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Dielectric barrier discharges (DBDs) driven by nanosecond high-voltage pulses can be quite different from those driven by alternating current (ac) sources, on both electrical characteristics and plasma microparameters. In this article, the power measurement strategy in nanosecond pulsed coaxial DBDs is optimized. The effect of different experimental parameters on energy deposition and reduced electric field in plasma is investigated. First, the results measured using the instantaneous power (from the production of voltage and current) method and that using the v – q trace (Lissajous curve) method are compared and the source of divergence is analyzed. By comparing monitor capacitors with different materials and capacitances, it is demonstrated that the high-frequency response of the monitor capacitor is essential and the capacitance value can be adjusted according to the applied voltage, taking both the dynamic range of oscilloscope and the signal-to-noise ratio into consideration. Second, the influence of morphology of ground electrode, voltage rising time, and pulse width on energy deposition per-pulse and reduced electric field, obtained from the nitrogen spectral line-ratio, is studied. It is found that the copper foil promotes the discharge energy coupling per-pulse compared with the copper grid, while the reduced electric field obtained from the nitrogen line-ratio with the copper foil is weaker. A shorter rising time will also enhance the energy coupling per-pulse, as well as the reduced electric field. [ABSTRACT FROM AUTHOR]

In this paper, an array wire-to-wire surface dielectric barrier discharge (DBD) structure is used to generate large-scale plasma by bipolar nanosecond pulse power in atmospheric air. The quantity of electrodes can be adjusted from 1 to 10 for varying discharge area. The applied voltage and discharge current are measured, the discharge images are captured, and optical emission spectra (OES) are collected. The discharge power, discharge area, and OES intensity for a single group of/and whole structure are calculated. The vibrational and rotational temperatures are simulated by the OES of N2 (C-B, $\Delta \nu =-2$), and the effects of the quantity of electrodes on discharge current, discharge power, discharge area, OES intensity, and vibrational and rotational temperatures are investigated. The results show that the discharge current has two main peaks during single applied pulse voltage, and decays in a fluctuant type. As the quantity of electrodes increases, the peak value of discharge current increases and the decaying time from peak value to zero around becomes longer. The discharge power, discharge area, and OES intensity have increasing tendencies, and those of the single group of wires decrease. In addition, when the quantity of electrodes increases, the vibrational temperature increases first and then decreases, and the rotational temperature varies in a fluctuant type. [ABSTRACT FROM AUTHOR]

Complexity of branching pattern and OH production of water surface discharges was investigated by comparing nanosecond water surface discharge (NWSD) and microsecond water surface discharges (MWSDs) that were defined as NWSD and MWSD, respectively. Experimental and analyzed results between NWSD and MWSD under comparable maximum discharge length ld are summarized as follows: 1) NWSD showed the greatest complexity of branching pattern by fractal analysis; 2) electron density of NWSD was approximately two times greater than MWSD and both orders were 10−17 cm−3; 3) emission intensity of OH (A-X) from MWSD was greater than NWSD; 4) rotational temperature of NWSD was almost constant around 1000 K irrespective of ld and rotational temperature of MWSD increased with increasing ld ranging from 2000 to 4000 K. It was found that the complexity of discharge pattern on water may be affected by the field intensity at water/air boundary. OH production was presumed to be caused by thermal dissociation in this experimental condition. [ABSTRACT FROM PUBLISHER]

This paper reports on the dependence of voltage rise rate on positive streamer branching and shock waves in supercritical carbon dioxide. Voltages with rise rates of 0.21 and 4.5 kV/ns were applied to a needle-to-plane electrode. Prebreakdown phenomena involving streamer growth and shock wave propagation were observed by means of a shadowgraph method. The results show that the spread angle of the streamer at the needle tip under 4.5 kV/ns was nearly twice as large as that under 0.21 kV/ns. The discharge may initiate without density reduction due to electrostrictive force. While shock wave Mach number was little affected by the voltage rise rate, it increased by increasing the negative voltage peak. Consequently, the velocity of shock wave is presumed to be influenced by the population of the vibrational state of carbon dioxide. [ABSTRACT FROM PUBLISHER]

A new design of the compact nanosecond pulse generator based of the drift step recovery diodes as an ignition system for internal combustion engines (ICEs) has been presented. Experimental results of the comparative analysis of the standard ignition system and an ignition system on the basis of the nanosecond pulsed discharge for the four-cylinder ICE have been presented. It was shown that a nonequilibrium plasma formed by the discharge was an effective way to reduce the specific fuel consumption as well as the concentration of nitrogen oxides in the engine exhaust gases. A numerical analysis of possible mechanisms of the nonequilibrium plasma influence on the suppression of the nitrogen oxides formation has been carried out. [ABSTRACT FROM PUBLISHER]

In the use of spark gaps as switching devices in pulsed power systems, spark resistance is a crucial factor influencing the voltage rise time and the efficiency of energy delivered to the load. However, current oscillations make it difficult to accurately measure the spark resistance. Therefore, we present a spectrographic method to measure the spark radius and the conductivity of spark channel according to its emission spectrums. A series of experiments has been carried out to investigate the time-dependent properties of spark resistance under oscillatory damped current pulses. The current and voltage waveforms have been recorded to calculate the energy deposited in the spark channel. Furthermore, the influences of gas pressure and gap length on spark resistance and the deposited energy have been discussed. The results indicate that lower resistance, and energy deposition is obtained with shorter gap lengths operating at higher pressures. [ABSTRACT FROM PUBLISHER]

Nanosecond pulsed discharges in a helium-adamantane mixture at atmospheric pressure were used for the synthesis of carbon nanomaterials, using the smallest diamondoid, adamantane, as a precursor. The discharges were generated using a point-to-plane electrode with a gap distance of 1–2 mm, at room temperature, 373 and 473 K. Scanning electron microscopy of the electrodes showed the formation of carbonaceous deposits with different morphologies, while micro-Raman spectra of the deposits covering the electrodes contained features characteristic of carbon structures of varying order. To study the effect of the temperature conditions on the plasma characteristics, time-resolved optical emission spectroscopy measurements were carried out at room temperature and 456 K. The optical emission spectra showed that at room temperature the afterglow has a duration of about several microseconds, whereas at 456 K, the plasma decay is on the order of only 1 \({\mu }\) s. This may be an indication that the carbon nanomaterials are predominantly formed in the afterglow of the nanosecond pulsed discharge, with the presence of hydrogen radicals facilitating the formation of carbonaceous materials. [ABSTRACT FROM AUTHOR]

This paper deals with the initiation mechanism of a negative nanosecond pulsed discharge in supercritical (SC) carbon dioxide that was examined in detail using Schlieren and photomultiplier techniques. A negative pulsed voltage with a rise time of about 90 ns and half-width of 410 ns was applied to the point electrode. The experimental results show that: 1) The negative primary streamer was of a form quite different than that of a positive one: a bushlike negative streamer and a filamentlike positive streamer; 2) a drastic change in the density dependence of streamer initiation voltage appeared around the subcritical phase in the characteristics for streamer initiation voltage versus medium density; and 3) a shock wave of speed 1–1.5 Mach began growth following a delay of around 50 ns from the streamer initiation. It was predicted from the analysis of the experimental results that the drastic change in the streamer initiation voltage versus medium density characteristics may be due to a transition between the two types of generation mechanisms of the initial electron: electron detachment from negative ions in the gas phase and field electron emission from the point electrode in the SC and liquid phases. The shock wave was presumed to be caused by thermal relaxation of the vibrational energy in carbon dioxide molecules in a decay process of the primary streamer. [ABSTRACT FROM PUBLISHER]