


Vol 62, No 7 (2019)
- Year: 2019
- Articles: 26
- URL: https://bakhtiniada.ru/1064-8887/issue/view/14966
Article
Study of the Cathode Sheath Dynamics After Arc Current Zero Crossing Using a Two-Dimensional Langmuir Probe System
Abstract
A two-dimensional system of miniature Langmuir probes operating in the electron saturation current mode is proposed. With its help, the spatial distribution of plasma density near the discharge gap and the dynamics of cathode-plasma sheath extension are investigated after the high-current vacuum-arc discharge crosses zero current. The proposed methodology and the results obtained are urgent for investigating processes of current interruption in vacuum circuit breakers.



Generation of Deuterium Ions in a Vacuum Arc with Composite Gas-Saturated Cathode and in a Low-pressure Arc
Abstract
Plasma containing deuterium ions is used, for example, to generate neutron flows. Neutrons are generated in nuclear reactions of interaction of accelerated ion beams extracted from deuterium plasma with a solid-state target containing heavy hydrogen isotopes – deuterium or tritium. Deuterium plasma was produced in two types of arc discharge with cold cathode: in a vacuum arc with composite cathode saturated with deuterium and in a low-pressure arc with inlet of gaseous deuterium into the discharge gap. Results of studies of the mass-to-charge fractions of the plasma of these discharges are presented, and a comparative analysis of the methods of deuterium ion generation in such discharge systems is performed.



Generation of Boron Ions for Beam and Plasma Technologies
Abstract
The urgency of the study of generation of beams and plasmas containing boron ions is caused by their application in ion-beam and plasma technologies of modification of the surface properties of not only semiconductors, but also structural materials. This is due to the fact that boron compounds are hard and chemically resistant materials that can be used to create hardening and protective surface coatings for a wide nomenclature of details. The operating principle and the characteristics of the experimental setup developed for generation of plasma and boron ion beams intended for creation of such coatings are presented, including an ion source based on vacuum arc with separation of boron isotopes in a magnetic field intended for highdose ion implantation, a plasma generator with boron target intended for obtaining coatings by magnetron sputtering, and a forevacuum electron source intended for synthesis of surface boron-containing coatings by electron beam evaporation.



Formation of Gradient Metalloceramic Materials Using Electron-Beam Irradiation in the Forevacuum
Abstract
Results of using an electron beam formed by a forevacuum plasma electron source to sinter metalloceramic materials in powder form are reported. As the materials to be sintered, we used mixtures of titanium powder and an aluminum-oxide or zirconium-oxide based ceramic powder. Sintering was performed using a narrowly focused beam directed onto the surface of the metalloceramic powder. It has been shown that using a mixture of finely dispersed zirconium dioxide or aluminum oxide powder with titanium allows one, by using the electron-beam method in the forevacuum pressure region, to obtain a metalloceramic sample with a titanium concentration gradient over the volume of the sample.



Sites of Initiation of Explosive-Emission Processes on the Surface of Single-Crystal and Coarse-Grained Polycrystalline Copper
Abstract
Results of a study of sites of vacuum breakdown initiation on the electrolytically polished surface of single-crystalline and coarse-grained polycrystalline copper are presented. The coincidence of cathode erosion traces with sites of dislocation outcrops has been established.



Variation in the Local Material Temperature During Electron Beam Treatment and its Influence on the Modified Layer Properties
Abstract
The local temperatures on the surface of a ceramic coating – aluminum substrate system are measured during its irradiation with a submillisecond pulsed electron beam. Certain parameters, such as electron beam density, pulse duration, and thickness of a TiCuN-coating, are varied. The optimal modes for electron beam irradiation of the TiCuN-coating – А7-substrate system are identified.



Influence of the Anode of a Non-Self-Sustained Glow Discharge with a Hollow Cathode on the Spatial Distribution of Plasma Concentration
Abstract
The results of investigation of the influence of geometrical dimensions, shape and position of the anode of a high-current non-self-sustained glow discharge with a hollow cathode at low (≈1 Pa) pressure with the current up to 100 A on the degree of inhomogeneity of azimuthal plasma-concentration distributions are presented. The azimuthal measurements demonstrate that upon injection of an electron flow with the current exceeding that of the electrons, formed as a result of the secondary ion-electron emission, the value of the average path length L of electrons before their escape to the anode is much lower than that in a self-sustained discharge burning mode. It is determined that in the case of injection of an electron flow into the hollow-cathode glow discharge the anode position considerably affects the degree of plasma concentration inhomogeneity. It is shown that placing the annular anode in the site preventing the line-of-site between the electron injection and anode surface directions allows achieving the minimum inhomogeneity coefficient value.



Study of the Stability of Triggering of a Controlled Multigap Spark Switch for Capacitive Energy Storage with Charging Voltage up to 100 kV and Energy Release Time on the Order of 100 ns
Abstract
Results are presented of a study of the stability of triggering of a six-channel seven-gap spark switch intended for switching a capacitive energy storage with a charging voltage of up to 100 kV and energy release time on the order of 100 ns. The working medium of the spark switch is air at atmospheric pressure. In the controlled triggering regime, we measured the breakdown delay time of all of the discharge gaps relative to the arrival of the voltage trigger pulse and calculated the mean square deviation of the delay time (jitter) under variation of the charging voltage and growth rate of the voltage trigger pulse. Jitter less than 1 ns was obtained for a charging voltage of 90–100 kV and a growth rate of the voltage trigger pulse ~650 kV/μs. When the growth rate of the trigger pulse is decreased, the jitter of the spark switch grows significantly. Thus, for a charging voltage of 100 kV and a trigger voltage pulse with growth rate ~200 kV/μs the jitter exceeds 20 ns, and for a voltage trigger pulse with growth rate ~100 kV/μs the jitter exceeds 30 ns. To reduce the jitter when using trigger pulses with low growth rate, blade electrodes were inserted into the spark switch, ensuring ignition of a corona discharge in the gaps during charging of the capacitive storage. It has been shown that for a growth rate of the trigger pulse of 100–200 kV/μs the corona discharge reduces the jitter by 2–3 times.



Complex Electron-Ion-Plasma Processing of Aluminum Surface in a Single Vacuum Cycle
Abstract
The principle of operation and major characteristics of a laboratory setup designed for electron-ion-plasma surface modification of materials and parts in a single vacuum cycle are presented. Using an Al–Ti system as an example, a possibility of a multiple improvement of mechanical (microhardness) and tribological (wear resistance and friction coefficient) properties of a material formed as a result of a comprehensive treatment in a single vacuum cycle, combining the formation of an A7 technical-grade aluminum surface layer alloyed with titanium via a deposition – irradiation scheme (number of cycles from 1 to 20; metal film thickness per cycle 0.5 and 1 μm) followed by a subsequent electron-ion nitriding (540°С, 8 h) of the resulting surface alloy in the plasma of a non-self-sustained discharge by the ion and electron plasma components is demonstrated.



Streamer Breakdown with Runaway Electrons Forming Diffuse Discharges in an Inhomogeneous Electric Field
Abstract
The results of experimental investigations of formation of diffuse nanosecond discharges in a nonuniform electric field in air and other gases at the pressures within 12.5–400 kPa are presented. The experiments were performed using a four-channel ICCD-camera, an ultra-fast streak-camera, and wideband oscilloscopes. It is found out that spherical streamers are formed in a sharply nonuniform electrical fields under high voltages in a pre-breakdown stage of the discharge, irrespective of the gas used (air, nitrogen, hydrogen, methane, neon, or helium). The data are obtained on the instantaneous streamer velocity in air at different voltages and on the displacement current caused by the electric field re-distribution in the gap during the streamer formation. Beams of runaway electrons are recorded. The mechanism of formation of anode- and cathode-directed streamers at high voltages is discussed.



On the Influence of Electron Energy on Characteristics of the Cherenkov Radiation and Cathodoluminescence
Abstract
A review of the results of investigations of the Cherenkov radiation (CR) and pulsed cathodoluminescence during irradiation of different crystals and polymethylmethacrylate (PMMA) with subnanosecond and nanosecond beams is made. The calculations demonstrating the influence of the electron energy and refractive index of a material on the CR intensity and spectrum, as well as its spatial characteristics are performed. The experimental data on the CR observation at the electron energy E up to 400 keV are presented. It is shown that an increase in E allows detecting the CR in diamond, leucosapphire, and quartz KU-1 crystals using a standard spectrometer. It is found out that is more difficult to register the CR in PMMA because of the radiation absorption in the region of wavelengths shorter than 300–350 nm and an internal breakdown of the specimens due to the accumulation of electrons in them at high beam current densities or (and) pulse durations, even with a monochromator and a PMT. Some recommendations are given for designing detectors of runaway electrons in tokomak-type facilities.



Packet-Pulse Dual Magnetron Sputtering
Abstract
The paper presents the results of experimental study of the discharge formed by a dual (DU) magnetron sputtering system (MSS) with aluminum targets in the mode of packet-pulse magnetron sputtering of high power (called deep oscillation magnetron sputtering (DOMS) in foreign literature). A special feature of the discharge in the DOMS mode is the use of unipolar sequence of micropulses with short durations and high powers that form macropulse duration 1000–3000 μs. This mode of sputtering was previously used only in single MSS. In this paper, the DOMS mode is first investigated with a dual magnetron sputtering system. The main plasma parameters are measured using triple and single Langmuir probes. Dependences of plasma parameters on the parameters of the pulsed discharge power supply: voltage and current amplitudes, current density, and power density on the target surface are established. The results of experiments show that the use of the dual packet-pulse magnetron sputtering can significantly increase the plasma density and the ion current density on the substrate in comparison with the traditional DC and MF modes of magnetron sputtering. The ratio of the ion flux density to the flux density of neutral atoms, characterizing the degree of ion bombardment of the growing coating in the DU DOMS mode, reached a value of 28, whereas in the DC mode it was 0.8.



Plasma Chemical Synthesis of Amorphous Hydrocarbon Films Alloyed by Silicon, Oxygen and Nitrogen
Abstract
The paper presents the synthesis of amorphous hydrocarbon (a-C:H) films alloyed by silicon, oxygen and nitrogen. The films obtained in polyphenyl methylsiloxane vapor and argon/nitrogen environment are deposited onto crystalline silicon surface using plasma chemical deposition. It is shown that the physical and mechanical properties of the films depend on the nitrogen concentration. The film composition is studied by Xray fluorescence spectrometry and Fourier-transform infrared spectroscopy. Raman spectroscopy is used to explore the film structure. A nanoindenter is used for testing the indentation hardness and other mechanical parameters of the films. It is shown that the chemical composition and properties of a-C:H:SiOx:N film can be maintained by changing the partial nitrogen pressure during the film deposition. The increase in the nitrogen content in a-C:H:SiOx:N film leads to the growth in the root mean square roughness and the contact angle. It also leads to the reduction in the carbon content and the film hardness caused by the lower content of sp3 carbon phase.



Selection of Symmetric and Asymmetric Modes in the Oversize Slow-Wave Structure of a Multiwave Cherenkov Generator
Abstract
The resonances of the slow-wave structure of a multiwave Cherenkov generator have been determined theoretically for a frequency range close to the π-type frequency of the lower symmetric mode. The ratio of the diameter of the slow-wave structure to the wavelength of the microwave radiation was equal to four. The resonance frequencies and Q factors of symmetric and asymmetric modes have been found in relation to the length of the drift tube. It has been shown that these relations strongly affect the spectral composition of the resonances. At some ratios of the slow-wave structure diameter to the radiation wavelength, high-Q resonances of higher oscillation types (locked modes) occur. Examples of the slow-wave structure geometries are given for which resonances of some modes have an advantage of higher Q factor.



Radiation Characteristics of an Offset Reflector Antenna Excited by a Combined Antenna Array
Abstract
The results of a numerical simulation of an ultrawideband hybrid antenna are presented. The antenna consists of an offset reflector and a 2×2 array of orthogonally arranged combined antennas optimized for excitation by bipolar voltage pulses of duration 0.5 ns at a center frequency of 2 GHz. The antenna is designed for use as a component of a high-power ultrawideband radiation source. The polarization characteristics of the radiation have been studied numerically for two 2-GHz modes. In the first mode, wave beams with linear orthogonal polarizations are sequentially formed. In the second mode, a wave beam with an elliptically polarized field is formed. It has been demonstrated that the radiation peak can be controlled in two mutually perpendicular directions within the width of the pattern.



Plasma Flows of High-Current Discharge in a Capillary and Their Propagation Across a Magnetically Insulated Line
Abstract
The characteristics of sources of plasma flows formed by high-current discharges in polyethylene, teflon, and ceramic capillaries are presented that allow high-velocity flows of high-density plasma to be obtained. The influence of the initial voltage on the capillary, rate of current growth, and current amplitude on the parameters of the formed plasma bunches and on the service life of the capillaries made of the examined dielectrics is shown. Experimental data on propagation of bunches in crossed electric and magnetic fields of a magnetically insulated transmission line are presented.



Optical Recording of the Surface Plasma of Cylindrical Conductors in Strong Magnetic Fields
Abstract
Results are presented of experiments on the explosion of aluminum and titanium conductors that were carried out on a multipurpose impulse generator (MIG) (current amplitude of 2 MA, current rise time of 100 ns) in magnetic fields up to 3 MG. The conductors consisted of two parts: a rod 3 mm in diameter and a tube with the same outer diameter and wall thickness of 250 μm. The surface plasma of the conductor was recorded with the help of an HSFC Pro high-speed optical camera. It has been shown that instabilities on the surface of material with high conductivity (aluminum) are formed later than on the surface of material with low conductivity (titanium). Instabilities appear on the surface of the tube earlier than on the surface of the rod and remain more pronounced over the course of the entire process for both conductor materials. The growth increments of the large-scale instabilities were determined and the mechanism of their formation was analyzed.



Peculiarities of an Electrical Explosion of Flat Conductors in the Current Skinning Mode
Abstract
The propagation of a nonlinear magnetic-field diffusion wave generated under the condition of an electrical explosion of flat conductors is investigated in the current skinning mode. Using a MIG terrawatt generator, a number of experiments are performed on electrical explosion of a copper foil, 100 μm in thickness and 5 mm in width, at the current amplitude up to 2.5 MA and its rise rate 100 ns. It is shown that under these conditions a plasma channel is formed by approximately 75-th ns from the current onset. The estimations, made considering the magnetic field enhancement on the foil edges, demonstrate that about 70–80 ns are required for the nonlinear magnetic-field diffusion wave to propagate from the foil edge to its center. A good agreement of the experimental data and the estimates suggested a conclusion that the plasma channel formation is due to the convergence of the nonlinear diffusion wave towards the longitudinal foil axis.



Generation of K-Shell Radiation of Noble Gases in the Microsecond Implosion Regime
Abstract
Studies of K-shell plasma radiation sources were performed on the GIT-12 generator (4.7 MA, 1.7 μs). In the experiments, a new type of load configuration was used to form the Z-pinch plasma – a gas puff with an outer plasma shell. Noble gases neon and argon were used as working gases. The gas puff consisted of two cascades: a hollow cylindrical shell outside and a solid jet inside. The outer plasma shell was created with the help of plasma guns located on a diameter of 350 mm. A distinctive feature of these studies is that the experiments with the plasma radiation source were carried out in the microsecond implosion regime usually leading to a significant decrease in the efficiency of generation of K-shell x-rays. The use of the Z-pinch load of a new type significantly improved the efficiency of the microsecond plasma radiation source. As a result of optimization of the initial gas-puff parameters, a neon K-shell radiation yield of 14.7 kJ/cm at a peak implosion current of 3.5 MA was achieved. In experiments with argon gas puffs, the radiation yield reached 1.9 kJ/cm at a peak implosion current of 3.1 MA.



High-Current Pulsed-Repetitive Electron Accelerator “Sinus-320”: Formation and Diagnostics of a Wide-Aperture Beam
Abstract
A wide-aperture (16 cm × 27 cm) source of e-beams for sterilization of plastic packages based on a“SINUS- 320” high-current accelerator with 450 keV energy, 9 kA current, 12 ns pulse duration, and repetition rate up to 100 Hz is described. A metal-dielectric cathode forms e-beam by explosive emission during a few nanoseconds of the voltage pulse. A high degree of e-beam uniformity can be achieved because of a large number of triple points (~800) and of electron scattering in a foil and in a 3 cm air layer for characteristic electron free paths in air up to half a meter.



Wide-Aperture Liquid Calorimeters for Measuring the Energy of Powerful Microwave Radiation Pulses
Abstract
An overview was done of the efficient devices designed to measure the energy of powerful electromagnetic radiation pulses of the relativistic microwave sources – liquid calorimeters with wide-aperture absorbing loads. Designs and characteristics of calorimeters are described along with their operational features.



Role of Prebreakdown Currents in a Static Breakdown of a Two-Sectioned Cold-Cathode Thyratron
Abstract
The data on prebreakdown currents and static breakdown voltages measured in two-sectioned sealed-off thyratron with a cold cathode TPI1-10k/50 are presented. The temporal behavior of the anode voltage and of the voltage at the separate sections has been investigated at the prebreakdown and breakdown stages. It is demonstrated that the prebreakdown current presented in the separate sections causes redistribution of the anode voltage between the sections. Due to this effect, a maximum thyratron breakdown voltage can be obtained. The other method to increase the breakdown voltage is based on the forced distribution of the anode voltage between the sections using a capacitive divider. Special features of transition from the prebreakdown to breakdown current are discussed for different thyratron-switching circuits.



Special Features of Parasitic Current Formation in a Sealed-Off Cold-Cathode Thyratron with Trigger Unit Based On an Auxiliary Glow Discharge
Abstract
Results of investigation of the glow discharge with hollow cathode and ring anode in the trigger unit of a coldcathode thyratron TPI1-10k/50 are presented. A specific feature of the discharge sustainment is that a highly emissive tablet containing cesium carbonate is placed in the cathode cavity. The current-voltage characteristics of the discharge in the trigger unit are obtained for different tablet compositions together with the measured parasitic current to the main cathode cavity. Stepwise transitions to the regime with decreased discharge burning voltage accompanied by an increase in the parasitic current are observed. A model of current sustainment in a hollow-cathode glow discharge is used to interpret the characteristics obtained. Instead of the conventional secondary emission yield, the model uses a generalized emission yield that takes into account not only the ion bombardment of the cathode, but also the emission current from an external source. Based on estimations of the discharge parameters in the trigger unit, a reason for a parasitic current increase is revealed.



Apokampic Discharge: Formation Conditions and Mechanisms
Abstract
The experimental and theoretical investigations of a new form of burning of a repetitively-pulsed discharge – apokamp – are performed. It is shown that an apokampic discharge represents a narrow streamer channel, growing from the point of plasma channel bending, having a characteristic propagation velocity of tens and hundreds of meters per second, depending on the applied voltage and gas pressure and type. The conditions necessary for an apokamp to evolve are formed in a comparatively weak macroscopic electric field. The channel bend ensures a local field enhancement, which prescribes the starting orientation of the growing streamer channel. The pulse-frequency power supply regime of the discharge reproduces the streamer channel in every pulse, but the preceding framework of the ion-ion plasma ensures reproduction of the channel shape from pulse to pulse.



The Synthesis of Ni–Al Surface Alloy by Low-Energy, High-Current Electron Beam Irradiation of Composite Coating
Abstract
The paper presents research results of the synthesis of Ni–Al surface alloy performed in a single vacuum cycle via magnetron sputtering of Ni (0.5 μm)–Al (1.5 μm)–Ni (0.5 μm) composite coating onto a steel substrate and its successive single pulse irradiation with low-energy, high-current electron beam (LEHCEB) of a microsecond duration. The numerical solution of the heat equation is used to determine the optimum LEHCEB modes which provide melting of all the films deposited. It is shown that the single pulse irradiation of thin films leads to the formation of the surface alloy consisting mainly of high-melting-point NiAl intermetallic phase. The structure of the surface alloy is a 2 μm thick homogeneous coating with 2×4 μm globules beneath separated by a thin layer of the substrate material. It is found that the wear resistance of the obtained Ni–Al surface layer is 2.7 times higher than that of the untreated steel substrate.



Plasma Source for Bipolar Electron-Optical System
Abstract
The paper presents the experimental study of pulsed plasma source with hollow cathode intended for bipolar electron-optical system with open-boundary plasma. The proposed configuration of the plasma source includes two ring-shaped discharge cells based on plasma thruster technology. The plasma source can operate both in a low-current high-voltage mode and a high-current low-voltage mode. The main attention is paid to the highcurrent mode which is similar to the glow discharge positive column. It is found that along with the discharge in closed electron drift the ignition of additional, non-self-sustained hollow cathode discharge with electron oscillation is possible, thus considerably reducing the voltage of the closed-drift discharge. Current–voltage characteristics of the high-current discharge are measured for these conditions. Using the double probe technique, the local plasma parameters are measured together with their spatial distribution in the discharge region. The maximum plasma density along the plasma source axis is (6.5–6.8)•1012 сm–3.


