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.     

Initiation of high-voltage discharge in air by a plasma filament produced by an intense femtosecond laser pulse

High-voltage discharge initiated in air by a plasma filament produced by an intense femtosecond laser pulse was studied experimentally. It is found that the threshold of a laser-induced discharge decrease three-fold as compared to that of a discharge in undisturbed air. It is shown that the formation time of a laser-induced discharge decreases by almost three orders of magnitude as the applied voltage increases by a factor of 2. A numerical model of the discharge process is developed that adequately describes the experimental results. In particular, simulations reproduce the experimentally observed steep dependence of the formation time of a laser-induced discharge on the applied voltage, as well as typical values of the electric field required for such a breakdown.     

Filament ring formation in the dimorphic yeast Candida albicans

Stationary phase cultures of Candida albicans inoculated into fresh medium at 37 degrees C synchronously from buds at pH 4.5 and mycelia at pH 6.5. During bud formation, a filament ring forms just under the plasma membrane at the mother cell-bud junction at roughly the time of evagination. A filament ring also forms in mycelium-forming cells, but it appears later than in a budding cell and it is positioned along the elongating mycelium, on the average 2 microns from the mother cell-mycelium junction. Sections of filament rings in early and late budding cells and in mycelia appear similar. Each contains approximately 11 to 12 filaments equidistant from one another and closely associated with the plasma membrane. In both budding and mycelium-forming cells, the filament ring disappears when the primary septum grows inward. The close temporal and spatial association of the filament ring and the subsequent chitin-containing septum suggests a role for the filament ring in septum formation. In addition, a close temporal correlation is demonstrated between filament ring formation and the time at which cells become committed to bud formation at pH 4.5 and mycelium formation at pH 6.5. The temporal and spatial differences in filament ring formation between the two growth forms also suggest a simple model for the positioning of the filament ring.     

Capillary discharges for guiding of laser pulses

The dynamics of a capillary discharge is studied to achieve optimum conditions for the guiding of ultrashort intense laser pulses. A dynamic regime is revealed in which, after a short transient process, the discharge plasma is in dynamic and thermal equilibrium. Such plasma configuration is stable against MHD perturbations. It is shown that the radial inhomogeneity of the discharge plasma composition can provide the improvement of the focusing properties of a plasma waveguide. The radius of the region where electromagnetic radiation is localized is governed by a contact discontinuity between the plasma that initially fills the channel and the plasma that is produced due to ablation of the capillary wall material.     

Microwave processes in the SPD-ATON stationary plasma thruster

Results of experimental studies of microwave processes accompanying plasma acceleration in the SPD-ATON stationary plasma thruster are presented. Specific features of the generation of microwave oscillations in both the acceleration channel and the plasma flow outgoing from the thruster are analyzed on the basis of local measurements of the spectra of the plasma wave fields. Mechanisms for generation of microwave oscillations are considered with allowance for the inhomogeneity of the electron density and magnetic field behind the edge of the acceleration channel. The effect of microwave oscillations on the electron transport and the formation of the discharge current in the acceleration channel is discussed.     

Beam-plasma interaction in a hybrid plasma waveguide in the pulsed mode of microwave generation

Results are presented from experimental studies of the energy spectra of an electron beam in a model beam-plasma oscillator based on a hybrid plasma waveguide in the pulsed mode of microwave generation with a pulse duration of 1 µs or shorter. The beam energy spent on sustaining the beam-plasma discharge in a slow-wave structure is measured. A correlation between the type of excited waves and the generation of a group of accelerated beam electrons with energies exceeding the injection energy is revealed. It is shown that the pulsed mode of microwave generation is related to the time variations in the plasma density profile in the waveguide and the trapping of beam electrons by the excited microwave field.     

Diagnostics of a nonequilibrium nitrogen plasma from the emission spectra of the second positive system of N2

A method is proposed for determining the electron density N _e and the electric field E in the non-equilibrium nitrogen plasma of a low-pressure discharge from the spectra of the second positive system of N₂. The method is based on measuring the specific energy deposition in the plasma and the distribution of nitrogen molecules over the vibrational levels of the C ³Π _u state, as well as on modeling this distribution for a given energy deposition. The fitting parameters of the model are the values of N _e and E. A kinetic model of the processes governing the steady-state density of the C ³Π _u nitrogen molecules is developed. The testing of this method showed it to be quite reliable. The method is of particular interest for diagnosing electrodeless discharges and provides detailed information on the processes occurring in the discharge plasma. Preliminary data are obtained on the plasma parameters in a cavity microwave discharge and an electrode microwave discharge. In particular, it is found that the electric field in an electrode microwave discharge in nitrogen is lower than that in a hydrogen discharge. This effect is shown to be produced by stepwise and associative processes with the participation of excited particles in nitrogen.     

A Subthreshold High-Pressure Discharge Excited by a Microwave Beam: Physical Basics and Applications

A new form of discharge excited by a microwave beam in a high-pressure (up to atmospheric and higher) gas in free space and in a closed chamber is discussed. For the first time, the discharge was implemented by means of a gyrotron with a pulse power of 200 ≤ P ≤ 600 kW, a pulse duration of 0.5 ≤ τ ≤ 20 ms, and a wavelength of λ = 0.4 cm. Under deeply subthreshold conditions in atmospheric-pressure air, a plasma column with a length of L = 50 cm was generated by a microwave beam formed with the help of a quasi-optical transmission line. With the use of the MIG-3 gyrotron complex with the above parameters, generation of a plasma column with a length of several meters is possible in principle. The parameters and structure of the formation of the plasma investigated make it possible to class it as a self-non-self-sustained (SNSS) discharge, discovered and described for the first time at the Prokhorov General Physics Institute, Russian Academy of Sciences. One of the important applications of this type of discharge is plasmachemical cleaning of the urban air environment of hazardous contaminants.     

A pulse-periodic torch in a coaxial waveguide: Formation dynamics and spatial structure

Results are presented from experimental studies of the formation dynamics, spatial structure, and parameters of a pulse-periodic microwave discharge excited in a coaxial waveguide. The experimental setup allows the stable generation of a plasma jet in molecular and atomic gas flows at pressures close to atmospheric pressure without applying additional initiators. The complicated sequence of processes leading to torch formation cannot be adequately described with conventional models of a discharge sustained by a surface electromagnetic wave.     

Effect of Dimensional Changes on Plasma Characteristics in Electrothermal Capillary Discharges for Optimized Performance in Fusion Pellet Injection

Geometrical changes in capillary discharges influence the plasma properties and can control exit parameters to certain desired values. For a fixed capillary radius of 2 mm and a 72-μs 43.9-kA peak discharge current, the plasma temperature is about 2.7 eV for different capillary lengths due to the constant input energy, while the number densities tend to saturate for capillary lengths greater than 12 cm. The electrical conductivity reaches 4.02 × 104 Ω^?1 m^?1 and then tends to saturate for 9-cm capillary length. The maximum bulk velocity at the capillary exit slightly increases with the increase in the capillary length from 6.15 to 6.26 km/s for lengths below 18 cm and decreases to 5.88 km/s for longer capillaries due to the higher amount of ablated mass and increased drag forces. For a 9-cm length with the same 72-μs 43.9-kA peak discharge current, the increase in the capillary radius reduces the energy density, which in turn reduces the total ablate mass, plasma density, electrical conductivity, and exit pressure. It is shown that the plasma temperature decreases from 4.6 to 2.1 eV by increasing the capillary radius and radiant heat flux also drops from 463 to 18.1 GW/m². The exit bulk velocity drops from 8.7 to 5.3 km/s as the radius increases from 0.5 to 3.6 mm, respectively. The design features of a capillary discharge can be adjusted for the radius and length, to produce specific plasma parameters for desired applications. Scaling laws relating exit peak plasma parameters to radius and length are obtained to facilitate quick estimate of plasma parameters. The validation of this model has been confirmed by confronting with experimental measurements.     

Surface microwave discharges in air

A microwave discharge excited on the outer surface of a dielectric antenna has been investigated. The transverse and longitudinal dimensions and propagation velocities of the discharge have been measured as functions of the air pressure and the power and duration of the exciting microwave pulse. The spatial distributions and time evolution of the gas temperature, electron density, and radiation intensity of the discharge have been determined. It is shown that the degree of ionization of the discharge plasma can exceed 10%. The spatial distribution of the electron density is found to depend strongly on the air pressure.     

Alternating nonsteady gas-discharge modes in an atmospheric-pressure air flow blown through a point-plane gap

The electric and spectral characteristics of a nonsteady discharge in an atmospheric air flow blown through a point-plane interelectrode gap were investigated experimentally. The discharge was produced by applying a constant positive voltage to the point electrode, the amplitude of the applied voltage being much higher than the corona ignition voltage. The nonsteady character of the discharge is due to the spontaneously repeating streamer-spark breakdown, followed by the formation of either a diffuse ultracorona or a filamentary glow discharge. In the latter case, the length of the plasma column increases progressively, being blown off by the gas flow from the discharge gap. The extinction of a filamentary discharge is unrelated to the break of the current channel: the discharge decays abruptly when the filament length reaches its critical value. The distribution of active particles (O, OH, and N*₂) carried out from the discharge gap is determined from the data of spectral measurements.     

Formation of a Plasma Ring by a Microwave Discharge in a Narrow Coaxial Cavity beyond the ECR Region

Results are presented from the experimental study of conditions for the formation of a plasma ring by a microwave discharge in a narrow coaxial cavity in an axisymmetric magnetic field the magnitude of which is below the electron-cyclotron resonance value. It is established that the necessary condition for this process is the presence of an electrostatic wave propagating in the azimuthal direction, the circumference of the plasma ring being a multiple of the wave half-length.     

Studies of pulsed and continuous microwave discharges used to deposit diamond films

Results are presented from optical measurements of the atomic hydrogen density and the gas temperature in a reactor for depositing diamond films from the plasmas of pulsed and continuous microwave discharges at a fixed mean microwave power. The results obtained make it possible to explain the fact that the growth rate of diamond films in the plasma of a pulsed microwave discharge is larger than that in a continuous microwave discharge.     

Microwave cellular discharge in fine powder mixtures

The excitation of microwave discharges in fine semiconductor powders and powder mixtures of metals and dielectrics is studied. The plasma is produced due to local sparks generated in microsecond microwave pulses. The time delay in the onset of a discharge amounts to several milliseconds for intensities on the order of 10 kW/cm². As the discharge develops, both microwave absorption and the discharge glow intensity increase. Intense nonuniform heating of the powder is observed.     

Plasma parameters controlled by remote electron shower in a double plasma device

The principal feature of this experiment is the electron showers consisting of three tungsten wires embedded by the plasma, which are heated up consequently emitting electrons inside the diffused plasma to control the plasma parameters in the discharge section of a double plasma device. These cold electrons emitted by the heated filament are free from maintenance of discharge which is sustained in the source section. The target plasma, where electrons are injected is produced as a result of diffusion from the source section. It is found that, plasma density and plasma potential can be effectively controlled in this way.     

Behavior of electrons under different biasing conditions in a multidipole plasma

This paper reports about the bi-Maxwellian nature of electrons in a filament discharge plasma that exist in a suitable working pressure range. The plasma is produced within a multidipole magnetic cage with a stainless steel mesh grid connected at one end of the cage. The variation in electron energy, plasma potential and electron energy probability function are studied by applying different bias voltage to the magnetic cage, mesh grid and the filaments. It is observed that the electron energies are highly influenced by the bias applied to the magnetic cage and the filaments. The plasma potential is found to be mostly affected by the cage bias voltage. A plane Langmuir probe is used to estimate the plasma parameters.     

Study of the dark phase in the initial stage of the positive column formation in an argon glow discharge

The initial stage of the positive column formation in an argon glow discharge is investigated both experimentally and theoretically. A decrease in the plasma radiation intensity (the so-called “dark phase”) was observed experimentally over a time period of about 1 ms. A similar dip was also observed in the time dependence of the electric field strength. The time evolution of the population of the lowest metastable state of Ar was measured. A relevant theoretical model has been developed and used to perform calculations for the actual experimental conditions. A comparison between the numerical and experimental results shows that the model adequately describes the processes that occur during the formation of the positive column in an argon glow discharge. Experimental and theoretical study shows that the dark-phase effect is related to an excessive amount of metastable Ar atoms at the beginning of a discharge and, consequently, to high rates of stepwise ionization and chemionization.     

Population of vibrational levels of krypton excimer states excited by an electric discharge

The effect of admixtures and excitation conditions on the population distribution of the vibrational levels of krypton excimer states is investigated. The emission spectra of a krypton discharge plasma in a supersonic jet, an extended capillary discharge, and a barrier discharge are considered. It is found that the population distribution in a barrier discharge best fits the equilibrium conditions.     

Nonuniformity of the chemical composition of a capillary discharge plasma

A steady-state distribution of the concentration of two ion species in a capillary discharge plasma is studied using MHD equations for a plasma with a spatially nonuniform, time-dependent chemical composition. In our case, the set of equations is significantly simplified because of the steady-state character and symmetry of the problem. Even with such simplification, however, some results could be obtained only by numerical integration. The factors affecting the distribution of heavy ions are studied. It is shown that the distribution of the heavy impurity over the discharge cross section can be much more nonuniform than the distribution of the main component (hydrogen). A simple criterion for such a nonuniformity is obtained.