High-pressure runaway-electron-preionized diffuse discharges in a nonuniform electric field

High-pressure nanosecond diffuse (volume) discharges in a nonuniform electric field are studied experimentally using a recording system with a ?100-ps time resolution. As the voltage pulse shrinks to a width of ≈100 ps, the initiation of a diffuse discharge without a source of additional ionization is facilitated; specifically, a runaway-electron-preionized diffuse discharge is ignited in atmospheric-pressure air in the case of short interelectrode gaps. It is found that a major energy deposit into the plasma of this discharge is from an abnormal glow discharge following a maximum of the gap voltage.     

FXR discharges at various pressures

The formation period time, current rise time, and photon energy of FXR discharges have been investigated at initial pressures of 5×10^?6 to 1×10^?2Torr. It has been found that the parameters studied are almost constant between 5×10^?6 and 1×10^?3Torr. The results obtained indicate that the upper limit of the interelectrode breakdown pressure is about 10^?3 Torr when the initial pressure is 10^?6 Torr.     

Formation of the microscopic structure of high-voltage nanosecond diffuse discharges in sharply nonuniform geometry

The possibility of the development of ionization instability in the avalanche and plasma phases (of the long-wave and short-wave type, respectively) is demonstrated for high-voltage nanosecond discharges in sharply nonuniform geometry. Specific implementation is determined by the electric field distribution in the cathode region and by the emissivity of the cathode. The spatial structure of the discharge formed in both cases is self-similar.     

Microstructure of the current channels in a nanosecond spark discharge in atmospheric-pressure air in uniform and highly nonuniform electric fields

The microstructure of a nanosecond spark discharge in atmospheric-pressure air in uniform and highly nonuniform electric fields is investigated. It is found that an 0.1-to 0.4-mm spark channel consists of a large number (from 100 to 1000) of 5-to 10-μm-diameter microchannels distributed nearly uniformly over the channel cross section. The current amplitude in the spark is 1.5–3 kA, and the current density in a microchannel is 10⁷ A/cm². It is shown that the formation of the microstructure cannot be attributed to ionization-heating instability. The instability of the ionization wave front is suggested as a mechanism for the microstructure formation.     

Neutron emission during a nanosecond discharge in deuterium in a nonuniform electric field

Neutron emission during a nanosecond deuterium discharge formed at the nonuniform electric field is investigated. A stable neutron yield is observed when the cathode is made of metallic plates covered by a layer of deuterated zirconium and the anode is made of stainless steel in the form of a tube. It is shown that, when the deuterium pressure equals several Torr, neutrons are emitted from both deuterated and deuterium-free cathodes. The influence of the anode design on the neutron yield is studied.     

Structure of the glow of a nanosecond diffuse discharge in a strongly nonuniform electric field

The structure of the glow of a diffuse discharge in air under atmospheric pressure is studied in detail in the “rod (cathode)-plane” geometry for an electrode gap of 10 cm and a cathode tip radius from 3 cm to 2.4 μm. The amplitude of voltage across the gap was ～ 220 kV for voltage growth rate of ～10¹³ V/s and a pulse duration of 180 ns. It is found that the shape of the discharge glow strongly depends on the radius of the cathode tip. For large values of the radius, the multichannel form of the glow prevails, which is statistically transformed into a volume glow as the radius decreases from 5 mm to 60 μm. This transition is accompanied by a decrease in the amplitude of the discharge current from 440 to 140 A on the average. For ultrasmall radii of the cathode (2.4–7.7 μm), the multichannel form of the glow prevails again and the amplitude of the discharge current increases up to ～300 A.     

On the formation of nanosecond volume discharges,subnanosecond runaway electron beams,and x-ray radiation in gases at elevated pressure

Properties of runaway electron beams and x-ray radiation in a nanosecond volume discharge in air at atmospheric pressure are investigated, and results of recent studies in this direction are analyzed. A physical nature of forming runaway electron beams and x-ray radiation in the nanosecond volume discharge is discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 40–51, December, 2005.     

Transition of a diffuse discharge to a spark at nanosecond breakdown of high-pressure nitrogen and air in a nonuniform electric field

The transition of a runaway-electron-induced diffuse discharge initiated in a nonuniform electric field under a high pressure of air and nitrogen to a spark is studied. High-voltage pulses with a rise time of 0.5 ns are applied to a discharge gap with a tubular cathode having a small radius of curvature. It is shown that the leader of the spark discharge propagates toward the tubular cathode along preproduced tracks and may pass from one track to another. For a pulse rise time of about 0.5 ns and a gap length of 12 mm or more, it is found that spark leaders originating at the cathode (which has a small radius of curvature) do not reach the anode and accordingly, do not cause the spark breakdown of the gap. It is confirmed that the spark breakdown of the gap is associated with a spark leader that moves away from the plane electrode after the appearance of a bright spot on it.     

Constraints of two-colour TiRe-LII at elevated pressures

The main objective of this work is to investigate the influence of high-pressure conditions on the determination of primary particle size distributions of laser-heated soot particles using pyrometrically determined temperature decays. The method is based on time-resolved laser-induced incandescence measurements carried out at two different wavelengths (two-colour TiRe-LII). The LII signals are transferred into a particle ensemble averaged (effective) temperature using Planck’s thermal radiation formula. Assuming that all particles within the size distribution possess a unique temperature at the end of the laser pulse, the size distribution can be determined by numerically simulating the measured temperature decay. From our investigations, for pressures up to a few bars it is obvious that this strategy can be successfully applied if standard laser pulses of nano-second duration are used as an LII-excitation source. At higher pressures the time scales of heat conduction are decreased to such an extent that a unique temperature for all particles within the ensemble cannot be assumed at the end of the nano-second laser pulse. However, further investigations show that the presented two-colour TiRe-LII technique can be successfully adopted under technical high-pressure conditions as well, if the pulse duration of the TiRe-LII-excitation source is reduced into the pico-second range.     

Optical characteristics of the plasma of a nanosecond atmospheric-pressure volume discharge in a nonuniform electric field

Optical characteristics of the plasma of nanosecond volume discharges in air, nitrogen, krypton, argon, neon, and Ar/N₂ and Ar/Xe mixtures at elevated pressures are investigated. The discharges are excited in a gap with a cathode of small curvature radius. The waveforms and spectra of plasma emission from discharges in different gases in the 230-to 600-nm spectral range are measured. Optical generation in an Ar/Xe mixture is achieved at an active length of 1.5 cm. A comparison is performed of the spectral characteristics of the emission from nitrogen, krypton, argon, and neon excited by a volume discharge in a nonuniform electric field, by a nanosecond electron beam, and by a pulsed volume discharge in a uniform electric field at a high initial voltage.     

A model for dielectrics experiencing partial discharges under high electric fields

This work is in the research field of partial discharges to electrical insulating materials. A theoretical model is proposed that simulates the partial discharge's development within a metal (solid insulating material) metal structure. A nonlinear, time-dependent resistance using a new formula simulates the activity of partial discharges in dielectrics and the suggested equivalent circuit is analysed. Comparison of the experimental results and the outcome of the model demonstrates satisfactory consistency. Slight deviations consist of a minor phase shift between the computed and measured waveforms, as well as small oscillations of the waveform measured in the vicinity of the potential steps. These deviations are attributed to a very small parasitic inductance, which is always present in the measuring circuit.     

Formation of electrohydrodynamic flows in strongly nonuniform electric fields for two charge-formation modes

The main features of the formation of electrohydrodynamic (EHD) flows are analyzed for two basic regimes of electrization of weakly conducting liquids: injection from the electrode surface and dissociation in the bulk. Analysis is carried out on the basis of the results of computer simulation of EHD flows in a strongly nonuniform electric field in the needle-plane electrode system. This system creates favorable conditions for the injection as well as dissociation mechanisms of charge formation. Typical features are revealed for each model of charge formation. The current-time characteristics of the transient process of stabilization of EHD flows are calculated.     

Ultrasonic velocity of binary systems at elevated pressures

Various empirical theories of ultrasonic velocity have been applied to three binary liquid mixtures, under pressures up to 200 MPa and their validity have been tested. A pressure dependent study of ultrasonic velocities has been made at 303.15 K. The agreement between theory and experiment is found to be quite satisfactory.     

Initiation and development of nanosecond discharges in liquids

The initiation and development of discharges in purified water and hexane were studied for pulse rise times ofΤ _r ≈ 2?3 nsec and pulse lengths ofΤ _p≈10–50 nsec by an oscilloscopic method and with an electron-optical image converter. The results are compared with data on the nanosecond breakdown of gases. A physical interpretation is offered for the observed behavior.     

Pulsed discharge in nitrogen and argon under an elevated pressure in a nonuniform electric field

The characteristics of a discharge and radiation in nitrogen and argon under pressures of 10–760 Torr and the discharge formation without pre-ionization of the gap from an auxiliary source are considered. A peak is detected on the pressure dependence of the radiation power of the second positive system of nitrogen for E ₀/p ～ 270 V/cm Torr and nitrogen pressure p ～ 70 Torr. In the pressure range 10–760 Torr and for a voltage pulse leading front duration of ～ 10 ns, an electron beam is formed behind the grid anode with various half-amplitude pulse durations. It is shown that, under the given conditions, the electron beam is formed at the voltage pulse front both in the case of a discharge gap breakdown and in the absence of a clearly manifested breakdown, as well as for a 10-ns delay of breakdown at the leading front of a discharge current pulse.     

Energy concentration in electric discharges between particles of semiconducting sulfide minerals under the action of high-power nanosecond pulses

A model for development of electric discharges between particles of sulfide minerals (pyrite) under the action of high-voltage nanosecond pulses is proposed. It is shown that through discharges in a layer of pyrite particles lead to energy concentration in small contact regions between particles; the concentrated energy is sufficiently high for local decomposition (disintegration) of mineral complexes.     

Fluorescence spectroscopy of anisole at elevated temperatures and pressures

Laser-induced fluorescence of anisole as tracer of isooctane at an excitation wavelength of 266 nm was investigated for conditions relevant to rapid compression machine studies and for more general application of internal combustion engines regarding temperature, pressure, and ambient gas composition. An optically accessible high pressure and high temperature chamber was operated by using different ambient gases (Ar, N₂, CO₂, air, and gas mixtures). Fluorescence experiments were investigated at a large range of pressure and temperature (0.2–4 MPa and 473–823 K). Anisole fluorescence quantum yield decreases strongly with temperature for every considered ambient gas, due to efficient radiative mechanisms of intersystem crossing. Concerning the pressure effect, the fluorescence signal decreases with increasing pressure, because increasing the collisional rate leads to more important non-radiative collisional relaxation. The quenching effect is strongly efficient in oxygen, with a fluorescence evolution described by Stern–Volmer relation. The dependence of anisole fluorescence versus thermodynamic parameters suggests the use of this tracer for temperature imaging in specific conditions detailed in this paper. The calibration procedure for temperature measurements is established for the single-excitation wavelength and two-color detection technique.     

Laminar burning velocities of 2-methyltetrahydrofuran at elevated pressures

The laminar burning velocities (LBVs) and cellular instability of 2-methyltetrahydrofuran (2-MTHF) were investigated at the unburned temperature of 423 K and pressures from 1 to 10 atm in a cylindrical constant-volume vessel. The LBVs of 2-MTHF/air flame exhibit a notably dropping with increasing pressure. The cellular instability analysis indicates that the critical flame radius of 2-MTHF/air mixture monotonically increases with increasing pressure and the flame surface suffers more badly cellularity under higher pressures. The critical flame radius exhibits non-monotonic variation versus ? and the most unstable flames appear at ? ≈ 1.3. It is observed that the measured Markstein length of 2-MTHF/air mixture decreases with increasing ? and P_u, leading to an earlier formation of wrinkling and cracks with respect to preferential-diffusional instability. Further investigation found that by using a mixture of 14.2% oxygen with 85.8% helium in place of air as bath gas at 10 atm can effectively suppress the cellular instability. Two recently developed models were used to simulate the experimental results and explore the chemical kinetic effects on LBV. Reaction path analysis reveals that the most consumption of 2-MTHF/air at stoichiometric conditions is through the abstraction of H-atom to form radical C₅H₉O-5. While the competitiveness of decomposition by CC scission yielding CH₃ and tetrahydrofuran radical is relatively weak. Sensitivity analysis illustrates that small-species reactions show a controlling effect on LBV. The increasing pressure leads to an evident increase in the sensitivity coefficient of the recombination reaction H + O₂ (+M)=HO₂ (+M). The reduction of H atom concentration will cause competition to the initiation reaction H + O₂ = O+OH. This could lower the overall oxidation rate and reduce the burning velocity.