Search

The effect of electrode polarity in the pin-to-rod geometry on the development of pulsed electric discharge in water with specific electrical conductivity of (90 ± 10) µS/cm with and without air microbubbles was experimentally studied. It was found that the initiation of the plasma channel at the anode in water occurs near the metal–liquid–insulation contact in both anode geometries under study. In the presence of microbubbles at an increased voltage, the development of plasma channels after initiation occurs in the direction opposite to the discharge gap along the insulated surface of the electrodes. In the presence of microbubbles, the breakdown voltage decreases, and the delay to discharge initiation and the total time of breakdown development by the channel propagating from the pin anode also decrease. When the voltage is increased, the development of the closing channel occurs from the cathode, regardless of its geometry.

The work is devoted to the study of thermophysical and radiation processes during the formation of a plasma channel under the passage of a series of current pulses of pulsed-periodic cesium–xenon–mercury discharge. The influence of the auxiliary discharge mode, temperature, and vapor pressure of metals on the development and relaxation of the plasma channel is shown. The spectral characteristics of the passage of each of the current pulses are studied.

The paper devoted to the research of the microdischarge dynamics in the dielectric barrier discharge. The discharge between rail electrodes in airflow along the electrodes at atmospheric pressure was studied. The aim of this work is to clarify the role of volume plasma and surface charges in the memory effect of microdischarges. Based on the analysis of microdischarge images obtained using high-speed camera, it is established that the transport of microdischarge plasma by a gas flow determines the microdischarge localization in each subsequent half-cycle of the applied voltage. An important part is played by the turbulence and the presence of vortices in the airflow. They determine both the speed of plasma channel transfer and the probability of the microdischarge appearence in a specific half-cycle. The results of the work show the possibility of the gas-dynamic control for parameters of a barrier discharge.

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.

A dc discharge in a cold (T = 200 K) supersonic air flow at a static pressure of 200–400 Torr was studied experimentally. The excited unsteady pulsating discharge has the form of a thin plasma channel with a diameter of ≤1 mm, stretched downstream the flow. Depending on the discharge current, the pulsation frequency varies from 800 to 1600 Hz and the electron temperature varies from 8000 to 15000 K.

The effect of an external magnetic field on the dynamics of shock waves generated in an argon plasma due to both explosive processes on the cathode and expansion of the spark channel has been studied experimentally. It is shown that the expanding plasma of the cathode spot forms a shock wave and that the application of a longitudinal magnetic field decelerates the radial expansion of the cathode plasma. It is found that the intensities of some argon spectral lines increase in the presence of a magnetic field.

Processes of spreading of the pulsed current from spherical electrodes and an electric breakdown in the quartz sand are studied experimentally. When the current density on the electrode exceeds the critical value, a nonlinear reduction occurs in the grounding resistance as a result of sparking in the soil. The critical electric field strengths for ionization and breakdown are determined. The ionization-overheating instability is shown to develop on the electrode, which leads to the current contraction and formation of plasma channels.

The time evolution of a nonequilibrium plasma channel created in a noble gas by a high-power femtosecond KrF laser pulse is investigated. It is shown that such a channel possesses specific electrodynamic properties and can be used as a waveguide for efficient transportation and amplification of microwave pulses. The propagation of microwave radiation in a plasma waveguide is analyzed by self-consistently solving (i) the Boltzmann kinetic equation for the electron energy distribution function at different spatial points and (ii) the wave equation in the parabolic approximation for a microwave pulse transported along the plasma channel.

Results of studies of a spark discharge initiated in argon in a point–plane electrode gap with limitation of the discharge current by a large ballast resistance are presented. It is shown that the current flowing through the plasma channel of such a low-current spark has the form of periodic pulses. It is experimentally demonstrated that, when a low-current spark transforms into a constricted glow discharge, current pulses disappear, the spatial structure of the cathode glow changes abruptly, and a brightly glowing positive plasma column forms in the gap.

Conditions for gas ionization in the field of a slow surface wave excited by a microwave source are considered. The gas ionization rate and the plasma density distribution over the radius of the discharge tube were studied by the optical method. The experiments were conducted in a dielectric tube with a radius much smaller than the tube length, the gas pressure being ~1–3 Torr. It is shown that the stationary distribution of the plasma density is determined by diffusion processes.

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.

The process of electron trapping by a wake wave excited by a laser pulse in a plasma channel in the case where the electron bunches are injected into the vicinity of the maximum of the wakefield potential at a velocity lower than the wave phase velocity is considered. The mechanism for the formation of a compact electron bunch in the trapping region when only the electrons of the injected bunch that are trapped in the focusing phase mainly undergo the subsequent acceleration in the wakefield is analyzed. The influence of the spatial dimensions of the injected bunch and its energy spread on the length of the trapped electron bunch and the fraction of trapped electrons is studied analytically and numerically. For electron bunches with different ratios of their spatial dimensions to the characteristic dimensions of the wake wave, the influence of the injection energy on the parameters of the high-energy electron bunch trapped and accelerated in the wake-field is studied.

The results of experiments on the filamentation dynamics of high-power ultrashort Ti: sapphire laser pulses with wavelengths of 800 and 400 nm upon their sharp focusing in air are presented. The dependences of the position and dimensions of the plasma channel that forms in the region of the nonlinear beam focus on the laser pulse power are obtained. The spectra of the laser pulse during its filamentation are measured.

The influence of the electrode system on the emission characteristics of a high-current low-inductance vacuum spark is investigated. It is shown that the structure and composition of the spark plasma radiating in the X-ray spectral range depend substantially on the geometry and relative position of the electrodes. A mechanism related to the effect of the initial distribution of the electric field in the electrode gap is proposed to explain such a dependence. The conditions in which the radiating plasma forms from the erosion products of one or both electrodes are determined.

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.

The contribution of the electrode material to the formation of the plasma of a low-inductive high-current vacuum spark and its influence on the process of discharge micropinching were studied using X-ray spectroscopy and laser diagnostics. Electrode system configurations are determined in which the contributions of the materials of both electrodes to the plasma emitting X-rays are comparable and in which the contribution of one electrode is dominating. It is found that discharge pinching occurs primarily in the vapor of the pointed electrode independently of its polarity. The experimental results indicate the formation of a suprathermal electron beam in the micropinch region.

Near-electrode processes in a coaxial plasma accelerator with equipotential impenetrable electrodes are simulated using a two-dimensional (generally, time-dependent) two-fluid MHD model with allowance for the Hall effect and the plasma conductivity tensor. The simulations confirm the theoretically predicted mechanism of the so-called “crisis of current” caused by the Hall effect. The simulation results are compared with available experimental data. The influence of both the method of plasma supply to the channel and an additional longitudinal magnetic field on the development of near-electrode instabilities preceding the crisis of current is studied.

The expansion of the cathode spot and the generation of shock waves during the formation and development of a pulsed volume discharge in atmospheric-pressure helium were studied by analyzing the emission spectra of the cathode plasma and the spatiotemporal behavior of the plasma glow. The transition of a diffuse volume discharge in a centimeter-long gap into a high-current diffuse mode when the gas pressure increased from 1 to 5 atm and the applied voltage rose from the statistical breakdown voltage to a 100% overvoltage was investigated. Analytical expressions for the radius of the cathode spot and its expansion velocity obtained in the framework of a spherically symmetric model agree satisfactorily with the experimental data.

The experimental and theoretical results of the investigation of an ion flow in a low-pressure discharge in crossed E and H fields are presented. It is shown that two quasi-stationary current states can be realized in a transonic collisionless flow of ions in a cold plasma.

A barrier electric discharge excited between a fixed electrode and a rotating electrode covered with a dielectric layer in atmospheric-pressure air is studied experimentally. A distinctive feature of this type of discharge is that it operates at a constant voltage between the electrodes. An advantage of the proposed method for plasma generation in the boundary layer of the rotating electrode (e.g., for studying the influence of plasma on air flows) is the variety of forms of the discharge and conditions for its initiation, simplicity of the design of the discharge system, and ease of its practical implementation

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.

X-ray diagnostics of the interaction of femtosecond laser pulses with intensities of 1016–1018 W/cm2 with CO2 clusters and frozen nanosize water particles is carried out. The stage of cluster expansion and the formation of a plasma channel, which governs the parameters of the formed X-ray radiation source and accelerated ion flows, is studied. The measurements are based on recording spatially resolved X-ray spectra of H- and He-like oxygen ions. Utilization of Rydberg transitions for spectra diagnostics makes it possible to determine plasma parameters on a time scale of t ∼ 10 ps after the beginning of a femtosecond pulse. The role of the rear edge of the laser pulse in sustaining the plasma temperature at a level of ∼100 eV in the stage of a nonadiabatic cluster expansion is shown. The analysis of the profiles and relative intensities of spectral lines allows one to determine the temperature and density of plasma electrons and distinguish the populations of “thermal” ions and ions that are accelerated up to energies of a few tens of kiloelectronvolts. It is shown that the use of solid clusters made of frozen nanoscale water droplets as targets leads to a substantial increase in the number of fast He-like ions. In this case, however, the efficiency of acceleration of H-like ions does not increase, because the time of their ionization in plasma exceeds the time of cluster expansion.

The processes of ion flux formation in the plasma of a high-current vacuum spark were investigated experimentally. It is shown that multicharged ions are generated in the neck formed in the erosion products of the inner electrode. The plasma escaping from the neck region plays a role of a piston dragging particles of the cold peripheral plasma into ambient space. As the discharge current increases, the flux of the evaporated electrode material grows, the degree of ionization of the plasma produced decreases, and the efficiency of plasma heating caused by the pinching effect is reduced.

A numerical model of the return stroke of a downward lightning is used to quantitatively analyze the error in recovering the parameters of a lightning current pulse from the electromagnetic field recorded in the far wave zone. It is shown that the error is mainly caused by the deformation of the current wave propagating along the plasma channel from the ground to the cloud and the dependence of the wave phase velocity on the current, rather than by the deviation of lighting from the vertical. The recovery error caused by the nonlinear dependence of the recorded magnetic field on the excitation current can reach a few hundred percent. Results of direct measurements of the lightning current at high-rise buildings and during trigger lightnings cannot be used to calibrate systems for remote monitoring of thunderstorm activity, because radiation from current-carrying metal structures substantially reduces the error caused by the damping of the return stroke wave in the plasma channel.

Results are presented from experimental studies of low-pressure inductive RF discharges (including those with a capacitive component) employed in plasma technology. It is shown that both the RF power absorbed in the plasma and the electron density depend nonmonotonically on the external magnetic field. Discharge disruptions occurring at critical values of the magnetic field and the spatial redistribution and hysteresis of the plasma parameters were observed when varying the magnetic field and RF generator power. The parameters of the plasma of low-pressure (0.5–5 mTorr) inductive RF discharges were investigated, and the discharge properties related to the redistribution of the RF generator power between the plasma and the discharge external circuit were revealed. The experiments were performed with both conventional unmagnetized inductive plasma sources and plasma sources with a magnetic field.

Results are presented from experimental studies and numerical simulations of a spark discharge excited in a short point-plane gap filled with atmospheric-pressure nitrogen. The discharge was powered from a high-voltage source connected to the discharge gap through a large ballast resistance. In this case, a short-term spark develops only due to the charging of the stray capacitance of the external circuit; therefore, the energy released by the spark and its intensity are both low. Rapid current growth in a weak spark is accompanied by the contraction of the current channel rather than by its gasdynamic expansion, as it occurs in long-duration kiloampere sparks. Simulations show that, because of the very short spark lifetime, the plasma in a weak spark is substantially nonequilibrium and the gas temperature is fairly high.

A one-dimensional axisymmetric time-dependent model of a discharge in a gas flow is developed. The model includes a fairly complete set of plasmochemical reactions and describes the heating and gasdynamic expansion of a plasma channel in air. The processes governing the distribution of nitrogen molecules in the N2(C3Πu, v) state over vibrational levels are considered. The parameters of a longitudinal glow discharge in a hot (T0 = 1500−3000 K) air flow at atmospheric pressure are calculated. It is found that gas preheating considerably influences the parameters of the discharge channel. The results of calculations are compared with the experimental data.

Results are presented from experimental studies of fast Z-pinches produced in plasmas of high-Z elements. An analysis of a plasma structure emitting X radiation and time-resolved measurements of the electron emission showed that a self-consistent regime of electron and ion motion is established in the plasma channel of the discharge. It was found that, in this regime, the electron component makes a negative contribution to the net current and an electrically neutral supersonic plasma flow propagates along the discharge axis in the direction of the net current.

Results are presented from experimental studies of the conversion of a steady-state negative corona into a spark. It is found that a spark in a negative corona in nitrogen and air is formed in the absence of fast primary streamers. It is shown that, in atmospheric-pressure nitrogen, the conversion of a corona into a spark begins with the propagation of a plasma channel (secondary streamer) from the point electrode (cathode) to the plane electrode (anode). In contrast, the plasma channel in air originates near the plane electrode and then propagates towards the point electrode. The propagation velocity of the secondary streamer is very low, V=103–104 cm/s. Two possible scenarios of the formation of the spark channel in a negative corona in nitrogen are described on the basis of the concept of a contracted volume glow discharge. Results are presented from time-resolved spectral measurements of plasma emission from different regions of the corona during its transformation into a spark.

Results are presented from numerical investigations of the potential of a test charge in a plasma-wall sheath. It is shown that like charges that are at the same distance from a floating electrode can attract one another.

The excitation of quasistatic magnetic fields by a circularly polarized laser pulse in a plasma channel is considered. It is shown that, to second order in the amplitude of the electric field of the laser pulse, circular rotation of the plane of polarization of the laser radiation in a radially nonuniform plasma gives rise to a nonlinear azimuthal current and leads to the excitation of the radial and axial components of the magnetic field. The dependence of the magnetic field distribution over the plasma channel on the spatial dimensions of the pulse and on the channel width is investigated for a moderate-power laser pulse. The structure of the magnetic fields excited by a relativistic laser pulse in a wide plasma channel is analyzed.

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.

The diffraction broadening of laser radiation restricts its efficient utilization in many applications. In this work, a method for laser radiation guiding in a density channel formed in a plasma by a relativistic electron beam is considered. The conditions and parameters of the relativistic beam ensuring the guiding are examined.

A mechanism for the formation of the structure of an optical discharge in Besselian laser beams is proposed on the basis of analyzing numerous experiments. The discharge structure is determined by the periodicity of the field of a Besselian beam in the radial and longitudinal directions and also depends on the power and duration of the heating pulse. In the initial stage of the plasma channel formation, the configuration of the channel inhomogeneities follows the discharge structure. If the spatial scale of the discharge structure is small, then the developing channel evolves into a homogeneous state. The time required for the structural inhomogeneities of the plasma channel to be smoothed out is estimated as a function of their scale length.

A new method for creating high-current plasma channels is developed. The method uses a narrow gas column formed by the leading particles of a nonsteady gas jet outflowing into a vacuum. An electric discharge device with a system for the formation of a narrow gas column is experimentally studied. The parameters of emission from the plasma channel are measured.

The problem of self-consistent motion of charged particles in a high-current plasma channel is solved using the kinetic model of a plasma with electron and ion beams whose motion is governed by the resulting electromagnetic field. It is shown that, in a high-density plasma, the ion motion makes the contribution of electrons to the current in the channel negative, in which case the ion current is higher than the net current and the plasma moves at a high speed as an electrically neutral axial stream whose direction coincides with the direction of the current in the channel.

A hot target plasma is obtained in the end cell of the AMBAL-M device with the use of an end gas-discharge plasma source. A fairly high longitudinal electron current flowing from the plasma source to the plasma receiver is detected experimentally. The electron current is studied in the region in front of the input magnetic mirror, where the longitudinal electric field is directed outward from the mirror. Different models for plasma description are considered, and possible plasma instabilities are discussed. It is shown that a fairly high longitudinal electron current in the region where the electric field accelerates electrons results in the generation of the flow of fast electrons responsible for the current in the mirror system.