Streamer-to-filament transition is a general feature of high pressure high voltage (HV) nanosecond surface dielectric barrier discharges. The transition was studied experimentally using time-and space-resolved optical emission in UV and visible parts of spectra. The discharge was initiated by HV pulses 20 ns in duration and 2 ns rise time, positive or negative polarity, 20-60 kV in amplitude on the HV electrode. The experiments were carried out in a single-shot regime at initial pressures P > 3 bar and ambient initial temperature in air, N 2 , H 2 :N 2 and O 2 :Ar mixtures. It was shown that the transition to filamentary mode is accompanied by the appearance of intense continuous radiation and broad atomic lines. Electron density calculated from line broadening is characterized by high absolute values and long decay in the afterglow. The possible reasons for the continuous spectra were analyzed.
Streamer-to-filament transition is a general feature of high pressure high voltage nanosecond surface dielectric barrier discharges (nSDBDs) for mixtures containing molecular gases. The transition is observed at high pressures and voltages in a single-shot experiment a few nanoseconds after the start of the discharge. A set of experimental results comparing streamer-to-filament transition and properties of plasma in the filaments for the identical high voltage pulses of negative and positive polarity is presented. The transition curves in voltage-pressure coordinates are obtained for N 2 :O 2 mixtures with different content of molecular oxygen, from 0 to 20%, at the pressure range 1-12 bar. Continuous optical spectra are compared for both polarities in 6 bar synthetic air. Electron density is calculated from Stark broadening of H α line at λ = 656.5 nm in the discharge and in early afterglow, 40 nanoseconds after the end of the high voltage pulse. Hydrodynamic perturbations are measured using schlieren imaging in 1-6 bar air for streamer and filamentary mode for both polarities. The review of common and distinctive features of the filamentary single-shot nSDBD for two polarities of the applied pulse is provided.
Weakly ionized plasmas in argon and nitrogen in a parallel-plate electrode configuration at a gas pressure of several Torr were sustained by repetitive nanosecond pulses with the pulse repetition frequency sufficiently high to ensure that the plasma does not fully decay between the pulses. In order to measure the electron number density decay in the afterglow of each pulse, a custom-constructed 58.1 GHz homodyne microwave interferometer was used. Initial analysis of the measured electron density decay indicates that dissociative recombination with molecular and cluster ions was the dominant electron loss mechanism for both gases, and that the electron thermalization was significantly faster than the decay of their density. The plasma with low electron temperature between the pulses could potentially be used to reduce the Johnson-Nyquist thermal noise in plasma antennas.
A diagnostic method for small and microcavity plasma discharges is proposed. The method is based on applying a weak variable-frequency probing signal to the same electrodes that are used to create the plasma and measuring the reflected signal's amplitude and phase over a wide frequency range. Thus, the discharge impedance at multiple probing frequencies may be found, and the key plasma discharge parameters, such as the electron density and temperature and the sheath thickness, can be inferred. The method is dubbed SPRIGHT (Small Plasma Reflection Inter-rogation with GigaHertz Transmitter) and is demonstrated for a small (2 mm interelectrode gap) radio frequency capacitively coupled discharge in argon at pressures of 1–5 Torr.
In this work, the effect of flashing corona enhancement by introducing Teflon dielectric enclosure in vicinity to the electrode assembly was studied. The discharge operating in air without the dielectric was able to operate within a very narrow voltage range of approximately 200 V. The pulsing frequency was below 1.2 kHz and current peaks were below 14 mA. Increasing the applied voltage onto the positive electrode beyond this range would result in sparks between the electrodes. When the Teflon tube enclosure surrounding the high voltage electrode was used, the window of stable flashing corona operation expanded up to 3-5 kV. The pulsing frequency increased up to 12 kHz and the current peak level increased to approximately 35 mA. Increasing voltage beyond the point with peak pulsing frequency would result in a drop of pulsing frequency until the discharge pulsations stopped completely. The Teflon enclosure was able to enhance the average power deposited into the discharge from 10 to 220 mWatt. In addition, the product gases of the enhanced flashing corona were tested to be mostly ozone with traceable amount of NO 2 . The discharge used about 150 eV and 1950 eV per one ozone molecule and nitrogen dioxide molecule respectively.
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