Влияние предыонизации на микроканальную структуру искрового разряда в воздухе в промежутке острие-плоскость

The initial phase of a spark discharge in the gap between the pin (cathode) and a plane 1.5 mm long in atmospheric pressure air under conditions of preliminary photoionization by an auxiliary discharge was investigated by the method of shadow photography. In the absence of preionization, the discharge from the first nanoseconds after breakdown is an aggregate of a large number of micron-diameter channels. It was found that the electron concentration resulting from preionization, estimated at 108  109 cm-3, increases the degree of uniformity of the discharge channel in the near-cathode region; however, in the near-anode region, the channel remains microstructured. Within the framework of the mechanism of microstructure formation due to the instability of the ionization wave front, a criterion for the formation of a uniform discharge is obtained and an explanation of the results obtained is presented.

Формирование микроканальной структуры искрового разряда в воздухе в промежутке острие-плоскость"

The formation of the microchannel structure of a spark discharge in the gap spike (cathode)-plane of 1.5 mm length in the air of atmospheric pressure was studied by the method of shadow photography. The images with the concurrent descent of microchannel diameter sand the number of microchannel increase in the cathode region on the time interval of 5 ns were recorded. The obtained data are explained in the frames of the microstructure formation mechanism at the expense of the front instability of the ionization wave. The parameters characterizing the process of the microstructure formation are estimated, and they agree with the experimental data.

Начальные стадии формирования импульсного разряда в промежутке с геометрией острие-плоскость в предварительно ионизированном аргоне

The study of the effect of the initial conditions on the features of the formation and development of the anodic ionization wave between two electrodes with a tip – plane gap geometry in argon at atmospheric pressure is performed on the basis of a two-dimensional axisymmetric drift-diffusion model.

Active species evolution in presence of different targets impacted by helium plasma jet at atmospheric pressure

A low-temperature plasma jet is generated by a dielectric barrier discharge poweredby a pulsed high voltage in helium flow (3 L/min) at atmospheric pressure inpresence of different targets (glass slide or ultra-pure water or a grounded metal plate)positioned perpendicular to the plasma propagation axis. Experimental electricalcharacterizations as discharge current, voltage and powerand optical ones as intensifiedcamera ICCD, Schlieren imaging and emission spectroscopy to follow specific excitedspecies have been achieved. The transition from laminar to turbulent regime wereobserved during the discharge ignition with a larger spreading of the plasma on thesurface target with lower dielectric permittivity and the generation of two dischargesduring each voltage pulse is highlighted during the propagation of the ionization wavethat has shown a variable speed along the plasma axis not depending on the target kind.The evolution of some active species (as OH, O and excited nitrogen and helium) areinvestigated using time resolved mapping of the emissions of radiative excited speciespropagating in ambient air between the plasma jet output andthe target. For a lowrelative permittivity target (glass), the volume ionization wave at its arrival on thetarget spreads on its surface thus behaving as a surface ionization wave. For thehighest relative permittivity (metal), a conductive channel appears between the targetsurface and the plasma jet during the first discharge, followed by a diffuse plasma plumefrom the target surface towards the plasma jet after the impact of the ionization waveon the target. A hybrid behavior is highlighted for the ultra-pure water which leadsto a short spreading of the ionization wave on the target surface, the formation of aconductive channel in ambient air between tube output and target and the formationof a plasma plume on the target surface.

Influence of different O2/H2O ratios on He atmospheric pressure plasma jet impinging on a dielectric surface

A two dimensional (2D) axisymmetric fluid model is built to investigate the effect of different O2 and H2O admixture on the plasma dynamics and the distribution of reactive species in He atmospheric pressure plasma jet (APPJ). The increase of O2: H2O ratio slows down both the intensity and the propagation speed of ionization wave. Due to the decrease of both H2O ionization rate and H2O Penning ionization as well as the stronger electronegativity of O2, the increase of O2: H2O ratio results in a significant reduction of electron density in the APPJ, which restricts the occurrence of electron collision ionization reactions and inhibits the propagation of plasma. The excitation energy loss of O2 is not the reason for the weakening of the plasma ionization wave. The densities of O2+, O- and O2- increase with the rise of O2 admixture while H2O+ decreases due to the decrease of electron density and H2O concentration. OH- density is affected by both the increase of O- and the decrease of H2O so it shows peak in the case of O2: H2O=7:3. O is mainly produced by the excitation reactions and the electron recombination reaction (e + O2+ → 2O), which is directly related to the O2 concentration. OH is mainly produced by e + H2O → e + H + OH so the OH density decreases due to the decrease of electron density and H2O concentration with the increase of O2: H2O ratio. On the dielectric surface when the propagation of streamer extinguishes, O flux shows an upward trend while the OH flux decreases, and the propagation distance of O and OH decreases with the increase of O2: H2O ratio.

The role of resonance radiation in the propagation of a positive pre-breakdown ionization wave in long discharge tubes

The work is devoted to calculating the flux of resonance photons towards the boundary of a cylindrical discharge tube of a finite size during the propagation of a pre-breakdown ionization wave of positive polarity. A cylindrical discharge tube of finite dimensions with argon at the pressure of p=1 Torr is considered. The propagation mechanisms of metastable and resonance atoms are compared. For the considered discharge conditions, the space-time distributions of metastable and resonance atoms are calculated. The manuscript presents a technique for calculating the flux of resonance photons onto the discharge tube wall with the account of the radiation trapping. It is shown that for the studied conditions the photon flux density towards the longitudinal boundary of the tube ahead of the ionization wave can reach 1013 cm-2s-1. The obtained results allow describing the appearance of seed electrons ahead of the positive ionization wavefront during its propagation due to the electron photoemission from the discharge tube wall.

Features of a negative ionization wave development preceding the full breakdown in a long capillary tube wrapped with a continuous or fine-sectioned grounded electrode

The experimental results on the study of negative ionization wave propagating along a long capillary tube are presented. The ionization wave was initiated by high-voltage pulse of negative polarity. The propagation of this surface ionization wave precedes and influences the establishment of complete electric breakdown within the tube. The spreading of this wave is accompanied by the surface charge deposition. The usage of the fine-sectioned outer electrode allows one to find out the general features of a negative ionization wave. The main of them is the tight correlation between local currents determining the formation of local surface charge and visual discharge images taken by the fast camera characterizing the pace of the ionization wave propagation.

Ionization wave propagation in a long capillary tube in the presence of a low-current glow discharge in low-pressure helium

The experimental results on the study of the positive ionization wave propagating along a long capillary tube are presented. The ionization wave was initiated by eigther high-voltage pulse of positive polarity applied over a long capillary tube with helium or high-voltage pulse applied over the same tube but in presence of low-current glow discharge. The spreading of this wave is accompanied by the surface charge deposition. The usage of the fine-sectioned outer electrode allows one to find out the features of the positive ionization wave in both cases (with or without low-current discharge).

Radial current distribution along the ionization wave propagating in a long dielectric capillary tube filled with a low-pressure helium

The experimental results on the study of the ionization wave propagating along a long capillary tube are presented. The ionization wave was initiated by high-voltage pulse of positive or negative polarity. The propagation of this surface ionization wave precedes and influences the establishment of complete electric breakdown within the tube. The spreading of this wave is accompanied by the surface charge deposition. The usage of the fine-sectioned outer electrode allows one to find out the general features of the ionization waves.

Subnanosecond switching of standard thyristors triggered in impact-ionization wave mode by a high-voltage PCSS driver

Power thyristors triggered in impact-ionization wave mode are capable to replace spark gap switches, bringing major advantages into repetitive pulsed power industrial applications. Low power thyristors remained for the moment out of the research focus, most likely because of the challenging driver, which must provide a sufficiently fast and powerful triggering pulse. This paper describes subnanosecond switching of standard off-the-shelf low-power thyristors in impact-ionization wave mode, running by the PCSS trigger generator based on the laser diode and GaAs switches. Several types of thyristors with a rated voltage from 0.6 kV to 2.2 kV have been tested running by both a commercial FID and by the tailored PCSS generators. The triggering and current flow stages were examined. For the 1.6 kV thyristor (TO-247 package), the following parameters have been obtained: switching time 250 ps, dI/dt up to 12 kA/μs, amplitude 85 A and FWHM about 60 ns. In this mode, the first 103 pulses have not revealed any thyristor degradation.