Self-sustained Volume Discharge Research Articles

Numerical Study of the Dynamics of an Electron Avalanche in the Forming Cathode Sheath of a Self-Sustained Volume Discharge

Numerical Study of the Dynamics of an Electron Avalanche in the Forming Cathode Sheath of a Self-Sustained Volume Discharge

Investigation on pulsed discharge mode in SF6-C2H6 mixtures

The non-chain chemical HF(DF) laser is one of the most powerful electrically-driven lasers operating in mid-infrared, in which SF6-C2H6 mixtures are often used as lasering media. Due to the electronegativity of SF6, the discharge in SF6-C2H6 presents a complicated discharge mode. To achieve reproducible pulsed laser output, pulsed discharge in SF6-C2H6 mixtures is investigated for discharge mode using plane electrodes assisted by array pre-ionization spark pins in cathode surface. Firstly, two modes can be distinguished. One mode is called the self-sustained volume discharge (SSVD), which is characterized by spatial uniformity in the discharge gap and pulse to pulse repeatability. On the contrary, another mode includes random arc passages in the discharge gap and therefore cannot conduct lasering. By varying discharge conditions (gap voltage, gas pressure, etc) two discharge modes are observed. Secondly, the holding scope of the SSVD mode is analyzed for the optimal mixture ratio of 20:1, and the boundary tend of the holding scope of SSVD indicates there exists maximum gas pressure and maximum charging voltage for SSVD. Finally, the peak current of SSVD relates positively to charging voltage, while negatively to gas pressure, from which it is drawn that synchronous electron avalanches initiated by the sliding array overlap spatially into SSVD and thus SSVD is essentially an α ionization avalanche.

Численное исследование возможности генерации убегающих электронов в формирующемся катодном слое самостоятельного объемного разряда высокого давления

Calculations of the formation of the cathode layer of an self-sustained high-pressure volume discharge with pre-ionization of the gas medium excited by nano-and subnanosecond voltage pulses are carried out. It is shown that at pressures of ~ 1 atm at the final stage of the cathode layer formation, conditions for the generation of escaping electrons arise. We also considered the transition of electrons into runaway mode from the area of electric field amplification in front of the plasma (streamer) channel, which originates from the top of the micro-spike on the cathode. It is shown that at pressures of ~ 10 atm, electrons can transfer into runaway mode immediately after emission from the top of micro-spike in amplified electric field. Thus obtained runaway electrons can create pre-ionization of the gaseous medium and provide the formation of the initial phase of the discharge in volume form in the system without external pre-ionization.

Self-sustained volume discharge in mixtures of SF6 with hydrocarbons, hydrogen and deuterium for non-chain HF(DF) lasers

This paper reports on the comprehensive studies on a self-sustained volume discharge in SF6-based working media for electrochemical non-chain hydrogen fluoride (deuterium fluoride) lasers. The majority of the experimental results have first been obtained at various values of gas pressure for the discharge gaps with a large interelectrode distance, including plane electrode systems with a strong enhancement of the edge electric field. Some special features of the observed physical phenomena are explained using the developed adequate mathematical models. In particular, a radically new mechanism of the ionization instability in SF6-based mixtures at an intermediate pressure has been first considered and a relevant mathematical theory was developed. On this basis, the experimentally observable break-up of the discharge current into separate filaments in a laser-heated SF6-mixture was qualitatively explained. Some practical applications and recommendations have also been drawn from the performed studies.

Pulsed Gas Lasers Pumped by a Runaway Electron Initiated Discharge

The generation parameters are investigated in a runaway electron preionized diffuse discharge (REP DD). Laser generation is produced in different spectral bands from the IR to VUV range. New modes of the nitrogen laser operation are obtained. Ultimate efficiencies of N2- and nonchain HF(DF)-lasers are achieved. A possibility of increasing the pulse durations of XeF-, KrF-, ArF- and VUV F2- lasers (157 nm) in an oscillating REP DD is shown. The efficiencies of VUV- and UV-generation comparable with that of a laser pumped by a self-sustained volume discharge with preionization are gained.

Environmentally friendly HF (DF) lasers

Dedicated to the 100th anniversary of the birth of Academician A M Prokhorov, this paper reviews the physics of self-sustained volume discharge without preionization—self-initiated volume discharge (SIVD)—in the working mixtures of non-chain hydrofluoride HF (deuterofluoride (DF)) lasers. The dynamics of SIVD in discharge gaps with different geometries is thoroughly described. The mechanisms for the restriction of current density in a diffuse channel in electric discharges in SF6 and SF6 based mixtures (which determines whether SIVD is possible) are proposed and analyzed using simple models. The most probable mechanisms are the electron impact dissociation of SF6 and other mixture components, electron–ion recombination and electron attachment to vibrationally excited SF6 molecules. Starting from a comparative analysis of the rate coefficients of these processes, it is shown that electron–ion recombination is capable of compensating for electron detachment from negative ions via electron impact. It is also established that SIVD is not only observed in SF6, but also in other strongly electronegative gases. The factors that determine the uniformity of the active medium in non-chain HF (DF) lasers are analyzed. Some special features of non-chain HF (DF) lasers with different apertures operating are carefully examined. Consideration is given to the problem of increasing the aperture and discharge volume of non-chain HF (DF) lasers. Based on our experimental results, the possibility of increasing the energy of such lasers to ~1 kJ and above is shown.

Non-chain pulsed DF laser with an average power of the order of 100 W

The design and performance of a closed-cycle repetitively pulsed DF laser are described. The Fitch circuit and thyratron switch are introduced to realize self-sustained volume discharge in SF6–D2 mixtures. The influences of gas parameters and charging voltage on output characteristics of non-chain pulsed DF laser are experimentally investigated. In order to improve the laser power stability over a long period of working time, zeolites with different apertures are used to scrub out the de-excitation particles produced in electric discharge. An average output power of the order of 100 W was obtained at an operating repetition rate of 50 Hz, with amplitude difference in laser pulses <8 %. And under the action of micropore alkaline zeolites, the average power fell by 20 % after the laser continuing working 100 s at repetition frequency of 50 Hz.

Discharge processes of UV pre-ionized electric-discharge pulsed DF laser

The discharge processes of ultraviolet (UV) pre-ionized electric-discharge pulsed DF laser operating with a SF6–D2 gas mixture are studied. A mathematical model based on continuity equation of electrons and Kirchhoff equations for discharge circuit is established to describe the discharge processes. Voltage and current waveforms of main discharge and voltage waveforms of pre-ionization are solved numerically utilizing the model. The calculations correctly display some physical processes, such as the delay time between pre-ionization and main discharge, breakdown of the main electrode and self-sustained volume discharge (SSVD). The results of theory are consistent with the experiments, which are performed in our non-chain pulsed DF laser. Then the delay inductance and peak capacitance are researched to analyze their influences on discharge processes, and the circuit parameters of DF laser are given which is useful to improve the discharge stability.

Investigation of a self-sustained volume discharge in c-C4F8

This paper reports the first experimental study of a self-sustained volume discharge (SSVD) in c-C4F8. The discharge voltage and current oscillograms are taken over a wide range of gas pressures. For the first time an SSVD in c-C4F8 preheated by CO2-laser radiation has been investigated. Some special features and temperature-dependent characteristics of this discharge are revealed. There is discussion on the peculiarities of an SSVD in a preirradiated c-C4F8. To refine the static limiting field in c-C4F8 the static dielectric strength of c-C4F8 is measured on changing the gas pressure by nearly two orders of magnitude.

Pulse volume discharges in high pressure gases

New approach for suppression of plasma inhomogeneities and instabilities in the volume self-sustained discharge is offered. The physical model is offered and conditions of obtaining extremely homogeneous self-sustained discharge are defined (with full suppression of plasma inhomogeneity and instability). Results of calculations agree with experiments.

Volume Self–Sustained Discharge in Atmospheric Pressure Gas with High Pulse Repetition Frequency

The paper describes a plasma source based on a self-sustained volume discharge for diamond and diamond-like film deposition. The suggested scheme of the electrode system enables to provide stable combustion of the self-sustained volume discharge with high pulse repetition rate in atmospheric pressure gases. Gas flow rate to blow off the interelectrode gap depending on the anode temperature has been considered.

Possibility of Using Nuclear Track Membrane for Ophthalmology

This work is devoted to study the possibility of using a nuclear track membrane for ophthalmology in epithelial-endothelial corneal dystrophy treatment. Experiments were conducted using the track membrane “TOMTREK” based on polyethylenterephthalate (PET) with pores diameters (0.2-0.8)micron and (5x106-2x109) pores/cm2 density. The pores were formed by irradiating the polymer PET 40Ar+8 ions with energy 41.5 MeV. After irradiation, the membrane was chemically treated in the alkaline solution. Imparting hydrorhilicity membrane was the next step. Contact angle of the wettability the surface track membrane was measured just after the chemical treatment in NaOH and after 10 hours in air. The surface had been processed by the plasma self-sustained volume discharge to give the surface the hydrorhilic properties of the membrane. The maximum voltage on plasma discharge was 20-22 kV. The energy density in the discharge was 6·10-4 J/cm2 per pulse. Pulse repetition rate was 103 s-1. Samples of 6x6 cm were attached in a special rotating device. Thus, the effects were both surfaces of the membrane. After surface treatment, TM samples were placed in plastic bags. After a night contact angle of wetting was measured with these samples. Disks treating by the plasma were cut. The disk’s diameter was 6-8 cm. The edges of the disks were incised and processed with hot temperature in order to obtain the model of an appropriate shape. It was a shape of a truncated sphere with a certain radius of curvature. Then, the membrane was sterilized and implanted into the anterior chamber of the pig’s eye. Thus, we tested the methods of barrier keratoplasty using the nuclear track membrane.

Diffuse volume-discharges without pre-ionization formed in SF6 and C2H6 mixtures

It has been shown that a volume discharge is forming in non-uniform electric-field rough cathode and even anode without pre-ionization in SF6 and C2H6 mixtures. The discharge is presented with many diffuse channels attached to bright circular cathode spots that diverge towards the anode, with the channels overlapping, form a spatially uniform glow discharge. Self-Initiated Volume Discharge (SIVD) has been performed at a total mixture pressure up to 8 kPa and energy deposition up to 200 J/L. The experimental results indicate that Self-Sustained Volume Discharge (SSVD) in SF6 and C2H6 mixtures develops in the form of SIVD, which is promising for creation of high energy and pulse-periodic HF laser.

Influence of gas temperature on self-sustained volume discharge characteristics in working mixtures of a repetitively pulsed COIL

The influence of gas temperature on the characteristics of a self-sustained volume discharge was studied in the working mixtures of a chemical oxygen – iodine laser with pulsed electric-discharge production of iodine atoms. In experiments, laser working mixtures were modelled by the mixture of air and iodide . It was established that mixture heating is accompanied by an increase in the voltage across the discharge plasma and by a decrease in the discharge current. By varying the temperature of the mixture with the iodine content of and initial pressure Torr from to , the current amplitude falls by , and at the instant corresponding to a maximal current the voltage raises by . Such a change in the discharge characteristics is explained by a higher rate of electron attachment to vibrationally excited iodide molecules at elevated temperatures.

On the current density limiting effect in SF6-based mixtures

This paper reports on the investigation of the current density limiting effect in a self-sustained volume discharge in SF6-based mixtures due to a self-action of the discharge current on the media it passes through. The mechanism of this phenomenon in terms of electron-impact dissociation is examined. The argument is advanced that in the planar geometry the limiting effect treated may provoke triggering a spatially uniform volume discharge in the media stated even in the absence of pre-ionization. It is shown that the dissociation mathematics developed by the authors allows the quantitative estimation of the averaged expenditures of energy per formation of a dissociation fragment. Specifically, when applied to pure SF6 this finding enables the assessment of the energy spent for F-atom formation. For the first time the influence of current pulse duration on the discharge voltage has been experimentally revealed. Both thorough qualitative and quantitative considerations including relevant numerical simulation have been performed.

Generation of plasma inhomogeneities and their total suppression in a volume self-sustained discharge

The principle types of inhomogeneities formed in a volume self-sustained discharge are analyzed. Possible mechanisms and conditions for suppression of their development are considered. An ultimately homogeneous volume self-sustained discharge in CO2: N2: He mixtures where local plasma inhomogeneities are absent is obtained. At the energy contribution of 170 J/l, the duration of steady discharge burning in the CO2: N2: He = 1: 2: 3 mixtures at atmospheric pressure is 10 μs.

On stability of self-sustained volume discharge in working mixtures of non-chain electrochemical HF laser

Burning voltage of a self-sustained volume discharge (SSVD) is studied as a function of the specific energy deposition in SF5 with C2—H6 and H2 mixtures, which are working media of a non-chain electrochemical HF laser. It is established that the voltage rises linearly with increasing the specific energy deposition, the relative voltage rise in the SF6—C2H6 mixtures being noticeably higher than in pure SF6 and SF6—H2 mixtures. An assumption is suggested and substantiated on determining the role of molecule dissociation by the electron impact leading to the observed voltage rise. From experimental data we have found approximate energy expenditures of producing dissociation fragments including atomic fluorine in a discharge in pure SF6: Ẽd(F)= 5±1 eV. The values of Ed well agree with literature data obtained by other experimental methods. A conclusion is drawn that the dissociation process is the main mechanism limiting the current density, which implies SSVD realisation without preliminary gas ionisation in working mixtures of a non-chain HF laser and determines a higher stability of the volume discharge in mixtures of SF6 with hydrocarbons (deuterocarbons) as compared to mixtures with hydrogen (deuterium). A method is suggested and substantiated for numerical estimation of the limitation effect of the current density and its influence on the SSVD stability.

Detachment instability of self-sustained volume discharge in active media of non-chain HF(DF) lasers

The development of detachment instability in active media of electric-discharge non-chain HF(DF) lasers due to the electron-impact detachment of electrons from negative ions is considered. This instability is initiated in large volumes of SF6-based gas mixtures, spatially separated from electrodes and heated by a pulsed CO2 laser. The self-organisation of self-sustained volume discharge upon laser heating, which results in the formation of quasi-periodic plasma structures within the discharge gap, is experimentally investigated. The evolution of these structures, depending on the gas temperature and specific deposition of electric energy, is analysed. The possible relationship between the self-organisation and detachment instability is discussed. A mechanism of development of single plasma channels in the working media of HF(DF) lasers, based on electron-impact destruction of negative ions is proposed.

Electrode system for electric-discharge generation of atomic iodine in a repetitively pulsed oxygen — iodine laser with a large active volume

Possibilities for increasing the active medium volume of a chemical oxygen — iodine laser (CCOIL) with a pulsed electric-discharge generation of atomic iodine are studied. The reasons are analysed of the low stability of the transverse self-sustained volume discharge in electrode systems with metal cathodes under the conditions of the electric energy input into gas-discharge plasma that are typical for CCOILs: low pressure of mixtures containing a strongly electronegative component, low voltage of discharge burning, low specific energy depositions, and long duration of the current pulse. An efficient electrode system is elaborated with the cathode based on an anisotropically-resistive material, which resulted in a stable discharge in the mixtures of iodide (CH3I, n-C3H7I, C2H5I) with oxygen and nitrogen at the specific energy depositions of ∼5 J L-1, pressures of 10 — 25 Torr, and mixture volume of 2.5 L.

Electron detachment instability and self-organization in strongly electronegative polyatomic gases

This paper reports on an investigation of the ionization instability and related phenomena in a self-sustained volume discharge (SSVD) in SF6-based mixtures pre-irradiated by a pulsed CO2 laser. The illumination geometry with a diaphragm slit arranged normal to the central electric field line of the discharge gap is considered. The delay times between the laser and voltage applications were varied from a few to several tens of microseconds. The formation and some special features of quasi-periodic filamentary structures in laser-heated SF6-based mixtures at different delays are studied. It is hypothesized that the self-organization observed is caused by the ionization instability developed in the SSVD plasma due to an imbalance between the rates of electron detachment by electron impact and electron–ion recombination. The mathematics of this instability in a zero-dimensional approximation and the characteristic time of its development are given. An imbalance-caused mechanism of conducting channel propagation in SF6-based mixtures at room temperature under the conditions approaching those of the HF laser performance is suggested. It starts from an appreciable decrease in electron–ion recombination rate in an enhanced electric field in the vicinity of the channel tip. The effect of a two-step ionization is assessed.