High-current Glow Discharge Research Articles

Computer Simulation of High-Current Non-Selfsustained Glow Discharge Plasma in Hollow Cathode at Low Pressure

Computer Simulation of High-Current Non-Selfsustained Glow Discharge Plasma in Hollow Cathode at Low Pressure

On the role of helium metastable (23S1) measured by time resolved tunable diode laser spectroscopy in high current magnetron discharge

Tunable diode laser absorption spectroscopy is applied to measure the density and gas temperature of helium metastable (23S1) in the ionization region of magnetron discharge operated at very high current density (), which is achievable in helium at 10 Pa with molybdenum target. Those measurements were combined to electrical characterization of the current and the voltage at the magnetron cathode. The discharge current waveform is anti-correlated with the time-evolution of helium metastable () density showing that the main depletion process of is its ionization. Counterintuitively, the higher the gas temperature the lower the discharge peak current. Gas rarefaction and gas recycling are discussed as major contributors for this phenomenon in very high current glow discharge.

Генерация плазмы с повышенной степенью ионизации в импульсном сильноточном тлеющем разряде низкого давления с полым катодом

Генерация плазмы с повышенной степенью ионизации в импульсном сильноточном тлеющем разряде низкого давления с полым катодом

INCREASING OF GLOW DISCHARGE STABILITY IN WELDING BY MEANS OF MULTISECTIONAL ELECTRODE SYSTEMS

Background. The glow discharge has been widespread as a source of surface heating for diffusion welding and soldering thanks to the possibility of regulation within a wide range of energy excursions in a heating spot. At the same time, due to the pressures inherent in these processes, certain violations have been identified, mainly due to the partial or complete loss of its stability and, as a result, its transition to a more stable form of gas discharge – an electric arc.Objective. The purpose of the paper is to provide stabilization of the burning process of a high-current glow discharge in regimes that are used in precision welding processes at discharge currents of 10 A, and gas pressures of 1.33–13.3 kPa.Methods. Using the methods of the theory of gas discharge physics, electrodynamics and electromagnetism, the main causes of the appearance of external perturbations and instabilities that lead to the emergence of a stable arc discharge on the local sections of the surfaces of the welded parts were determined.Results. It is established that at a constant current of discharge the application of a 5-section anode unit with a traveling electric field increases the region of stable existence of a glow discharge by 1.5–1.7 times in comparison with a similar electrode system with a stationary electric field. The application of this system, when heated by glow discharge, can also increase the energy density in the heating spot by a stable glow discharge by 2.6 to 2.9 times compared with the use of the anode of a continuous section.Conclusions. The stabilization of the power current flare discharge in the conditions of welding heat can be achieved by applying multisection electrode systems, namely by replacing the solid (contour) anode with a sectional one. However, such a system does not exclude the possibility of melting of parts due to the stabilization of the arc discharge on a separate anode section (and the corresponding section of the cathode).

Distinctive features of the high-current glow discharge with the hollow cathode at low gas pressures

A pulsed hollow cathode glow discharge with current up to 200 A, with the pulse width amounting to a few milliseconds, is studied in a wide range of the gas pressure p from 0.01 to 10 Pa. The high current discharge is established due to the application of a high-voltage pulse with amplitude Uo up to 5 kV between the anode and the hollow cathode of a continuous discharge with a current of Io = 0.5 A. The establishing time τ ranges from 10 to 300 μs, diminishes with the pulse amplitude growth, increases in the gas pressure, and decreases in the output orifice area of the hollow cathode. At Uo > 2.5 kV, an abnormal mode of the discharge establishing process is observed with intensive oscillations of the discharge current and voltage in the frequency range from 1 to 10 MHz. In the abnormal mode, the high current establishing time τ can be reduced by hundred times down to τ ∼ 1 μs. At a low gas pressure p < 0.1 Pa, the high current can be reached only in the abnormal mode. Without oscillations at the pulse front, only low-current high-voltage discharge is established.

Stabilization of high-current glow discharge by the active (ballast) resistor while the precision welding

Purpose.Thepurposeofthis work is to determine the value of the external active (ballast) resistance switched on the gas-discharge gap that provides stabilization of the burning process of a high-current glow discharge in regimes that are used in precision welding processes: discharge currents 1 ... 30 A, gas pressures of 1.33 ... 13.3 kPa.Approach.Using the methods of the theory of gas discharge physics, electrodynamics and electromagnetism, the main causes of the appearance of external perturbations and instabilities that lead to the emergence of a stable arc discharge on the local sections of the surfaces of the weldedpartsweredetermined.Findings.The result of the work is the developed method of analytical determination of the optimal value of the external stabilizing resistance, which ensures a discrepancy between the calculated data and the similar ones obtained during the experiments at the level of 15 ... 20%.Practically value.The results of the research are of interest for the enterprises of machine-building and instrument-making, power and electrical engineering industries in the manufacture of diffusion-welded assemblies of dissimilar materials in a gas-discharge plasma of a glow discharge of medium pressures

Time-resolved analysis and optical diagnostics of Trichel corona in atmospheric air

Trichel pulses of negative corona discharge in atmospheric air are investigated in a needle-to-plate configuration. Time-resolved images of the pulsed discharge are recorded by using an ICCD camera. Light emission spectra of N2 molecules and ions during the discharge are measured by optical diagnostics. The distributions of the positive ions and the reduced electric field along the discharge channel are analyzed accordingly. The results show that the positive ions that can accumulate in space and distort the electric field play an important role on the formation of the Trichel pulse. The Trichel pulse is a mode transition between low-current Townsend discharge and high-current glow discharge. The rising time of the pulse corresponds to the breakdown and formation of glow discharge, and the decay time is related to destroy the process of positive ion clouds and the quenching of glow discharge. The time interval between the pulses is determined by the re-accumulation of positive ions near the cathode in the low-current Townsend discharge to form the positive ion clouds as well as a strongly-distorted electric field.

Multifunctional bulk plasma source based on discharge with electron injection

A bulk plasma source, based on a high-current dc glow discharge with electron injection, is described. Electron injection and some special design features of the plasma arc emitter provide a plasma source with very long periods between maintenance down-times and a long overall lifetime. The source uses a sectioned sputter-electrode array with six individual sputter targets, each of which can be independently biased. This discharge assembly configuration provides multifunctional operation, including plasma generation from different gases (argon, nitrogen, oxygen, acetylene) and deposition of composite metal nitride and oxide coatings.

Processes and parameters of diamond films deposition in AC glow discharge

We report about a new PACVD reactor construction for diamond films deposition from the alternating high-current glow discharge plasma. Argon–hydrogen–methane gas mixture was used as the precursor gas. Argon percent in the gas mixture could reach 75%. The discharge was generated between two tungsten electrodes with the total area no more than 1cm2. The electrode temperature was about 2000°C. The discharge was in the form of plasma line and could reach 20cm lengthwise and more. The discharge voltage was varied from 90 to 700V depending on the argon partial pressure, the discharge current and the interelectrode spacing used. The current of discharge reached 30A. The maximum growth rate of optical grade films was about 2μm/h. OES spectrometry showed that hydrogen activation degree decreases from the center of plasma line to its edges. X-ray diffractometry and SEM showed the high quality of diamond films. The uniformity of the films in the transverse direction was measured.

A plasma generator based on nonself-sustained low-pressure glow discharge with a large-volume hollow cathode

The results of studying nonself-sustained glow discharges in an electrode system with a hollow cathode with a volume of 0.25 m3 are presented. A high-current (up to 35 A) nonself-sustained glow discharge at low pressures (0.3–1.0 Pa) is initiated and sustained with the help of an auxiliary cold-hollow-cathode arc discharge. When the current of a nonself-sustained glow discharge increases from 2 to 35 A, its burning voltage changes from 40 to 300 V. These values are much lower than the voltage for a self-sustained glow discharge in the same electrode system. At a discharge current of 30 A, the electron concentration at the center of the hollow cathode is ne ∼ 1010–1011 cm−3 and the electron temperature is Te ≈ 2 eV. The discharge considered can be used in the system for modification of materials and products.

Spatiotemporal Development of Low-Pressure Gas Discharges

We present an analysis of time and space resolved development of all typical regimes of low-pressure DC discharge in argon - low current Townsend discharge, oscillations and constrictions of discharge and high current glow discharges. Our work is based on time-resolved ICCD recordings of the discharge structure, synchronized with current and voltage measurements. Special care is given to radial effects and influence of dielectric walls during development of the glow discharge structure.

Evanescent wave fields at the plasma frequency in a microwave-generated hollow-cathode plasma

The different discharge regimes of the double-plate hollow-cathode are analysed with respect to the high plasmon levels, which are measured by the spectroscopic plasma-satellite method. The high-current glow discharge at start up creates a preplasma, which lets charging half-cycles start in an alternating order from both cathodes. They follow a cylindric boundary surrounding the central plasma cylinder, which exceeds the cut-off density, and thus constitute a self sustaining microwave oscillator. The ions from anodic space are radially attracted by the scattered Hertzian electrons and approach the central plasma, where they get neutralized upon penetrating the boundary, which is steadily displacing. Below the spiking threshold these ions of a high final radial velocity are responsible for the rapidly growing plasma core, whose expansion velocity has a strong cooling effect on the radial electron kinetic energy. Then a Lorentzian line shape indicating critical damping over a single oscillation is found for the near and the far satellites to the forbidden component of the helium I 447 nm line, corresponding to a field of up to 4 kV cm−1. The rotating wave approximation allows us to derive the exponential damping of the field envelope. The other solution of the rotating wave approximation is associated with the reflection of microwaves at the plasma boundary. Above the spiking threshold the production of wave fields around the plasma frequency in the central region is continued by the 4 ns current spikes. But the boundary conditions of high-current electron-neutral scattering require a spatially evanescent Green's function, therefore within both regions 2 mm adjacent to the cathode surfaces no plasma-satellites are then found.

Plasma display panel with micro-discharge array

A novel discharge device is designed on the basis of the configuration of micro-hollow cathode discharge (MHCD). By using many MHCDs in parallel connection, a micro-discharge array can be constructed. With the micro-discharge array, a high-pressure high-current density glow discharge plasmas can be formed to make a plasma display panel (PDP). An air discharge experiment is finished with the discharge device. The stable direct current glow discharge is formed under the pressure from 20 Torr to 500 Torr. The voltage-current characteristic curve and the discharge photograph are noted. The voltage-current characteristic curve has a positive differential resistance coefficient on the whole discharge range. The estimated current density reaches 70. 1A/cm3, the power density is 3.6 × 104 W/cm3, and the electron density is in the order of 1013 cm−3 atp=200 Torr andI D=10 mA. The experimental results indicate that the designed discharge device is appropriate for PDP.

DD reaction enhancement and X-ray generation in a high-current pulsed glow discharge in deuterium with titanium cathode at 0.8–2.45 kV

The DD reaction yield (3-MeV protons) and the soft X-ray emission from a titanium (Ti) cathode surface in a periodic pulsed glow discharge in deuterium were studied at a discharge voltage of 0.8–2.45 kV and a discharge current density of 300–600 mA/cm2. The electron screening potential Ue = 610 ± 150 eV was estimated in the range of deuteron energies 0.8 keV < Ed < 2.45 keV from an analysis of the DD reaction yield as a function of the accelerating voltage. The obtained data show evidence for a significant enhancement of the DD reaction yield in Ti in comparison to both theoretical estimates (based on the extrapolation of the known DD reaction cross section for Ed ≥ 5 keV to low deuteron energies in the Bosch-Halle approximation) and the results of experiments using accelerators at the deuteron energies Elab ≥ 2.5 keV and current densities 50–500 μ A/cm2. Intense emission of soft X-ray quanta (1013–1014 s−1 cm−2) was observed at an average energy of 1.2–1.5 keV. The X-ray emission intensity and the DD reaction yield enhancement strongly depend on the rate of deuterium diffusion in a thin subsurface layer of Ti cathode.

Dependence of Voltage-Current Characteristics of High-Current Nitrogen Glow Discharge on Gap Length and Gas Pressure

Voltage-current characteristics and plasma parameters of high-current nitrogen glow discharge for various gap lengths and gas pressures are described in this article. The transient glow discharge was produced with high-current larger than 100 A by static breakdown in nitrogen gas. Damping resistor was used to control the glow discharge current and was altered in order to obtain the voltage-current characteristics. The voltage-current characteristics of the normal glow phase and the abnormal glow phase were obtained in a wide current range from 50 mA to 100 A. The experimental results show that the current at phase transition of a normal to abnormal glow increases with increasing the gas pressure. The current at the phase transition is independent of the gap length. The electron density and temperature in positive column of the glow discharge were estimated using the voltage-current characteristics. The electron density and temperature increased with increasing the glow current and these values were 1010-1012 cm-3 and a few eV, respectively. The number densities of ions, metastable molecules and atomic radical particles in plasma were also estimated based on the each voltage-current characteristics. Metastable molecules density was 2-3 orders higher than electron density. Especially, the N2(A) metastable molecule density was 3 orders higher than the other metastable molecule densities.

Cathode fall parameters of a self-sustained normal glow discharge in atmospheric-pressure helium

Results from comprehensive studies of a high-current self-sustained glow discharge in atmospheric-pressure helium are presented. The main parameters of the cathode fall, namely, the electric field pro-file, cathode fall thickness, current density, gas temperature, and heat flux to the cathode are determined. The results obtained are discussed using one-dimensional models of the cathode fall with allowance for volumetric heat release.

Voltage–current characteristics of high-current glow discharges

The voltage–current characteristics of glow discharges in gas mixture (N2:O2=8:2) at a pressure of 10 Torr were obtained with the discharge current up to 150 A. Parallel-plane electrodes with a diameter of 10.7 cm and a discharge chamber with co-axial geometry were used to produce glow discharge with high current. The glow discharge voltage was almost constant until the whole surface of the cathode was covered with glow, i.e., until the discharge current became 3.7 A in our experimental condition (a normal glow discharge mode). The voltage, however, increased with the current when the glow covered over the cathode (an abnormal glow discharge mode). The electron density in positive column of the high-current glow discharge was obtained to be 3×1011 cm−3 from Langmuir probe measurements.

Characteristics of a high-current transient glow discharge in dryair

The transition of a high-current glow to an arc and the energydissipation during the glow phase have been investigated in dry air by staticbreakdown. The measurements have been made for uniform-field gaps up to 2 cmat gas pressures of 1-20 Torr. A low-inductance capacitor of 1.89 µF and a discharge circuit with a coaxial system were used to produce atransient glow discharge with a current in excess of 400 A. The experimentalresults showed that the high-current glow discharge occurred at a cathodecurrent density of 3.2 A cm-2, which is almost two orderslarger than the value deduced from the formula for Cu-air normal glow;jn = 240×10-6p2, where jn and p are the current density and gaspressure, respectively. The transition from the glow to an arc starts todevelop in the discharge region near the cathode. The energies dissipated inthe discharge or in the cathode fall region during the glow phase depend onthe gas pressure and/or the electrode separation. However, the dissipatedenergy density in the cathode fall region during the glow phase is independentof the gas pressure and the electrode separation, and is approximately0.035 J cm-3 in the present experiment.

High-current pulse sources of broad beams of gas and metal ions for surface treatment

This paper reviews the experimental study, development, and improvement of various types of processing ion sources undertaken in association with the joint program performed in recent years by the Institute of Electrophysics and the Institute of High-Current Electronics of the Russian Academy of Sciences. The beam parameters (type and energy of ions, current density, cross-sectional area of the beam, permissible content of impurities, etc.) should meet the requirements of particular ion beam treatment conditions, while the ion source itself should be simple and reliable in operation. Technical and service characteristics of the developed ion sources permit their use for ion-beam modification of materials, preparation of surfaces and ion-assisted deposition of thin films, and in some other applications. The sources under consideration employ high-current glow discharges with a hollow cathode or in crossed electric and magnetic fields, and low-pressure arc discharges and vacuum arc. Cold cathodes enhance reliability of the ion sources when they work at a high residual gas pressure or in the reactive gas media. The repetitive pulse mode of the plasma and beam generation provides optimum conditions for stable operation of the discharge, control of the average beam current over a wide range, and formation of homogeneous large-cross-section beams. The paper describes techniques used to realize high-current discharges at a reduced pressure, methods for producing a stable, dense and homogeneous plasma in a large volume, techniques of formation of large-cross-section homogeneous beams, and also findings on the mass-charge composition of the plasma and beams produced. Some design versions of the sources are given. At voltages from 10 to 100 kV, the pulse duration of 10 to 1000 μs, and the pulse repetition rate of 1 to 500 Hz these sources provide the current density of ∼1–10 mA/cm 2 in beams having the cross-sectional area of a few hundreds of square centimeters. The achieved parameters of the sources permit their use both in commercial and research applications. The use of these sources for modification of surface properties of materials has been exemplified.

Parameters of the discharge plasmas of surface-plasma sources of negative hydrogen ions

The main parameters of the plasma of high-current hydrogen-cesium glow discharges of surface-plasma (planotron and Penning) sources of negative hydrogen ions are determined using contact-free spectroscopic methods and compared for identical discharge current densities. The elemental and charge composition of the plasma is established. The temperature of the hydrogen atoms and the energy of the visible-range radiation of the plasma discharge are measured and estimates of the electron density in the plasma are made. The dynamics of the change in the parameters of the discharge plasma of a Penning source — the densities of hydrogen atoms, cesium atoms and ions, and molybdenum atoms — is tracked during a discharge pulse with spatial resolution along two coordinates. It is observed that cesium atoms and ions and molybdenum atoms are pent up near the cathode surface.