge with an inner grid anode has been studied. The region between the two electrodes is seen dark, while a brightly glowing region forms inside the grid anode up to the center. The current-voltage characteristic of a dc cylindrical glow discharge in nitrogen is similar to that of a normal glow discharge, while the normal glow discharge voltage decreases with increasing pressure. The minimum plasma potentials are observed in the hollow cathode region due to the accumulation of electrons at the back of the grid anode. At the center, some of the passed electrons are converged, so their potential is decreased. These electrons have a sufficient time to be redistributed to form one group with a Maxwellian electron energy distribution function. The electron temperature measured by electric probes varies from 1.6 to 3.6 eV, while the plasma density varies from 3.9 × 10¹⁶ to 7 × 10¹³ m⁻³, depending on the discharge current and probe position. The plasma density increases as the electrons move radially from the grid toward the central region, while their temperature decreases.     

Effect of hydrogen ratio on plasma parameters of N2-H2 gas mixture glow discharge

A dc plane glow discharge in a nitrogen-hydrogen (N₂-H₂) gas mixture has been operated at discharge currents of 10 and 20 mA. The electron energy distribution function (EEDF) at different hydrogen concentrations is measured. A Maxwellian EEDF is found in the positive column region, while in both cathode fall and negative glow regions, a non-Maxwellian one is observed. Langmuir electric probes are used at different axial positions, gas pressures, and hydrogen concentrations to measure the electron temperature and plasma density. The electron temperature is found to increase with increasing H₂ concentration and decrease with increasing both the axial distance from the cathode and the mixture pressure. At first, with increasing distance from the cathode, the ion density decreases, while the electron density increases; then, as the anode is further approached, they remain nearly constant. At different H₂ concentrations, the electron and ion densities decrease with increasing the mixture pressure. Both the electron and ion densities slightly decrease with increasing H₂ concentration.     

Plasma decay in high-voltage nanosecond discharges in oxygen-containing mixtures

Plasma decay in high-voltage nanosecond discharges in CO₂: O₂ and Ar: O₂ mixtures at room gas temperature and a pressure of 10 Torr is studied experimentally and theoretically. The time dependence of the electron density during plasma decay is measured using microwave interferometry. The time evolution of the charged particle density, ion composition, and electron temperature is simulated numerically. It is shown that, under the given conditions, the discharge plasma is dominated for the most time by O ₂ ⁺ ions and plasma decay is determined by dissociative and three-body electron?ion recombination. As in the previous studies performed for air and oxygen plasmas, agreement between measurements and calculations is achieved only under the assumption that the rate of three-body recombination of molecular ions is much greater than that for atomic ions. The values of the rate constant of three-body recombination of electrons with О₂ ⁺ ions in a wide range of electron temperatures (500–5500 K), as well as for thermal (300 K) electrons, are obtained by processing the experimental results.     

Heat transport from the region of local electron heating in a plasma

The initial stage of the one-dimensional expansion of a hot electron cloud into a “warm” plasma (i.e., into a plasma with a finite electron temperature) is studied with allowance for plasma turbulence. It is shown that, in a nonturbulent plasma or in a plasma with sufficiently weak turbulence, counterstreaming warm plasma flows interpenetrate one another; in this process, the plasma flows are accelerated and form beams escaping from the heating region. A stronger turbulence gives rise to electron heat waves that also propagate away from the heating region.     

Influence of the plasma density and heating power on the intensity of electron cyclotron emission in the L-2M stellarator

Results are presented from experiments on studying the plasma behavior in the L-2M stellarator in regimes with a high power deposition in electrons during electron cyclotron heating at the second harmonic of the electron gyrofrequency (X mode) at heating powers of P _in=120–400 kW and average plasma densities from n _e≤3×10¹⁹ to 0.3×10¹⁹ m^?3. It is shown that, as the plasma density decreases and the heating power increases, the electron cyclotron emission spectrum is modified; this may be attributed to a deviation of the electron energy distribution from a Maxwellian and the generation of suprathermal electrons. At low plasma densities, the emission intensity at the second harmonic of the electron gyrofrequency increases, whereas the plasma energy measured by diamagnetic diagnostics does not increase. This poses the question of the correctness of determining the plasma electron temperature by electron cyclotron emission diagnostics under these conditions.     

Langmuir probe study of an inductively coupled magnetic-pole-enhanced helium plasma

This study reports the effects of RF power and filling gas pressure variation on the plasma parameters, including the electron number density n _e , electron temperature T _e , plasma potential V _p , skin depth δ, and electron energy probability functions (EEPFs) in a low-pressure inductively coupled helium plasma source with magnetic pole enhancement. An RF compensated Langmuir probe is used to measure these plasma parameters. It is observed that the electron number density increases with both the RF power and the filling gas pressure. Conversely, the electron temperature decreases with increasing RF power and gas pressure. It is also noted that, at low RF powers and gas pressures, the EEPFs are non-Maxwellian, while at RF powers of ≥50 W, they evolve into a Maxwellian distribution. The dependences of the skin depth and plasma potential on the RF power are also studied and show a decreasing trend.     

Evolution of a Langmuir wave in a weakly inhomogeneous plasma in a longitudinal electric field

The spatial evolution of a Langmuir wave excited by external sources in a weakly inhomogeneous electron plasma in a longitudinal electrostatic field is considered. It is shown that, in a longitudinal electrostatic field, a Langmuir wave can only be amplified in an inhomogeneous plasma provided that the current of trapped electrons exceeds that of untrapped electrons. In this case, as the wave propagates through the inhomogeneous region where its phase velocity increases, some untrapped electrons become trapped in the wave potential wells. As a result, the current of trapped electrons increases and the wave is amplified. Moreover, in the regions where the bulk electrons are localized, the minima of the wave are amplified to a greater extent than its maxima.     

Quasilinear relaxation of a low-density electron beam in a plasma

The quasilinear relaxation of a low-density electron beam under the action of plasma turbulence, which is generated during the development of a beam instability, when the beam is formed due to rapid local electron heating (acceleration) is analyzed in the one-dimensional approximation. It is shown that quasilinear diffusion results in the formation of a local plateau at the top of the electron distribution function without causing any significant spread in velocities of the beam electrons and that the relaxation process proceeds primarily through the spatial expansion of electrons with different velocities.     

Flow of a plasma of multielectron elements along a magnetic field

An analysis is made of a flow of Ar plasma imitating plasma flows in ion separation systems such as systems for processing spent nuclear fuel or ion cyclotron resonance isotope separation systems. It is found that the electron temperature is equalized along the flow by electron heat conduction. When the electron temperature is not too low (T _e ≥ E _ion/10, where E _ion is the ionization energy), multicharged ions are intensely produced along the entire flow. It is shown that this process is accompanied by the flow acceleration. Difficulties in describing a supersonic flow by hydrodynamic equations are pointed out.     

Estimation of the electron density and radiative energy losses in a calcium plasma source based on an electron cyclotron resonance discharge

The parameters of a calcium plasma source based on an electron cyclotron resonance (ECR) discharge were calculated. The analysis was performed as applied to an ion cyclotron resonance system designed for separation of calcium isotopes. The plasma electrons in the source were heated by gyrotron microwave radiation in the zone of the inhomogeneous magnetic field. It was assumed that, in such a combined trap, the energy of the extraordinary microwave propagating from the high-field side was initially transferred to a small group of resonance electrons. As a result, two electron components with different transverse temperatures—the hot resonance component and the cold nonresonance component—were created in the plasma. The longitudinal temperatures of both components were assumed to be equal. The entire discharge space was divided into a narrow ECR zone, where resonance electrons acquired transverse energy, and the region of the discharge itself, where the gas was ionized. The transverse energy of resonance electrons was calculated by solving the equations for electron motion in an inhomogeneous magnetic field. Using the law of energy conservation and the balance condition for the number of hot electrons entering the discharge zone and cooled due to ionization and elastic collisions, the density of hot electrons was estimated and the dependence of the longitudinal temperature T _e∥ of the main (cold) electron component on the energy fraction β lost for radiation was obtained.     

Time evolution of the exponential wavenumber spectra of turbulence upon helium injection into a hydrogen discharge at the FT-2 tokamak

The effect of variations in the key parameter of short-wavelength turbulence—the ion-acoustic Larmor radius ρ _s , which determines the position of the maximum of the drift instability growth rate over poloidal wavenumbers—was studied experimentally at the FT-2 tokamak. For this purpose, helium was injected to hydrogen plasma, which resulted in a change in the electron temperature at the plasma edge. The universality of the exponential shape of the turbulence spectra over radial wavenumbers q and a substantial excess of the characteristic turbulence scale L over the ion-acoustic Larmor radius was confirmed with the help of correlative diagnostics of enhanced scattering. This excess at the discharge periphery reaches a value of 3–5 at a low electron temperature, apparently, due to an increase in the dissipation of drift waves upon their cascade transfer toward short scale-lengths.     

Role of secondary electrons and metastable atoms in the electron-beam activation of argon-silane mixtures

The energy and spatial degradation of the primary beam electrons and the production of high-energy secondary electrons in ionizing collisions are analyzed by solving the Boltzmann integral equation for the electron distribution function. The effect of the primary and secondary electrons on the direct ionization of an Ar-SiH₄ mixture, the production of metastable argon atoms, and the dissociation of monosilane molecules is investigated over a wide range of the beam electron energies, argon pressures, and monosilane concentrations. The influence of metastable Ar* atoms on the dissociation of SiH₄ is studied by using the balance equation for metastable argon atoms and the equation for the ambipolar diffusion of ions and low-energy secondary (plasma) electrons in the beam plasma. It is shown that the main contribution to the activation of an Ar-SiH₄ mixture in an electron-beam plasma is provided by secondary electrons with energies higher than the excitation threshold for argon and the dissociation threshold for monosilane, whereas the contribution from metastable argon atoms, though potentially being comparable with that from secondary electrons, is less than in gas-discharge plasmas.     

Magnetic field excitation during electron beam injection from the Intercosmos-25 satellite (APEX)

Results of active experiments on electron beam injection from the Intercosmos-25 satellite into the ionospheric plasma are presented. A quasistatic magnetic field and the VLF-wave magnetic component are excited when an unmodulated electron beam with a current of I _be ?0.1 A and energy of ? _be =mv ²/2?10 keV is injected into the ambient plasma. The magnetic field excitation is attributed to the onset of plasma gradient instabilities.     

Measurements and calculations of the electron distribution function in the electronegative plasma of a hollow-cathode discharge in N<Subscript>2</Subscript>: SF<Subscript>6</Subscript> mixtures

Results are presented from measurements and numerical calculations of the electron energy distribution function in the plasma of a hollow-cathode glow discharge in N₂: SF₆ mixtures. It is shown that, when a small amount of SF₆ is added to nitrogen, the number of electrons in the inverse region of the distribution function (2–6 eV) increases by about one order of magnitude. As the electric field in plasma increases to ≈0.5 V/cm, the dip in the distribution function disappears and the inversion region vanishes.