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Surface charges have significant impact on the evolution of surface dielectric barrier discharge (SDBD). In this work, the role of residual surface charges on repetitively nanosecond pulsed SDBD in atmospheric air is investigated using a two-dimensional fluid model, based on the assumption of preserving the distribution of surface charges at the end of the previous high voltage (HV) pulse. In the bipolar mode when the polarity of residual surface charges is opposite to that of the current HV pulse, a lower breakdown voltage and more deposited energy can be observed, showing an obvious enhancement of SDBD. In the unipolar mode, residual surface charges suppress the development of discharges and energy deposition. It is found that more residual surface charges are accumulated during the negative pulsed discharge, which have a more pronounced effect on the subsequent positive pulsed one. This is explained by the fact that the negative surface streamers directly contact the dielectric and charge it, while the positive surface streamers float above the dielectric, forming a ion-rich region near the surface. The results in this work demonstrate the mechanism of how residual surface charges affect discharge dynamics, which can be utilized to regulate energy deposition in SDBDs. [ABSTRACT FROM AUTHOR]

In this work, the reaction mechanism in non-thermal plasma enabled methane conversion driven by different sources, i.e. microwave discharge, nanosecond pulsed dielectric barrier discharge, and spark discharge, is investigated. The behaviour of reactive species is monitored using the optical emission spectroscopy (OES), while the final gaseous products are analysed with gas-chromatograph. A zero-dimensional (0D) reaction kinetics model is established to reveal the underlying reaction processes. By changing pressure, different plasma modes (diffuse and filamentary) are achieved and the relative emission intensity of different species and distribution of gaseous productions are manipulated. It is found that there is a strong correlation between the intensity-ratio of C2A → X and CH A → X, gas temperature, and the relative concentration of C2 products in different discharge forms, i.e. higher intensity ratio of C2/CH is associated with higher gas temperature and more unsaturated hydrocarbon products. Assisted with the 0D reaction kinetics model, the relationship between the intermediate species and final products is illustrated. Based on the investigation in this work, it is proposed that intensity ratio of OES can be widely adapted as a non-intrusive and real-time process monitoring in the plasma-enabled methane conversion. [ABSTRACT FROM AUTHOR]

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 work, the propagation of the surface ionization wave (SIW) in the nanosecond pulsed surface dielectric barrier discharge with different dielectric materials and pulse repetition rates is investigated. The current waveforms at different locations along the route of the SIW propagation are obtained, based on a specially designed ground strip array geometry. The temporal evolution and spatial distribution of the electric field during the SIW propagation are measured by using the electric field induced second harmonic generation method. The distribution of the residual surface potential after the discharge is mapped with a Kelvin electrostatic probe, which verifies both the existence of the residual electric field and its opposite direction to that during the SIW propagation. It is found that with the dielectric material on which the surface charges decay faster, there are the well-pronounced primary and secondary SIWs with a higher velocity on the voltage rising edge and both the peak current and the peak electric field are also higher, with a less spatial attenuation along the SIW propagation route. It is demonstrated that the residual surface charges with the same polarity as the high-voltage pulse suppress the development of the SIW. [ABSTRACT FROM AUTHOR]

In this work, the propagation of fast ionisation wave (FIW) discharges driven by negative nanosecond high-voltage (HV) pulses and the x-ray spectrum from the FIW discharges are investigated. The measurement using a capacitive probe shows that the peak axial electric field () at each position rises with the increase of the applied voltage amplitude (Umax), resulting in an increase of the FIW velocity (VFIW). The influence of the pre-ionisation on the FIW propagation is estimated from the relationship between VFIW and The pre-ionisation effect appears to be enhanced with the increase of Umax and it is suspected to be due to the presence of more high energy electrons. This is supported by the measured x-ray spectrum, showing a higher count rate and a more elevated high energy tail with a larger Umax. The spatially resolved measurement of the x-ray spectrum shows that, with the increase of the distance away from the HV electrode (as the cathode), the lower energy part (< ∼15 keV) of the x-ray spectrum keeps decaying, while its high energy tail (> ∼25 keV) rises at first and then decays. Based on the spatially resolved x-ray spectrum, it is inferred that, as high energy electrons move away from the HV electrode, the peak of their energy distribution is shifted toward a higher value, while their total number decreases. Further analysis implies that the electrons emitted from the cathode obtain a large percent of the applied potential in the vicinity of the cathode and become runaway electrons. The electrons with a high initial energy travel freely in the discharge region, due to a decreasing electron collisional cross section with its energy. [ABSTRACT FROM AUTHOR]

Pulsed plasma actuators are used for an active flow control application since the 2000s. In this paper, we discuss shock wave and vortex characteristics in pulsed plasma actuators after an introduction of research progress on atmospheric-pressure discharge plasma actuators. First, the shock wave characteristics in surface dielectric barrier discharge (SDBD) actuator operating in diffuse-like and multi-streamer modes are discussed. In the most general case, a shock wave in a diffuse-like SDBD actuator is stronger and faster than in a multi-streamer SDBD actuator. Improved plasma actuators, such as the three-electrode SDBD actuator and the plasma synthetic jet actuator have the enhanced shock wave characteristics. Second, in order to investigate the effects of pulse parameters on the shock wave characteristics in nanosecond-pulse SDBD actuator, a particle image velocimetry system is used to capture the formation of starting vortexes at different pulse rise times, pulse durations, and pulse repetition frequencies (PRFs). It is shown that the velocity of a starting vortex significantly increases when the pulse rise time decreases from 400 ns to 50 ns due to a more significant hydrodynamic effect generated during a shorter rise time. This phenomenon is confirmed by calculating a reduced electric field E/N at a short rise time, which turns out to be higher at a shorter rise time than that at a relatively longer rise time. It is also shown that the velocity of a starting vortex increases and its active area enlarges when the PRF increases. [ABSTRACT FROM AUTHOR]

The plasma produced by a pulsed generator is an efficient tool for an airflow control. In this paper, a nanosecond pulsed generator constructed with a fast solid-state switch is designed for triggering synchronous discharges in plasma synthetic jet (PSJ) actuators connected in series. The generator delivers high-voltage pulses with a peak voltage varying from 0 to 20 kV, a rise time of 20 ns, a full-width at half-maximum of 200 ns, and a wide range of pulse-repetition rate varying from 0 to 5.5 kHz. It is proved that this generator is able to trigger synchronous discharges in three PSJ actuators in atmospheric air with a peak discharge current more than 90 A and a deposited energy per pulse of 39 mJ. A schlieren system is used to observe the airflow dynamics, which shows the high synchronization among the three PSJ actuators with a time triggering delay of about 20 ns. The synchronous operation of three PSJ actuators produces a plasma jet with a maximum velocity of 60 m/s and a precursor wave with a velocity of 350 m/s. [ABSTRACT FROM AUTHOR]

• Catalyst-free approach for activating potent greenhouse gas methane. • Nanosecond pulsed plasma enables new reaction pathways at low temperature (300 K). • Time-resolved and in-situ measurements for the first time reveal reaction kinetics. • Energetic argon species function as an energy pooling for methane activation. Methane activation at low temperature remains a major challenge for its utilization in the field of reduction of greenhouse gases and generation of value-add products, while non-thermal plasmas provide a promising catalyst-free methane activation approach. In this work, an innovative energy pooling mechanism for catalyst-free methane activation at low temperature enabled by the nanosecond pulsed non-thermal plasma in argon and methane gas mixture is investigated by both optical diagnostic and kinetics modelling. Importantly, the evolution of the absolute density of the hydrogen atom at the ground state, as a real-time and in-situ indicator of the methane dissociation degree, is measured using the two-photon absorption laser induced fluorescence (TALIF) method. A simple zero-dimensional reaction kinetics model is built-up to perform a quantitative interpretation on the density evolution of active species. Especially, a good agreement is achieved between the measured hydrogen atom density and that from the kinetics model, validating the reaction pathways of active species. It is demonstrated that, except for the direct electron impact dissociation during the pulse-on period, both the charge transfer between argon ion and methane and the quenching of argon metastable species by methane contribute to a further increase of the hydrogen atom density during the pulse-off period, i.e. the argon ions and metastable species can function as an energy pooling for the formation of hydrogen atom and hydrocarbon radicals. The innovative mechanism proposed in this work may contribute to the catalyst-free and cost-effective strategies for methane utilization at low temperature. [ABSTRACT FROM AUTHOR]

Non-thermal plasmas show great potential in low-temperature activation of methane (CH4) owing to the abundant energetic active species. Motivated by the fact that the chemical reactions in plasma-based CH4 conversion are dominated and regulated by the energetic electrons and various radicals, the temporal evolution of the electron energy distribution function (EEDF) and its relation to hydrogen (H) radical generation in an atmospheric-pressure CH4 needle–plane discharge plasma have been investigated numerically. The simulations are carried out using one-dimensional particle-in-cell Monte-Carlo collision and fluid dynamic models. It can be shown that during the formation and development of the streamer, a characteristic time exists, before and after which the evolution characteristic of the EEDF is reversed. This is mainly attributed to the competition between the energies continuously obtained from the electric field and the increasingly strong inelastic collisions and fast-growing low-energy electron population. When the amplitude of the applied voltage is increased, the fraction of electrons with high enough energy to participate in dissociation or ionization reactions of CH4 increases, leading to an increased H density. Besides, the characteristic time decreases exponentially, and the energy efficiency of the activation of CH4 molecules is decreased. An appropriate electron energy distribution and H radical density should be chosen to ensure acceptable product selectivity and conversion rate without excessive energy consumption; this will depend on the required products. The results presented in this work provide a partial theoretical basis for effectively optimizing the content of high-energy electrons and H radicals. [ABSTRACT FROM AUTHOR]

CeO2 was prepared using a hydrothermal method, modified by radio-frequency plasma in the form of glow discharge, and then the solid superacid S 2 O 8 2 − /CeO2 was prepared by the impregnation method. A series of properties such as pore structure was characterized by N2 adsorption–desorption experiments, surface morphology was characterized by TEM, crystal phase was characterized by XRD, and surface acidity of the catalyst was characterized by Py-IR and Hammett titration. The methyl esterification reaction of tryptophan was used to evaluate the activity of the solid superacid. The results showed that the catalyst modified by radio-frequency plasma had a larger specific surface area, more surface oxygen vacancies, smaller particle size, and higher total acid content. The yield of tryptophan methyl ester reached a higher level of 94.5% (150 °C, 1 MPa, 2 h), catalyzed by the modified S 2 O 8 2 − /CeO2. This work verified the feasibility of plasma technology in the field of catalytic activity enhancement of solid superacid. [ABSTRACT FROM AUTHOR]

[Display omitted] • Without any chemical agent or rare gas, microwave plasma surface modification only consuming low-power electricity and ambient air. • Rapid hydrophobicity recovery of seriously contaminated silicone rubber on the order of 10 s, with a nice long-term aging performance. • Competition between plasma-enabled bond-breaking/etching and oxidization dictates hydrophobicity. • Targeted at power system in severe environment, highly-efficient, environmentally-friendly, and on-line treatment to reduce the risk of pollution flashover. The loss of hydrophobicity of silicone rubber (SR) insulators due to contamination accumulation will increase the risk of flashover and threaten the stability of electrical power system. To solve this problem, a highly efficient and environmentally friendly microwave plasma surface treatment method is developed, which only consumes low-power electricity and ambient air without any chemical agent or rare gas. The hydrophobicity of seriously contaminated SR can be recovered on the order of 10 s, with a nice long-term aging performance under natural transfer. The underlying mechanism is revealed via carefully designed control experiments and systematic surface analysis. It is illustrated that the plasma-generated active species break the long chain of SR into non-crosslinked low-molecule-weight siloxanes (LMWs) and accelerate their transfer to contamination layer, while the bombing and etching effects of plasma manufacture a porous structure and increase the surface roughness. All of these factors will contribute to the increase of hydrophobicity. On the contrary, the oxidation effect of plasma will turn LMWs into polar groups and destroy the hydrophobicity, which should be restricted in practice. The microwave plasma treatment method developed in this work has the on-line application potential in power system for hydrophobicity recovery under severe contamination conditions. [ABSTRACT FROM AUTHOR]

In this study, a microbubble‐enabled plasma reactor driven by a nanosecond‐pulsed generator was developed to provide an effective means for transferring highly reactive plasma species into liquids for plasma‐activated water (PAW) production. The physicochemical characteristics of PAW at different temperatures were evaluated, and the corresponding antimicrobial effects of PAW against Escherichia coli cells were investigated. Results show that the microbubble‐enabled PAW at a lower temperature (10°C) had the highest antimicrobial activity, resulting in a 2.43 ± 1.02‐log10 reduction of E. coli in PAW. The excellent energy efficiency of the total reactive oxygen and nitrogen species production in PAW is 10.37 g kW−1 h−1. Overall, this study provides much‐needed insights into the microbubble‐enabled plasma chemistries for optimizing the biochemical activity of PAW. [ABSTRACT FROM AUTHOR]

The conditions of the very first breakdown happening in a periodical dielectric barrier discharge (DBD) are not the same as those in the discharge established regime. The main intriguing problem in the very first breakdown is the source of the initial seed electrons. In this work, the role of atmospheric small negative ions in the appearance of seed electrons is investigated. The very first breakdown was studied by using a pin-to-plane barrier corona and plane-to-plane DBD operated with ambient air. These discharges were driven by fast- and slow-growing applied voltage. The experimental data related to the very first breakdown are presented. A short summary sounds as follows. In the case of a barrier corona initiated by a fast-growing voltage, seed electrons can appear due to their detachment from background negative ions in the bulk. Under a slow-increasing voltage, background ions have the time to be fully adsorbed on the cathode and the anode long before breakdown occurs. In such a case, seed electrons can appear from negative ions desorbed from the cathode. This desorption is induced by a strong electric field in the gap. The latter mechanism can work in a plane-to-plane DBD driven by both fast- and slow-growing applied voltage. Based on numerical calculations, a qualitative explanation of the obtained results is given. The influence of UV irradiation on the very first breakdown was also studied. [ABSTRACT FROM AUTHOR]

We develop a quasi-one-dimensional theoretical model describing the oscillatory plasma MHD conversion in a channel with parallel electrodes. Based on the model, a thermoacoustic plasma MHD generation system with a looped configuration is proposed and the characteristics of the operation of the system are investigated. Our results show that, for efficient oscillatory MHD generation, the channel dimension of the MHD duct should be much larger than the penetration depth (a proper Womersley number is above 100), otherwise the thermoacoustic effect will play a major role and consume considerable energy. Further, the electromagnetic induction parameter should be larger than 100 S ⋅ T 2 / m , so that a majority of the kinetic energy of the oscillatory plasma can be harnessed for energy conversion. Moreover, a load ratio between 0.6 to 0.8 is suggested for the presented system, with which an efficiency above 20% can be achieved when the electromagnetic induction parameter is 500 S ⋅ T 2 / m. In the optimal case, an efficiency of 24% can be obtained, with the corresponding output electricity of 1644 W. These results shed light on the feasibility of the closed-cycle MHD generation without any moving part, which endows the system with advantages of high reliability and long lifetime. • A theoretical model for oscillatory plasma MHD conversion was developed. • A novel thermoacoustic plasma MHD generation system was proposed. • Characteristics of the oscillatory Hartmann flow and the thermoacoustic MHD generator were investigated. • An efficiency of 24% with an output electricity of 1644 W could be achieved. [ABSTRACT FROM AUTHOR]

• The flashover voltage and current of G10 in LN 2 are obtained through experiments and the flashover data is analyzed by Weibull model. • The flashover process is observed by OES, and the surface evolution after G10 flashover is analyzed by optical microscope and FTIR. • The relationship between polymer flashover and trap in LN 2 is established through test of surface trap distribution and calculation of charge transport equation. The insulation failure is a bottleneck to restrict the safe operation and development of direct current (DC) superconducting power devices. The solid–liquid flashover test is an important evaluation index, but the previous researches mostly focus on engineering application, and less involve in the mechanism analysis. As a characterization method for defects existing in the dielectric, the trap has been applied to the study of flashover to occur polymer surface in liquid nitrogen (LN 2) in recent years. However, the mechanism of trap action in solid–liquid flashover is still confusing. Therefore, this paper carries out the following research. Firstly, the flashover voltage and current of glass epoxy laminate sheet (G10) in LN 2 were obtained through experiments and analysis using the Weibull model in this paper. Secondly, the flashover process of G10 was monitored. The changes of G10 surface morphology and chemical bond before and after flashover were also observed. Finally, the relationship between polymer flashover and trap in LN 2 was analyzed based on the surface trap distribution test and theoretical calculation of the charge transport equation. The results are beneficial to guide the selection and performance improvement of dielectric materials and the design of insulating structures in superconducting devices. [ABSTRACT FROM AUTHOR]

Methane conversion for higher hydrocarbons was carried out in the diffuse, filamentary, and spark regimes of nanosecond repetitively pulsed discharge with negative polarity in a pin‐to‐plate reactor. The diffuse discharge was characterized as a regime with low‐plasma energy, and only trace hydrogen and C2H6 were found in this regime. In filamentary regime, the conversion rate reached 2.2–7.6% with energy conversion efficiency (ECE) of 8.8–12.7%. Meanwhile the selectivity of C2H6, C2H4, C2H2, and C3 were 24.3–18.6%, 9.3–7%, 5.6–7.3%, and 0–5%, respectively. As for the spark regime, C2H2 became the main hydrocarbon product with the selectivity of 12.8%, conversion rate of 44.7–74.4% and ECE of 12.3%. Electrical measurements and ICCD photographs showed that the short pulse could prevent the streamer turning into spark discharge, and thus realize smooth transitions among each regime. Optical emission spectroscopy was used to investigate the plasma chemistry in each regime. It found that the highest spectral line, gas temperature, and electron density varied in these regimes. These results suggested that, comparing with electron impact reactions, the thermal chemistry became more and more important as the plasma energy increased. Highlights: The discharge regimes of nanosecond repetitively pulsed discharge with negative polarity for methane conversion has been investigated in a pin‐to‐plate reactor. Smooth transition among the diffuse, filamentary, and spark regimes are realized by changing voltage and pulse width of the negative nanosecond repetitively plasma (NRP).The short pulse can inhibit the growth of the unstable bright spots around the electrodes, and thus prevents the streamer turning into spark discharge.The main final products can be controlled from C2H6 to C2H2 with different discharge regimes.The detected maximum CH4 conversion was 7.6% in filamentary regime and 74.4% in spark regime, respectively.The plasma chemistry in each regime was analyzed with the optical emission spectroscopy (OES). [ABSTRACT FROM AUTHOR]