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The characteristics of a corona discharge in atmospheric pressure air are studied using pulsed power generators that produce voltage pulses of different durations, polarities and shapes. The characteristics are measured in the single pulse, batch, and repetitively pulsed modes. It is shown that no matter what the voltage pulse polarity is, a corona discharge starts developing as a conical diffuse discharge near the electrode tip with a voltage rate of increase of ~1015 Vs-1 across an electrode of small curvature radius. With lower voltage rate of increase (~1013 Vs-1 or lower), one or several diffuse jets develop from this electrode. The diameter of the jets at their front is less than 1mm and depends on many factors (voltage pulse amplitude and increase, interelectrode gap width, pulse repetition rate, etc). It is found that at long voltage pulse durations, the radiation spectrum of the corona discharge changes, and the bands and lines of the material of the electrode appear in the UV region at 200-300 nm. It is demonstrated that a runaway electron beam in a corona discharge is generated and detected at a distance several times greater than the brightly glowing plasma region of the corona discharge. It is shown that x-rays are generated from a corona discharge at high pulse repetition rates of up to 1 kHz. [ABSTRACT FROM AUTHOR]

Pulsed discharge in a nonuniform electric field accompanied by the appearance of bright spots due to explosive electron emission on electrodes has been investigated. The experiments were carried out using three experimental setups, a voltage pulse duration at a matched load of 2 ns, 40 ns, and 130 ns, respectively. Data on the formation of electrode spots during diffuse discharges in tube-plate or needle-plate gap configurations filled with gases at elevated pressures (air, nitrogen and argon) were obtained. It was found that in the air and other gases, bright spots arise on the flat electrode, and on the negative polarity of the electrode with a small radius of curvature, during the direction change of the current through the gap and the increase of the voltage pulse duration. It was shown that at the positive polarity of the electrode with a small radius of curvature, bright spots on the flat electrode arise due to the participation of the dynamic displacement current in the gap conductance. [ABSTRACT FROM AUTHOR]

We report the experimental results with nanosecond-pulse discharges formed in the air gap between a flat electrode and a sharp electrode. The appearance of anode and cathode spots on the electrodes is studied experimentally. It is considered that bright spots on the flat cathode with positive polarity of the sharp electrode are formed due to the explosive electron emission on the cathode and the dynamic displacement current in the gap. It is also shown that with negative polarity of the sharp electrode, bright spots on the flat anode are formed after changing the polarity of the flat electrode due to the discharge oscillatory mode. Under these conditions, the explosive electron emission firstly forms on the sharp cathode. With negative polarity of the sharp electrode of the subnanosecond-pulse pulser, the runaway electron beam current is measured behind the anode foil with a time resolution of no more than 100 ps. [ABSTRACT FROM AUTHOR]

In this paper, the generation and discharge modes of 2D atmospheric pressure plasma jet arrays in economic argon feeding gas with a honeycomb configuration is first reported. Two coupling and collimated discharge modes are achieved with the same array structure. The discharge modes are shown to depend on the gas flow rate and center-to-center distances of jets in the array. Stronger plasmas with higher plasma density than single jet can be obtained in coupling mode array at close proximity of jets in the array and small gas flow rate, while plasmas with moderate plasma density and relative large area can be obtained in the collimated mode array at far proximity of jets in the array. The power density and emission spectra from the centered plasma jet for the coupling mode array are both larger than those of the collimated mode. The appearance of the two discharge modes may be due to the hydrodynamic interactions between the seven individual Ar channels emerging from individual tubes with the air surrounding them. [ABSTRACT FROM AUTHOR]

High-energy electron beam can be generated during the breakdown process in a gas diode at elevated pressures. The design of electrodes in the gas diode affects the parameters of these electron beams. In this paper, investigation on parameters of the runaway electron beam (REB) in air and nitrogen excited by nanosecond pulsed generators are presented. Experiments were carried out with gas diodes by using cathode and anode made of different materials, including stainless steel, aluminum, copper and titanium. Experimental results show that, in the case of nanosecond-pulse breakdown, the cathode material significantly affects the amplitude of the REB current pulse when either pressure or gap spacing are small. When the pressure is low, the REB current is highest for the stainless steel cathode due to its small electron work function. However, the REB current for the aluminum, copper and titanium cathodes is higher than that of stainless steel cathode at atmospheric pressure because of the Malter effect on their surfaces. The dependence of the amplitude of the REB current on the anode foils is investigated by using aluminum, titanium, copper and tantalum. The experimental results show that the atomic number of the metal, as well as mass mean free path of the electron in it contribute to the amplitude of the REB current pulse. Based on the REB currents of different anode foils, energy distribution of runaway electrons is estimated. Electron distribution with an average energy of ~55 keV is obtained when the amplitude of the applied voltage pulse across the discharge gap is 110 kV. The results may contribute to the design of the gas diode for the generation of high-energy electron beams in nanosecond-pulse breakdown. [ABSTRACT FROM AUTHOR]

A double layer DBD plasma jet driven by a pulsed generator is used for SiOx thin film deposition. The effects of dielectric and conductive targets on discharge behavior and film properties are analyzed. Different fraction of N2 is added to the working gas (argon) to modulate film growth rate. The electrical property and optical emission of the plasma jet at different positions are characterized. Film surface morphology, composition and thickness are measured for different targets. The excitation temperature (Texc), vibrational temperature (Tvib) and rotational temperature (Trot) are calculated. The relationship between the discharge behavior and the film properties is studied. Results show that the relative optical emission intensities of Si, O, Ar and N2, as well as Texc, Tvib and Trot are higher for the PMMA target between the high voltage electrode and the grounded electrode (referred to as the H–G region). However, Tvib and Trot appear to be higher on the surface of the copper (conductive) target. The morphology of SiOx film is different: nanoparticles are formed on the PMMA (dielectric) target while dense amorphous film is formed on the copper target. With the same depositing parameters, the thickness of the SiOx film is higher on the copper target. The presence of PMMA at the downstream of the plasma can be considered as a small capacitor with a large resistance, which inhibits the current into the primary circuit passing through the H–G region. Although the presence of a dielectric target enhances the discharge in the H–G region, the charge accumulation on the PMMA surface limits the film growth. These results may contribute to the development of next-generation plasma surface modification technologies for diverse applications in electronics, material synthesis and other related areas. [ABSTRACT FROM AUTHOR]

Repetitively pulsed discharge is an emerging non-thermal plasma technology in the chemistry field given its advantages of high electron energy and moderate gas temperature. The focus of this paper is on the temporal evolution of physical and chemical processes in the plasma-assisted non-oxidative conversion of methane (CH4). Positive and negative streamer discharge plasmas were generated by a microsecond-pulse power source (pulse rising time ~500 ns and full width at half maximum ~8 μs) in a well-designed dielectric barrier discharge reactor. The performance of CH4 conversion (conversion rate, product yield, energy conversion efficiency, etc) was investigated in the first instance. A CH4 conversion rate of 6.2%–9.6% was acquired in the negative streamer discharges, which is on average 6% higher than that in the positive streamer discharges. To explain this phenomenon, the temporal evolution of voltage–current waveforms and ultrafast ICCD photographs and optical emission spectra were recorded. As expected, significant differences between the two different forms of discharges were found. Compared with those of the positive streamer discharges, characteristics of only one, big breakdown, higher initial voltage and current peak, a more stable and nearly uniform luminescent process, and higher CH (A2Δ  −  X2Π) emission intensity and rotational temperature were observed in the negative streamer discharges, which coincide with the promotion of CH4 conversion performance. A simplified chemical kinetics analysis of CH4 discharge was also implemented to demonstrate the influence of characteristic differences on CH4 conversion performance. [ABSTRACT FROM AUTHOR]

In this paper, deposition by non-thermal plasma is used as a surface modification technique to change the surface characteristics of epoxy resin exposed to DC and nanosecond-pulse voltages. The corresponding surface characteristics in both cases of DC and nanosecond-pulse voltages before and after the modification are compared and investigated. The measurement of the surface potential provides the surface charge distribution, which is used to show the accumulation and dissipation process of the surface charges. Morphology observations, chemical composition and electrical parameters measurements are used to evaluate the treatment effects. The experimental results show that, before the plasma treatment, the accumulated surface charges in the case of the DC voltage are more than that in the case of the nanosecond-pulse voltage. Moreover, the decay rate of the surface charges for the DC voltage is higher than that for the nanosecond-pulse voltage. However, the decay rate is no more than 41% after 1800 s for both types of voltages. After the plasma treatment, the maximum surface potentials decrease to 57.33% and 32.57% of their values before treatment for the DC and nanosecond-pulse voltages, respectively, indicating a decrease in the accumulated surface charges. The decay rate exceeds 90% for both types of voltages. These changes are mainly attributed to a change in the surface nanostructure, an increase in conductivity, and a decrease in the depth of energy level. [ABSTRACT FROM AUTHOR]

In this paper, we report on results from a computational investigation of nanosecond pulsed discharges in air of negative polarity using a two-dimensional fluid and fluid-Monte Carlo simulations. The computational parameters and conditions are taken from an experiment. The discharge is initiated near the cathode having a small radius of curvature and propagates towards the flat anode. The simulations are done in atmospheric pressure air with a nanosecond pulse of 120 kV amplitude in a 12 mm gap with a sharp cathode. The essential difference between discharges initiated by low- and high-energy beams of electrons originating in the vicinity of the cathode was observed. The beam of electrons with the initial energy of 20 keV was shown to result in a diffuse discharge, while the 20 eV beam had a negligible influence on the discharge evolution. By analyzing the experimental and computational results, we conclude that a diffuse discharge is formed due to fast electrons without any assumptions on the critical role of the runaway electrons. More study is required to explore the intermediate range of energies and investigate the transition to the diffuse mode of the discharge. [ABSTRACT FROM AUTHOR]

A single needle-electrode plasma jet driven by a home-made microsecond pulse power supply is studied. The electrical characteristics and optical emissions of the plasma jets driven by positive- and negative-polarity pulses are compared. With the same magnitude of applied voltage, the plasma jet driven by positive pulses shows a higher discharge current, a higher optical emission intensity and travels to a longer distance. The temporal-spatially resolved He (706.5 nm), N2 (337.1 nm) and (391.4 nm) emissions behave differently in the plasma jets driven by different polarity pulses: They appear to be discrete emission packets in the positive plasma jet, but continuous emission in the negative plasma jet (under the time resolution in this study). The emission front propagates at a faster speed in the positive plasma jet than in the negative plasma jet. The different behavior of the plasma jets is attributed to the electric field distribution under different polarity pulses. [ABSTRACT FROM AUTHOR]