Hollow cathode glow discharge in long tubes

An analysis is made of some burning characteristics of a hollow-cathode glow discharge with a long tube (L≫D) used as the cathode. It is shown that, as in the case L∼D, the main factor imposing a lower limit on the range of operating voltages is the drift of fast electrons through the aperture in the cavity. Assuming that the electrons move along the cavity as a result of diffusion, it was possible to calculate the critical pressure at which the discharge can no longer burn and to determine the optimum ratio L/D for which the discharge can be sustained at the lowest voltage. The calculations showed satisfactory agreement with the experiment. Zh. Tekh. Fiz. 69, 36–39 (June 1999)     

Characteristic features of the formation of the vibrational distribution function of H2 molecules in a hydrogen stream

A theoretical analysis is made of the flow of vibrationally excited hydrogen in a channel. It is shown that coverage of the channel walls with adsorbed hydrogen atoms can substantially increase the concentration of vibrationally excited molecules in the stream. The possibility of applying these results to bulk sources of negative H⁻ hydrogen ions is discussed. It is shown that the rate of H⁻ ion generation in the source may be enhanced appreciably under conditions where this generation is achieved by dissociative attachment of thermal electrons to H₂ molecules injected into the discharge chamber, whose vibrational distribution function has been pre-enriched in excited molecules by suitably organizing the hydrogen flow in the channel. Zh. Tekh. Fiz. 69, 15–21 (June 1999)     

Effect of helium/argon gas ratio in a He-Ar-Cu<Superscript>+</Superscript> IR hollow-cathode discharge laser: modeling study and comparison with experiments

The He-Ar-Cu⁺ IR laser operates in a hollow-cathode discharge, typically in a mixture of helium with a few-% Ar. The population inversion of the Cu⁺ ion levels, responsible for laser action, is attributed to asymmetric charge transfer between He⁺ ions and sputtered Cu atoms. The Ar gas is added to promote sputtering of the Cu cathode. In this paper, a hybrid modeling network consisting of several different models for the various plasma species present in a He-Ar-Cu hollow-cathode discharge is applied to investigate the effect of Ar concentration in the gas mixture on the discharge behavior, and to find the optimum He/Ar gas ratio for laser operation. It is found that the densities of electrons, Ar⁺ ions, Ar_m ^* metastable atoms, sputtered Cu atoms and Cu⁺ ions increase upon the addition of more Ar gas, whereas the densities of He⁺ ions, He₂ ⁺ ions and He_m ^* metastable atoms drop considerably. The product of the calculated Cu atom and He⁺ ion densities, which determines the production rate of the upper laser levels, and hence probably also the laser output power, is found to reach a maximum around 1–5 % Ar addition. This calculation result is compared to experimental measurements, and reasonable agreement has been reached. Received: 14 October 2002 / Revised version: 28 November 2002 / Published online: 19 March 2003 RID="*" ID="*"Corresponding author. Fax: +32-3/820-23-76, E-mail: annemie.bogaerts@ua.ac.be     

Conditions for the existence of a positively charged structure in a Penning discharge

An analytical model is developed for the high-current form of a low-pressure glow discharge in a magnetic field. Expressions are derived for the critical magnetic induction and critical pressure, below which it becomes impossible for this form of discharge to exist. It is shown that the transition from the high-voltage form to the high-current form of discharge with increasing pressure is not attributable to an increase in the ionization rate, but to an increase in the drift velocity of plasma electrons across the magnetic field. Estimates based on the expressions derived in the article agree in order of magnitude with the experimental data. It is shown that the region in which discharge exists can change considerably in the presence of electron emission. Zh. Tekh. Fiz. 68, 56–63 (July 1998)     

Study of the limiting energy characteristics of a combined discharge in a gas flow

A combined method of excitation of a gaseous medium is used in an investigation of the energy characteristics of a non-self-sustaining discharge as functions of the capacitance of the capacitor bank, the number of pulses per burst, and the burst repetition rate for different time intervals between the pulses in a burst and different flow velocities of the gas. It is shown for infrared lasers that under optimal pumping conditions the average discharge power can reach 8.5 W/cm³ for a pulse power of 25 W/cm³. Zh. Tekh. Fiz. 68, 33–38 (May 1998)     

Wide-aperture high-current electron beam in a discharge with cathode plasma at elevated pressure

This work pursues investigations into the discharge with a cathode plasma in a cavity one wall of which is an insulating plate with a hole D in diameter (the cavity is 0.5 or 1.5 mm wide). This discharge is thoroughly analyzed in comparison with the high-voltage hollow-cathode discharge. Owing to the reduced emission of electrons from the cathode plasma, the discharge becomes more stable against transition to the low-voltage form, as a result of which an electron beam can be generated under higher gas pressures. Such a beam formed at the entrance to the cavity is used as an auxiliary one that propagates over the remaining (flat) surface of the cathode and adds to the gas ionization. Accordingly, the beam current from the main discharge to the anode rises (high-current regime). Wide-aperture (D = 22 mm) ≈1-μs-long pulsed beams with a current an order of magnitude higher than the total current of the equivalent anomalous discharge are obtained. Experiments are carried out at a helium pressure to 20 Torr and a voltage from 1 to 20 kV.     

Plasma parameters of a nonself-sustained microwave discharge created by a programmed pulse

The kinetics of the electrons in the plasma of a nonself-sustained discharge formed at the focus of a microwave beam when the gas is exposed to electromagnetic radiation with an energy flux density that varies with time in a programmed-pulse mode are investigated. It is shown that the temperature of the electrons in the plasma of the localized microwave discharge is of the order of 1 eV and varies weakly during the pump pulse and as the air pressure is varied, while the electron density is an order of magnitude or more lower than the critical density and depends on the level of the pump generator power. It is shown that the degree of ionization of the plasma can be regulated by altering the programmed-pulse mode. Zh. Tekh. Fiz. 67, 19–23 (July 1997)     

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)     

Dynamics of an electron beam from a high-current picosecond accelerator

The dynamics of a high-current (10²–10⁴ A) electron beam with energies of 10⁵–10⁶ eV and picosecond duration (10⁻¹⁰ s) at the output of the accelerator tube is investigated. The slowing of electrons by the residual positive charge on the surface of the tube is found to have a significant influence in the case of short pulse durations. The distance of the electron beam from the surface of the tube in vacuum is estimated on the basis of a one-dimensional model. It is shown that the electron radiation can travel to a distance of several centimeters from the surface at current densities below 20 A/mm², whereas at high current densities the beam is trapped near the surface. Zh. Tekh. Fiz. 69, 111–115 (May 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)     

Build-up of F and M color centers in KCl single crystals under combined electron and proton irradiation

The F and M color-center build-up kinetics in KCl crystals under combined irradiation with electrons of energy 15 and 100 keV and 100-keV protons have been studied in the flux range of 10¹³–10¹⁵ cm⁻² and at a flux density of 3×10¹¹ cm⁻² s⁻¹. It is shown that consecutive irradiation with electrons and protons produces results not obtainable under electron or proton irradiation alone. Fiz. Tverd. Tela (St. Petersburg) 40, 2015–2018 (November 1998)     

Beam-plasma discharge in the propagation of a long-pulse relativistic electron beam in a medium-pressure rarefied gas

A theoretical model is given, along with a numerical analysis of the evolution of beam-plasma discharge in the propagation of a long-pulse relativistic electron beam in a rarefied gas at medium pressure. It is shown that the self-stabilization of beam-plasma discharge as a result of longitudinal inhomogeneity of the density of the discharge plasma makes it possible for the beam to traverse the beam chamber with relatively low total energy losses, including ionization losses and energy losses in the generation of oscillations. During the dissociative recombination of electrons and ions of the discharge-driven plasma, heat is released and spent in raising the temperature of the gas. The investigated collective-discharge mechanism underlying heating of the gas for a relativistic beam can be more efficient than the classical heating mechanism due to ionization losses of the beam in pair collisions of its electrons with gas particles. Zh. Tekh. Fiz. 67, 94–98 (May 1997)     

Negative space charge and electric field intensity in polymeric insulators at low temperatures

The effect of negative space charge accumulation due to injection of electrons from cathode microprotrusions on the steady-state and transient electric field distributions in polymer dielectrics is discussed. An isolated microprotrusion is modeled by a spherical capacitor in which an electrode of smaller radius is the cathode. The calculations include the fact that the distribution of negative space charge depends on the rate of capture and liberation of electrons by traps, while the activation energy of this process depends on the electric field intensity. An exponential energy distribution is proposed for the traps. It is shown that significant electrical overvoltages can only appear near the cathode microtips immediately after switching on the voltage. In the course of 10⁻⁶–10⁻⁵ s, the coefficient of electrical overvoltage drops to a few units and approaches its steady-state value. The region of significant electrical overvoltage is localized, and is the same order as the dimensions of the microtip. Fiz. Tverd. Tela (St. Petersburg) 40, 1167–1172 (June 1998)     

Emission spectroscopy of the A<Superscript>1</Superscript>Π–X<Superscript>1</Superscript>Σ<Superscript>+</Superscript> system of AlH

The high-resolution emission spectrum of the A¹ Π–X¹Σ⁺ transition of AlH was observed in the 18 000–25 000 cm^-1 spectral region using a conventional spectroscopic technique. The AlH molecules were excited in an Al hollow-cathode lamp filled with a mixture of Ne carried gas and a trace amount of NH₃. The emission from the discharge was observed with a plane grating spectrograph and recorded by a photomultiplier tube. In total 163 transition wave numbers belonging to six bands (0-0,1 and 1-0,1,2,3) were precisely measured and rotationally analysed. In the final fit the present data have been combined with available high-resolution measurements of the vibration-rotation bands by White et al. [J. Chem. Phys. 99, 8371 (1993)]. This procedure enabled extracting molecular constants for the A¹ Π and X¹ Σ⁺ states of AlH. A very slight local perturbation has been discovered in the v=1 vibration level of the A¹ Π state at J=5. This was probably caused by the interaction with the a³Π state.     

Electron kinetics in a discharge plasma produced by a focused microwave beam in free space

A study is made of the electron kinetics in a discharge plasma produced by a high-power beam of electromagnetic radiation in the centimeter-wave region under conditions approaching free space, when the dimensions of the chamber are much greater than the wavelength of the microwave radiation. Two regimes of discharge production are investigated: the regime of short microsecond pulses at a repetition rate of 200 Hz, and a single millisecond pulse regime. It is shown that at threshold values of the microwave energy flux density the electron density in the initial stages of discharge formation reaches the critical value, and that the average energy of the electrons is of the order of 1.5–3 eV. Zh. Tekh. Fiz. 67, 10–14 (June 1997)     

Excitation of nuclei in a hot, dense plasma: Feasibility of experiments with 201Hg

It is shown that in a plasma produced on the surface of a sample consisting of a natural mixture of mercury isotopes, ∼10⁴−10^{5 201}Hg nuclei can be excited into the low-lying isomeric level 1/2⁻ (1.561 keV) by an ultrashort laser pulse with energy ≈1 J, duration ≈200 fs, and intensity ≈10¹⁶ W/cm² and the lifetime of the level can be determined. Possible mechanisms leading to the excitation of ²⁰¹Hg nuclei by photons and electrons in a dense, hot plasma are examined and the cross sections of the processes are estimated. Schemes for detecting the effect are proposed. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 5, 312–316 (10 September 1997)     

Propagation of axions in a strongly magnetized medium

The polarization operator of an axion in a degenerate gas of electrons occupying the ground-state Landau level in superstrong magnetic fields H≫H ₀=m _e ² c ³/eℏ=4.41×10¹³ G is investigated in a model with a tree-level axion-electron coupling. It is shown that a dynamic axion mass, which can fall within the allowed range of values 10⁻⁵ eV≲m _a≲10² eV, is generated under the conditions of strongly magnetized neutron stars. As a result, the dispersion relation for axions is appreciably different from that in a vacuum. Zh. éksp. Teor. Fiz. 115, 3–11 (January 1999)     

Scanning of a laser beam and purification of materials through the use of light-induced particle drift in semiconductors

The light-induced drift of electrons, light-absorbing impurities, and defects in II-VI semiconductors is investigated experimentally, along with some potential practical applications of the phenomenon. It is shown that the light-induced drift of electrons induces a very pronounced change in the refractive index, |Δn|∼0.01, and can be used to implement effective scanning of nanosecond and picosecond laser pulses through frustration of total internal reflection. The light-induced drift of absorbing particles increases their density in the surface layer of the crystals, and this effect can be exploited in semiconductor technology. Zh. Tekh. Fiz. 68, 121–124 (April 1998)     

Penetration of a low-pressure reflex-discharge plasma into a hollow electrode

It is shown experimentally that the plasma of a hollow-cathode reflex discharge is characterized by a nonequilibrium electron velocity distribution. The parameters of the electron distribution, which is approximated by a superposition of two Maxwellian distributions with different temperatures, are estimated. The penetration of the discharge plasma into the hollow cathode at various cathode potentials and a gas pressure of ∼10^{\t− 2} Pa is studied. It is shown that the plasma parameters in the hollow electrode depend not only on the parameters of the reflex-discharge plasma, but also on the magnitude and configuration of the magnetic and electric fields in the plasma expansion region. It is shown that the plasma penetration can be accompanied by quasineutrality violation and the formation of space-charge double layers. Experiments confirm that the ion current from the nonequilibrium plasma exceeds the Bohm current.     

Water-vapor-based source of UV radiation

The results of studying a capacitive discharge in water vapor are reported. This discharge is an effective and environmentally friendly source of UV radiation due to hydroxyl OH (A ²Σ⁺ → X ²Π transition) in the wavelength range 280–330 nm. It is shown that, at E/N < 210 Td, the discharge is greatly influenced by dissociative attachment of electrons, while at E/N > 210 Td, ionization dominates over dissociation. The formation rate of excited OH* radicals (A ²Σ⁺) in the water-vapor discharge is much higher than the formation rate of emitting states in hydrogen and oxygen atoms. As E/N grows, the ratio between the intensities of OH lines and atomic hydrogen Balmer series decreases. According to estimates, E/N values optimal for the OH radical line excitation fall into the range 250–400 Td.