Peculiarities of grid control over the current in a discharge with a cathode spot on the cesium liquid-metal cathode

We report on the results of successful implementation of full grid control in a cesium discharge with a cathode spot. The discharge is quenched by a negative grid pulse for a discharge current density up to 75 A/cm² in the grid plane for voltage switching up to 100 V in the pressure range 0.5–1.5 Pa for a voltage drop of 5–6 V in the discharge. It is shown that quenching is the break-off type. The discovered effect of “evacuation” of the heavy component (ions) from the grid-anode gap to the cathode region leads to an unusually long (hundreds of microseconds) time of stabilization of the steady state in the discharge under the experimental conditions.     

Gas-filled electron diode based on a glow discharge

Stable ignition and sustention of a pulsed discharge with a current of up to 180 A and duration of 12 μs at a pressure of 10⁻¹–10⁻² Pa are achieved in a glow-discharge plasma cathode with the help of an auxiliary initiating discharge. An electron emission current density of up to 100 A/cm² and accelerating voltageof 15 kV are obtained in a gas-filled diode based on this type of a plasma cathode. An electron beam witha neutralized space charge can be transported almost without losses in a weak axial magnetic field alonga plasma channel formed due to the gas ionization by the accelerated electrons over a distance of up to 30 cm.     

Effect of cathode surface state on the characteristics of pulsed hollow-cathode glow discharges

A study is made of the effect of pulse repetition rate (0.1−10³ s⁻¹) and average discharge current (0–1 A) on the breakdown delay time and burning voltage of low-pressure glow discharges (p<0.1 Pa) in an electrode system of the reverse magnetron type with a large cathode surface area (≈10³ cm²). It is shown that increasing the repetition rate leads to a many-fold reduction in the statistical spread in the delay time and in the discharge formation time, while the average discharge current has a significant effect on the burning voltage. The mechanism for the observed phenomena is interpreted qualitatively in terms of the presence of thin dielectric films on the cathode surface. Zh. Tekh. Fiz. 69, 20–24 (May 1999)     

Expansion of spark channels in air and possibilities of plasma crowbar in high pulsed current and magnetic field technique

The radial expansion velocities and diameters of spark channels in air are measured by the plasma-metal contact method. An LC discharge circuit with energy up to 50 kJ and with current frequencies ranging from ∼10² to ∼10⁴ Hz served as the source of the channels. It is concluded on the basis of the results obtained that the previously proposed automatic plasma crowbar based on the expansion of spark channels in gases is a universal switch for an inductive load in LC discharge circuits that could find very wide applications in high pulsed current and magnetic field technology. Zh. Tekh. Fiz. 68, 133–136 (February 1998)     

Analysis of a hollow-cathode reflex discharge in a hydrocarbon flow

Variation of parameters of a reflex discharge with a hollow cathode operating continuously in propane with a flow rate of 1.3–5.6 (m³ mPa)/s and a discharge current of 0.1–0.4 A is analyzed. It is shown that for a hydrocarbon flow rate of 2.4 (m³ mPa)/s and higher, an increase in the discharge voltage takes place after a time interval depending on the discharge current and gas pressure; this is explained by the formation of coating of the dissociation products of hydrocarbon molecules on the electrodes of the discharge chamber. An increase in the thickness of the carbon coating of the cathodes with time and their charging with ions lead to electric breakdown of coatings and the formation of cathode spots. The oscillograms of the discharge current and voltage indicate a short-term transformation of the glow discharge into the arc discharge. The energy spectra of ions emerging from the discharge are measured, and the effect of the discharge current and the gas flow rate on the energy spread of ions is analyzed. The operation time of the discharge in hydrocarbon after which the cleaning of the discharge chamber is required is determined. The possibility of using an ion source based on the reflex discharge with a hollow cathode for technological purposes is established.     

Formation of narrow low-energy high-intensity electron beams

The process of formation of high-density low-energy (5–10 keV) pulsed electron beams of small diameter (on the order of a few millimeters) in a gun of the “channel-spark” type is studied. It is shown that beams with a rate of rise of the current exceeding 10¹¹ A/s and an amplitude exceeding the Alfvén current by a factor of 1.5–2.0 can be obtained in experiments with intense preionisation of the transport channel combined with a pulsed supply of the accelerating voltage to the cathode. In the optimal pressure mode, the current density at a distance of 2–3 cm from the gun outlet is 40–25 kA/cm², which will ensure ablation of most solid targets.     

High-power discharge with a plasma cathode in dense gases

An attempt is made to create an electric-discharge source for pumping argon, krypton, and xenon dimer lasers. The device is based on a method proposed previously by the authors, wherein confinement of the discharge is achieved by removing the cathode spot from the main discharge region and closing the discharge to the spot along a narrow extended auxiliary plasma channel. The conditions for the formation of such a discharge are investigated. The high stability of the sparkless stage of the discharge permits the first-ever attainment of energy depositions at the level of 100 J/cm² at pressures ∼10 atm, a level several orders of magnitude higher than is attainable by conventional methods. A discharge cell and power supply system are designed for a multisectional discharge with an active length of 200 mm, and the reliability of the entire apparatus is demonstrated in long-term use. Zh. Tekh. Fiz. 67, 10–14 (November 1997)     

Generation of high-energy electrons in a transverse slot-cathode nanosecond discharge at working gas medium pressures

Experimental data for the electrical and optical characteristics of a transverse slot-cathode nanosecond discharge are reported. The discharge is initiated in He at a discharge current of 1–500 A and a working gas pressure in a discharge chamber ranging from 10² to 10⁴ Pa. It is shown that the cathode current density is much (several orders of magnitude) higher than the total current density of an equivalent abnormal discharge. The electrical characteristics of an open discharge and a discharge confined by dielectric walls are found to differ considerably. Electrons passing through the cathode fall region acquire a high energy (on the order of 1 keV) under the given conditions. The fast electron relaxation conditions correlate with the initiation and evolution of the discharge. A pattern of the discharge evolution is derived from experimental data. A way of estimating the coefficient of electron emission from the cathode plasma is suggested.     

Plasma-emitter electron accelerators

This paper considers the physical processes associated with the extraction of electrons from the gas discharge plasma in plasma emitters where the emission boundary is stabilized with a fine grid. The ways of improving the uniformity of the emission current density distribution are discussed. Accelerators designed on the basis of plasma emitters are capable of producing pulsed beams of current 10–10³ A, current density 0.1–1.0 A/cm², pulse duration 10⁻⁷–10⁻³ s, and cross-sectional area up to 10⁴ cm². Institute of High-Current Electronics, Siberian Division of the Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 92–96, April, 2000.     

Mechanism for the influence of the geometric parameters on the electrical characteristics of Penning ion sources

A steady-state Penning ion source is studied experimentally. Depending on the geometric parameter l _a (the anode length to diameter ratio) and pressure, maxima are observed in the discharge current and in the ion beam current extracted from an aperture at the center of the cathode. It is shown that at pressures of the order of 10⁻⁴ Torr, two maxima appear in these currents: one, for short discharge gaps with l _a =1–1.5, corresponds to a diverging ion beam, and the other, for longer anodes with l _a =4–5, to a collimated ion beam. At pressures of the order of 10⁻⁵ Torr, only one maximum appears in these currents, for short anodes l _a =2–3 with a diverging ion beam. A physical explanation is proposed for these findings. Zh. Tekh. Fiz. 68, 29–32 (September 1998)     

Nanostructuring of surfaces of metalloceramic and ceramic materials by electron-beams

An electron-emitting source generating a low-energy beam measuring 1–3 cm in diameter, with current up to 300 A, pulse duration within 50–200 μs, and pulse repetition frequency up to 10 Hz is investigated in a gas-filled diode with a mesh plasma cathode at the accelerating voltage up to 25 kV. The beam is transported in a longitudinal pulsed magnetic field to a distance of up to 30 cm towards the region of its interaction with a solid. For the current densities up to 100 A/cm², it provides the power density as high as 10–100 J/cm² sufficient to melt surfaces of metals, alloys, and composite (metalloceramic) materials within one or a few pulses. This makes this beam useful for modification of material surfaces and articles made thereof. Using the methods of optical, scanning and diffraction electron microscopy, by building micro-and nanohardness profiles, and via identification of the treated surface roughness, the phase composition and the substructure state of the materials subjected to pulsed low-energy e-beam of sub-millimeter durations are investigated. Formation of submicro-and nanocrystalline multi-phase structure is observed, which ensures a multiple increase in physico-mechanical and tribological characteristics of the treated material. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 60–70, May, 2008.     

Effect of the gas dynamic structure of a flow on the parameters of a self-sustained discharge. I

A study is made of self-sustained glow discharges in transverse gas flows and jets. The distributions of the discharge current and voltage over the elements of a sectioned cathode array are measured along with the temperature of the cathode array. The limiting current and discharge voltage corresponding to the transition from a uniformly burning discharge to a contracted state are measured. Two-dimensional and one-dimensional systems of equations for the gas dynamics and vibrational kinetics are used for a numerical analysis of the experimental data, and the results are used to determine the character of the distribution of E/N in the discharge, where E is the electric field and N is the molecular density. The heat balance of the cathode array is calculated. A model is proposed for self-consistently calculating the parameters of the gas flow, the distribution of the current over the cathode array, and the discharge voltage, as well as the values of the ballast resistances. Zh. Tekh. Fiz. 69, 42–48 (November 1999)     

Non-self-sustained hollow-cathode glow discharge for large-aperture ion sources

A discharge system is proposed in which an auxiliary gas discharge is used to inject electrons into the cathode cavity of a hollow-cathode glow discharge. A study is made of the region of stable existence of a non-self-sustaining hollow-cathode discharge. It is shown that the injection of electrons permits a reduction to <10⁻² Pa in the minimum pressure at which a discharge can exist. It is shown experimentally that this discharge can be used to generate wide-aperture ion beams. Zh. Tekh. Fiz. 67, 27–31 (June 1997)     

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. Zh. Tekh. Fiz. 68, 32–38 (October 1998)     

Electrical and optical characterization of pulsed plasma of N<Subscript>2</Subscript>–H<Subscript>2</Subscript>

This paper considers the electrical and optical characterization of glow discharge pulsed plasma in N₂/H₂ gas mixtures at a pressures range between 0.5 and 4.0 Torr and discharge current between 0.2 and 0.6 A. Electron temperature and ion density measurements were performed employing a double Langmuir probe. They were found to increase rapidly as the H₂ percentage in the mixture was increased up to 20%. This increase slows down as the H₂ percentage in the gas mixture was increased above 20% at the same pressure. Emission spectroscopy was employed to observe emission from the pulsed plasma of a steady-state electric discharge. The discharge mainly emits within the range 280–500 nm. The emission consists of N₂ (C-X) 316, 336, 358 nm narrow peaks and a broad band with a maximum at λ_max = 427 nm. Also lines of N₂, N₂ ⁺ and NH excited states were observed. All lines and bands have their maximum intensity at the discharge current of 0.417 A. The intensities of the main bands and spectral lines are determined as functions of the total pressure and discharge current. Agreement with other theoretical and experimental groups was established.     

Effect of the gas dynamic structure of a flow on the parameters of a self-sustained discharge. II

The limiting current of a self-sustained glow discharge is calculated. Two-dimensional equations for the flow of a viscous, vibrationally nonequilibrium gas and a model of the cathode sheath are used. The validity of the approximations which form the basis of the cathode sheath model was tested with experimental data for anomalous and normal currents. The effects of laminar and turbulent gas flow and of the geometric dimensions of the channel on the limiting discharge current are examined. Zh. Tekh. Fiz. 69, 49–55 (November 1999)     

Luminescence of short-lived color centers induced in LiF crystals by a pulsed microwave discharge

A new phenomenon — intense luminescence of noncolored lithium fluoride (LiF) crystals excited by an electrodeless pulsed microwave discharge at the prebreakdown stage of development — is observed. This luminescence consists of the luminescence of short-lived aggregate F₂ and F ₃ ⁺ color centers at room temperature. It is shown that the density of short-lived color centers induced in the surface layer of LiF crystals by a microsecond microwave discharge reaches values of ∼10¹⁹−10²⁰ cm⁻³. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 3, 163–167 (10 August 1997)     

Plasma focus as a current switch for a capillary discharge

Experiments are described in which a plasma focus is used simultaneously as an inductive store and a current switch. The obtained rates of current growth on a load of 0.01 Ω is 10¹² A/s, and the maximum values of the switched current lie in the range 50–100 kA. The technique is seen as promising for employing a capillary discharge as a source of laser medium for the soft x-ray region. Zh. Tekh. Fiz. 68, 110–113 (November 1998)     

Generation of a pulsed high-current low-energy beam in a plasma electron source with a self-heated cathode

The transition of a low-current discharge with a self-heated hollow cathode to a high-current discharge is studied, and stability conditions for the latter in the pulsed–periodic mode with a current of 0.1–1.0 kA, pulse width of 0.1–1.0 ms, and a pulse repetition rate of 0.1–1.0 kHz are determined. The thermal conditions of the hollow cathode are analyzed, and the conclusion is drawn that the emission current high density is due to pulsed self-heating of the cathode’s surface layer. Conditions for stable emission from a plasma cathode with a grid acting as a plasma boundary using such a discharge are found at low accelerating voltage (100–200 eV) and a gas pressure of 0.1–0.4 Pa. The density of the ion current from a plasma generated by a pulsed beam with a current of 100 A is found to reach 0.1 A/cm². Probe diagnostics data for the emitting and beam plasmas in the electron source are presented, and a mechanism behind the instability of electron emission from the plasma is suggested on their basis.     

Initiation of a low-pressure glow discharge in a plasma electron source with a ribbon beam

Results are presented from an experimental investigation of a low-pressure glow discharge with a wedge-shaped hollow cathode in a plasma electron source, where this discharge is initiated by reflex and magnetron discharges. Zh. Tekh. Fiz. 69, 135–137 (July 1999)