Increase in the discharge duration under the action of ion-cyclotron plasma heating in the T-11M tokamak

Results are presented from experimental and theoretical studies of the heating of a hydrogen plasma with a lithium admixture at the fundamental ion-cyclotron frequency of hydrogen in the T-11M tokamak. It is found experimentally that the action of RF radiation on a hydrogen plasma containing a small amount (less than 4%) of lithium increases the duration of the discharge current pulse. The effect of the increase in the discharge current pulse under the action of RF radiation is simulated numerically.     

Parametric turbulence-sustained gas discharges

A brief review is given of papers on the RF production of a plasma whose electrons are heated due to the parametric turbulence driven by an alternating electric pump field and maintain the discharge by ionizing the working gas atoms. Results are summarized from studies of low-frequency parametric turbulence, specifically, ion-acoustic plasma turbulence in a magnetic field, ion-cyclotron turbulence associated with the excitation of ion Bernstein modes, and lower hybrid turbulence in a plasma with ions of one or two species. The turbulence level and the rate of turbulent heating of the electrons and ions are presented, and the results of modeling of these phenomena are described. Attention is focused on experiments in which low-frequency parametric turbulence may be observed.     

Chirped dissipative ion-cyclotron solitons in the Earth’s low-altitude ionospheric plasma with two ion species

Conditions for the excitation of small-scale nonlinear ion-cyclotron gradient-drift dissipative structures in cold ionospheric plasma are considered. The solution for the wave electric field in this structure in the form of a chirped soliton satisfying the equation of the Ginzburg-Landau type is derived in the electrostatic approach. The dissipative structure as a whole represents the chirped soliton accompanied by the comoving quasineutral plasma hump. The possibility of the excitation of two modes of this type (the high- and low-frequency ones) in plasma containing light and heavy ion impurities is considered. The role of electromagnetic corrections and the possible contribution introduced by these structures to the transport processes in the ionosphere are discussed.     

Density measurements in a tenuous plasma by a high-frequency resonator with a periodic structure

It is proposed to measure plasma densities in the range of 10⁵–10⁹ cm^?3 by a high-frequency resonator with a periodic structure consisting of annual high-frequency electrodes mounted on oppositely directed racks. The method proposed substantially increases the proportionality factor between the electron density and the shift of the resonator eigenfrequency. This factor is determined by the calibration method using an electron beam with given parameters. The calibration ensures an accuracy of about 10% for density measurements in a plasma produced by a 5-MeV proton beam propagating in air at pressures of 10^?2–10^?5 torr.     

Creation of a long magnetized plasma column in a metal chamber

A method for creation of a long magnetized column of dense hydrogen plasma in a metal chamber by means of a high-current linear discharge is considered. It is the main method for the formation of preliminary plasma in the GOL-3 multimirror trap, in which a plasma column with a length of up to 12 m and diameter of 8 cm, suitable for conducting experiments on the injection of a relativistic electron beam, was obtained. Conditions for stable discharge operation in the density range of 3 × 10¹⁹–10²² m^?3 are determined, including a discharge with a uniform longitudinal plasma density profile and incomplete initial ionization of hydrogen. It is demonstrated that the system is capable of operating in a magnetic field with a variable configuration and strength of up to 6 T in the solenoidal section and up to 12 T in the end mirrors. It is shown that an important role in the development of a discharge is played by fast electrons with energies corresponding to the initial applied voltage (about 25 kV), which provide primary gas ionization. The properties of low-temperature plasma in such a discharge are discussed.     

Energy characteristics of beam-plasma interaction in a closed volume

Energy exchange between an electron beam and plasma during a beam-plasma discharge in a closed cavity excited by the electron beam is analyzed using computer simulations by the KARAT code. A method allowing one to analyze the beam-plasma interaction in the quasi-steady stage of the discharge is proposed. Qualitative characteristics of energy exchange (such as beam energy losses and the energy distributions of beam electrons and plasma particles leaving the discharge) both during spontaneous discharge excitation and in the presence of initial beam modulation by regular or noiselike signals are determined. The results obtained enable one to estimate the energy characteristics of a plasma processing reactor based on a beam-plasma discharge.     

Anomalous Doppler effect and stimulated Cherenkov effect in a plasma waveguide with a thin-walled annular electron beam

A dispersion relation for the complex frequencies of the E modes excited by a thin-walled annular low-density beam in a cylindrical plasma waveguide is derived using the methods of perturbation theory. The cases of an annular and a uniform plasma filling are considered, and the corresponding wave growth rates are determined. A condition is obtained under which the primary mechanism for the excitation of the waveguide is the anomalous Doppler effect. The possibility is discussed of suppressing Cherenkov generation in a plasma resonator at the expense of the normal Doppler effect.     

Optical studies of plasma inhomogeneities in a high-current pulsed magnetron discharge

Results are presented for experimental studies of the plasma glow in a high-current pulsed magnetron discharge by using a high-speed optical frame camera. It is found that the discharge plasma is inhomogeneous in the azimuthal direction. The plasma bunches rotate with a linear velocity of ∼1 cm/μs in the direction of electron Hall drift, and their number is proportional to the discharge current. Plasma inhomogeneities in the form of plasma jets propagate in the form of plasma jets from the cathode region toward the anode. It is shown analytically that the formation of inhomogeneities is caused by the necessity to transfer high-density electron current across the magnetic field.     

Structure of a discharge induced by a coaxial microwave plasmatron with a gas-supply channel in the inner electrode

The structure of a discharge induced by a coaxial microwave plasmatron with a gas-supply channel in the inner electrode of a coaxial waveguide is investigated. A plasmatron with a power of up to 10 W operates at a frequency of 10 GHz. Depending on the operation regime, the discharge takes either a filament or torch form. A plasma filament arises at low flow rates of the working gas (argon) and occurs at the border of the potential core of the gas jet. A torch discharge occurs at high flow rates and has the form of a hollow cone. In both cases, the discharge arises in the potential core of the gas jet and does not spread beyond it. The distribution of the microwave field in the discharge plasma is determined.     

Excitation of RF Oscillations in a Discharge with Negative Differential Conductivity

The excitation of oscillations in a discharge with negative differential conductivity is studied experimentally. The possibility is demonstrated of amplifying oscillations in the cathode dark space at frequencies close to the electron plasma frequency of the positive-column plasma. The phase velocities of waves at these frequencies are determined. When the waves pass from the cathode dark space to the discharge positive column, their phase velocities decrease; the closer the frequency is to the electron plasma frequency, the more pronounced the decrease in the phase velocity. As the intensity of oscillations increases, the discharge becomes non-steady-state. This is confirmed by the time evolution of the current-voltage characteristic. The shape of the current-voltage characteristic, its splitting, and the rate at which it varies depend on the input RF power. The decrease in the cathode dark space indicates that the ionization processes in the discharge are strongly influenced by electron plasma oscillations excited due to the collective interaction of the electron beam formed at the cathode with the discharge plasma. It is these processes that determine the maximum values of both the frequency of the excited oscillations and the power that can be withdrawn from the discharge.     

Stimulated ionization scattering of a wave beam forming a discharge channel in a magnetic mirror trap

The stimulated scattering of a whistler wave beam forming an extended discharge channel in a magnetic mirror trap is discovered and investigated experimentally. It is shown that the beam is scattered by relaxaction oscillations of the lattice of plasma inhomogeneities excited by the beam field. The spectrum of the pump field in the RF discharge plasma is found to broaden considerably and to contain individual modulation peaks corresponding to lattice oscillations. The peaks are observed at working gas pressures at which the electron mean free path is close to the wavelength of the standing wave forming the discharge channel. A physical model describing the phenomena observed is developed.     

Self-consistent model of a pulsed air discharge excited by surface waves

A complete self-consistent electrodynamic model of a pulsed gas discharge excited by surface waves is developed. The model allows one to calculate both the initial phase of the discharge front propagation and the parameters of the produced plasma. The spatiotemporal evolution of the electromagnetic field and plasma parameters at the discharge front is investigated for the first time. It is shown that discharge propagation is mainly governed by a breakdown wave in an inhomogeneous electric field at the leading edge of the ionization front. It is found that the effect of the electric field enhancement in the plasma resonance region significantly affects the velocity of the breakdown wave. The results of calculations agree well with experimental data.     

Properties of a low-pressure inductive RF discharge II: Mathematical simulations

Self-consistent numerical simulations of a low-pressure inductive RF discharge have been carried out. It is shown that, on the one hand, the plasma parameters are determined by the RF power absorbed in the plasma and, on the other, they themselves govern the power absorption. This results in a nonmonotonic dependence of the plasma parameters on the magnetic field, as well as in discharge disruptions, similar to those observed experimentally in such discharges. An inductive RF discharge with a capacitive component is simulated. The experimentally observed characteristic properties of the discharges are explained based on the regular features of the absorption of RF power in the plasma. Traditional inductive plasma sources (both without and with a magnetic field) are considered.     

Diagnostics of a magnetized plasma by the field of surface waves guided by a discharge channel

A diagnostic method for determining plasma density from the dispersion of surface waves guided by a discharge channel in an axial magnetic field is discussed. The diagnostic characteristics that are the easiest to record experimentally are determined by analyzing the theoretical dispersion curves, and the ways of exploiting these characteristics for plasma diagnostics are suggested. To determine the slowing-down factor of a probing wave in a plasma channel, it is proposed to use diagnostic-signal resonances that occur when the wavelength of the slowed wave becomes equal to the length of the emitting or receiving antenna. The dependence of the plasma density averaged over the cross section of the plasma column on the strength of the external magnetic field is determined for a discharge channel formed as a result of the ionization self-channeling of plasma (lower hybrid) waves and whistlers.     

Charging of dust grains in a nonequilibrium plasma of a stratified glow discharge

A theoretical model is presented that describes the charging of dust grains in the positive plasma column of a stratified glow dc discharge in argon. A one-dimensional self-consistent model is used to obtain axial profiles of the electric field, as well as the electron energy distribution function along the axis of the discharge tube. Radial profiles of the electric field are determined in the ambipolar diffusion approximation. It is assumed that, in the radial direction, the electron distribution function depends only on the total electron energy. Two-dimensional distributions of the discharge plasma parameters are calculated and used to determine the potential and charge of a test dust grain at a certain point within the discharge and the electrostatic forces acting on it. It is shown that the grain charge distribution depends strongly on the nonequilibrium electron distribution function and on the nonuniform distribution of the electric field in a stratified glow discharge. A discussion is presented on the suspension of dust grains, the separation of grains by size in the discharge striations, and a possible mechanism for the onset of vortex dust motion at the edge of a dust cloud.     

One-dimensional model of a helicon discharge

A simplified model describing the steady state of a helicon discharge in a low-pressure plasma is considered. The electron Langmuir frequency of the plasma produced by the discharge is shown to be much higher than the electron gyrofrequency. It is found that the gas medium is ionized and the electrons are heated primarily by the extraordinary mode. The calculated electron density depends nonmonotonically on the magnetic field, in agreement with the results of numerous experiments.     

Effect of the Air Flow Velocity on the Characteristics of a Pulsating Discharge Produced by a DC Power Source

Results are presented from experimental studies of a pulsating discharge produced by a dc power source in subsonic and supersonic cold (T = 150–300 K) air flows at static air pressures in the flow of 40–760 Torr. Two modes of pulsating discharge were implemented experimentally: without and with (from one to five) intermediate breakdowns. The discharge pulsation frequency, the maximum attainable voltage across the discharge gap, the length of the plasma channel, and the electric field in the discharge plasma were studied as functions of the air flow velocity and discharge current.     

Electron beam relaxation in nonuniform plasma of a steady-state beam-plasma discharge at moderately low gas pressures

Electron beam relaxation in plasma under conditions typical of laboratory plasma devices based on a steady-state beam-plasma discharge was investigated. It is shown that the measured dependences of the beam loss factor in a discharge operating at a moderately low gas pressure disagree with theoretical dependences calculated for a longitudinally uniform plasma. Analytic dependences obtained in the framework of quasilinear theory with allowance for longitudinal plasma inhomogeneity agree with experimental data. Some effects caused by the influence of the main discharge parameters on electron beam relaxation are analyzed.     

Scaling of a fast spherical discharge

The influence of the discharge cavity dimensions on the properties of the spherical plasma formed in a fast discharge was studied experimentally. The passage of a current pulse with an amplitude of 30–40 kA and a rise rate of ~10¹² A/s (a fast discharge) through a spherical ceramic (Al₂O₃) cavity with an inner diameter of 11 mm filled with argon at a pressure of 80 Pa results in the formation of a 1- to 2-mm-diameter spherical plasma with an electron temperature of several tens of electronvolts and a density of 10¹⁸–10¹⁹ cm^–3. It is shown that an increase in the inner diameter of the discharge cavity from 11 to 21 mm leads to the fourfold increase in the formation time of the spherical plasma and a decrease in the average ion charge. A decrease in the cavity diameter to 7 mm makes the spherical plasma unstable.     

Investigation of the helicon discharge plasma parameters in a hybrid RF plasma system

Results of an experimental study of the helicon discharge plasma parameters in a prototype of a hybrid RF plasma system equipped with a solenoidal antenna are described. It is shown that an increase in the external magnetic field leads to the formation of a plasma column and a shift of the maximum ion current along the discharge axis toward the bottom flange of the system. The shape of the plasma column can be controlled via varying the configuration of the magnetic field.