Pulsed Streamer Discharge Research Articles

Characteristics of spatiotemporal variations of primary and secondary streamers under pulsed-voltage in air at atmospheric pressure

A pulsed positive streamer discharge was simulated using a two-dimensional axisymmetric model to investigate the characteristics of primary and secondary streamers in air at atmospheric pressure and ambient temperature. The spatiotemporal variations of the reduced electric field and the electron density during propagation of the primary streamer were clarified, and their relationships with the applied voltage were discussed. The phenomenon of the secondary streamer was introduced according to the previously developed “attachment instability” theory, and the spatiotemporal variations of the net-attachment frequency were presented to validate the theory. The results indicated that variations in the reduced electric field and electron density can be approximately estimated by the theory even in conditions involving a pulsed voltage and non-uniform Laplacian field. Because the primary and secondary streamers have spatiotemporal characteristics related to the chemical reactivity in the streamer discharge, understanding these characteristics is valuable for the development of atmospheric-pressure plasma applications.

Generation of the single-filament pulsed positive streamer discharge in atmospheric-pressure air and its comparison with two-dimensional simulation

Axisymmetric two-dimensional (2D) simulation is often used in case of streamer discharge in atmospheric-pressure air. To validate the 2D simulation of streamer discharge, the simulation has to be compared with the measurement results. However, there is difficulty in performing the comparison because the streamer is not a 2D phenomenon but a three-dimensional phenomenon. In this study, a pseudo-2D axisymmetric single-filament pulsed positive streamer discharge was generated in atmospheric-pressure air and compared with our 2D simulation. The propagation, emission intensity, shape, and cathode current of the streamer were compared. The ozone density was also measured using laser spectroscopy and compared with the simulation because our simulation intends to simulate the dynamics of reactive species. The measurements and simulations exhibited some agreements and disagreements. The measurement also showed a cathode fall and other cathode phenomena, which were not considered in the simulation.

Investigation of the Electrode Erosion in Pin-to-Liquid Discharges and Its Influence on Reactive Oxygen and Nitrogen Species in Plasma-Activated Water

Although the erosion of high-voltage electrodes was extensively studied in in-liquid electrical discharges, to the best of our knowledge, the erosion produced by discharges generated above water has been barely explored. This work studies the erosion of three pin electrode materials (hafnium, copper, stainless steel) in two gas atmospheres (oxygen, air). They are powered by repetitive high-voltage nanosecond pulses, producing pulsed streamer discharges above water. The electrode material does not affect the energy deposited per pulse. The surfaces of all three electrodes erode, releasing metal particles into the water. Stainless steel is the material with least erosion, in both gas atmospheres. Overall, copper in air shows the highest erosion. We also examine how the metals released into the water affect three long-lived reactive oxygen and nitrogen species (RONS), H2O2, NO2− and NO3−, during four weeks post-discharge. After treatment with air plasma, NO2− and NO3− are measured in the treated water, but H2O2 is not detected. NO2− is almost completely converted into NO3− after two weeks. H2O2 is measured for samples prepared with an oxygen plasma. Neither the RONS nor the conductivity of plasma-treated water are significantly affected by the use of different electrode materials.

Characteristics of pulsed streamer discharge with MgO cathode and enhanced toluene degradation

Herein, characteristics of pulsed streamer discharge ignited by a MgO/stainless steel (MgO/SS) cathode and toluene degradation at atmospheric pressure are investigated. The voltage-current characteristics, spatiol-temporal distribution characteristics of streamer propagation and optical emission spectroscopy (OES) of the streamer discharge are inspected. The results show that the discharge peak current with MgO/SS cathode is 20% higher than that with stainless steel (SS) cathode at the optimal condition. Compared with the SS cathode, MgO/SS cathode will suppress the intensity of primary streamer but enhance the intensity of secondary streamer. The emission spectral intensity of N2C3∏u,υ'→B3∏g,υ'' with MgO/SS cathode is higher than that with SS cathode, implying that the MgO/SS cathode can increase the production of active species in the pulsed streamer discharge. The electron density can reach 6.364 × 1021 m−3 for the discharge with MgO/SS cathode, which is higher than that with SS cathode (4.799 × 1021 m−3). MgO/SS cathode can improve the O3 production, toluene removal efficiency and energy yield of pulsed streamer discharge.

Shockwaves from pulsed streamer discharge imaged at ultra-high speeds

A high-speed camera system revealed the individual shockwaves created by a pulsed electrical discharge in water, unveiling some of the fundamental physics.

Shock wave release behavior of a pulsed positive streamer discharge in water

We performed experimental observations to distinguish an individual shock front released by a pulsed positive streamer discharge in water. The step length between two adjacent shock wave events was measured and its dependence on water conductivity and applied voltage was investigated. We found that the step length between two adjacent shock wave events is ∼160 μm, independent of both water conductivity and applied voltage, within the range studied. Based on the bubble theory of electrical discharge in liquid, we concluded that the individual shock front is released by the electrical breakdown of a gaseous micro-segment near the head of a streamer filament in water. After launching a shock wave, the filament head pauses and waits for another gaseous micro-segment to form in front of it.

Propagation behavior of microsecond pulsed positive streamer discharge in water

The propagation behavior of microsecond pulsed positive streamer discharge in low-conductivity water was investigated via photography and shadowgraph imaging. Eight successive images were acquired from a single discharge pulse, allowing for the study of the overall streamer propagation behavior; both the emission and the shadow emission-combined images of the streamer discharge were obtained. The experimental results confirmed that the streamer at the later stage of propagation is weak in illuminance and has a propagation velocity of ∼2 km/s, which is slower than that of the secondary streamer (∼30 km/s) but as fast as that of the primary one (∼2 km/s). The streamer filaments propagated stepwise in water with two different behaviors: (1) a filament head launches a shock wave, quietly moves a short distance (or short time) ahead, and then launches the next shock wave and (2) a filament head pauses for a relatively long time while the filament illuminance decays, then restarts launching shock waves and moving ahead, and the decayed filament illuminates again. These pause and reactivation behaviors randomly alternate among the filaments, resulting in different shock-wave patterns and filament illuminance. The drop of the propagation velocity from ∼30 km/s to ∼2 km/s and the many irregular spikes in the discharge current waveform can be associated with this alternation of pause and reactivation behaviors of the filament heads.

Two-dimensional electron density measurement of pulsed positive secondary streamer discharge in atmospheric-pressure air

Highly sensitive Talbot-type laser wavefront sensors with a temporal resolution of 5 ns were used for the single-shot visualisation of two-dimensional electron density distributions over pulsed positive secondary streamers generated in a 13 mm pin-to-plate gap installed in air under atmospheric pressure. The single-shot imaging demonstrated that the electron density in the primary streamer channel increased after passage of the secondary streamer front. The electron density inside the propagating secondary streamer showed almost uniform axial distribution and it kept approximately constant at 0.7– cm−3 for about 40 ns. This temporally stable electron density suggested that the reduced electric field inside the secondary streamer was approximately 110–120 Td during the propagation. In the end of the secondary streamer phase, the electron density decreased at a time constant of ∼10 ns.

A Study on the Primary Mode of Pulsed Positive Streamer Discharge in Water

The primary mode of the pulsed positive streamer discharge in water has been studied by the shadowgraph imaging techniques utilizing an ultrahigh speed frame camera system. The propagation speed and the flare angle of the fan-shaped primary streamer were measured and their dependence on the water conductivity and the applied voltage were investigated. The initial propagation speed of the primary streamer is about 4–6 km/s and rapidly decreases to about 1.5 km/s. The water conductivity and the applied voltage have no significant influence on the propagation speed of the primary streamer. The flare angle of fan-shaped structure of the primary streamer ranges from 150° to 210°, and increases with increasing the applied voltage. In the voltage range 15 to 17 kV, the water conductivity has no significant influence on the flare angle of the fan-shaped structure, but as the applied voltage increases beyond 18 kV, the flare angle of the fan-shaped structure decreases with increasing the water conductivity. Based on the plasma discharge in a bubble and the fact that the radius of primary streamer is about 200–400 $\mu \text{m}$ , the primary streamer in water seems to be incepted by the positive ions at the head of the discharge filament generated in the microbubble, which penetrates into the water through spattering process and forms a gaseous trajectory in the bulk water.

Simulation of pulsed positive streamer discharges in air at high temperatures

Two-dimensional simulations of an atmospheric-pressure streamer discharge at high gas temperatures were performed in humid air at initial gas temperatures, T0, in the range 300 K–600 K with the same electrode and applied voltage conditions as those used in Ono and Kamakura (2016 Plasma Sources Sci. Technol. 25 044007). The simulation was validated by comparing its results to experimentally obtained discharge currents, primary streamer velocities, and secondary streamer diameters and lengths. This paper discusses the mechanisms underlying the temperature effects in terms of the behaviour of the charged particles in the primary and secondary streamers. At T0 = 300 K, the main decay processes in the primary streamer are the electron recombination reactions with cluster ions and electron three-body electron attachment with O2 and H2O, while the main decay process in the secondary streamer is the two-body electron attachment to O2. Although, at T0 = 600 K, the main decay processes in both streamers are still recombination and two- and three-body electron attachment reactions, the rates of these reactions decrease owing to the increase in the gas temperature, which leads to the increased conductivity of streamer discharge channels at high gas temperatures.

Plasma-Induced Suppression of Recurrent and Reinoculated Melanoma Tumors in Mice

Plasma irradiation of cancer is expected to activate a cancer-specific immune response; however, its long-term systemic effect in animal models has not been sufficiently investigated. In this paper, the long-term systemic effect induced by plasma irradiation was examined using C57BL/6 mice inoculated with B16F10 melanoma. Melanoma tumors on the right hind legs of the mice were resected after treatment with nanosecond pulsed streamer discharge. The results showed that plasma treatment delayed the growth of recurrent tumors at the site of resection. In addition, plasma treatment delayed the growth of tumors reinoculated at the nonirradiated site on the left leg of the mice after resection. Histological and single-cell analyses revealed that plasma had no direct effect on primary tumors, whereas it enhanced cytotoxic T cell infiltration into reinoculated tumors. These results suggest that plasma treatment induces cancer-specific long-term immune memory in mice.

Rapid temperature increase near the anode and cathode in the afterglow of a pulsed positive streamer discharge

The spatiotemporal evolution of the temperature in the afterglow of point-to-plane, pulsed positive streamer discharge was measured near the anode tip and cathode surface using laser-induced predissociation fluorescence of OH radicals. The temperature exhibited a rapid increase and displayed a steep spatial gradient after a discharge pulse. The rate of temperature rise reached 84 K μs−1 at mm, where z represents the distance from the anode tip. The temperature rise was much faster than in the middle of the gap; it was only 2.8 K μs−1 at mm. The temperature reached 1700 K near the anode tip at s and 1500 K near the cathode surface at s, where t represents the postdischarge time. The spatial gradient reached 1280 K mm−1 near the anode tip at s. The mechanism responsible for the rapid temperature increase was discussed, including rapid heating of the gas in the early postdischarge phase (s), and vibration-to-translation energy transfer in the later postdischarge phase (s). The high temperatures near the anode tip and cathode surface are particularly important for the ignition of combustible mixtures and for surface treatments, including solid-surface treatments, water treatments, and plasma medicine using pulsed streamer discharges.

The effect of temperature on pulsed positive streamer discharges in air over the range 292 K–1438 K

The effect of temperature on pulsed positive streamer discharges in air is measured by comparing atmospheric-pressure, high-temperature discharges with low-pressure, room-temperature discharges at the same air densities n and discharge voltages. Both discharges have the same reduced electric field E/n, so the differences between the two discharges only depend on the temperature, which is varied from 292 K to 1438 K. Temperature affects the discharge pulse energy most significantly; at 1438 K, the energy of an atmospheric-pressure discharge pulse is approximately 30 times larger than that of the corresponding 20.5 kPa, room-temperature discharge. Temperature also affects the shapes of the streamers when K, but no significant effect is observed for K. There is also no significant temperature effect on the spatially integrated intensity of N2(C–B) emission. However, temperature strongly affects the ratio of the integrated emission intensity to the discharge energy. No effect of the temperature is observed on the propagation velocity of the primary streamer or on the length of the secondary streamer.

Quantitative relationship between the maximum streamer length and discharge voltage of a pulsed positive streamer discharge in water

A linear relationship has been realized between the maximum streamer length and discharge voltage of a pulsed positive streamer discharge by measuring the streamer length in water with conductivity of 100 μS cm−1 using high-speed photography. Based on Ohm’s law, a quantitative equation has been derived for the dependence of the maximum streamer length on the discharge voltage of a pulsed positive streamer discharge in water. According to the equation, our results suggest that the streamers spontaneously stop propagating through water due to the voltage at the streamer head dropping below the ignition voltage of a pulsed positive streamer discharge.

Fractal analysis of positive pulsed streamer pattern underwater

A positive pulsed streamer discharge underwater in the rod-to-ring electrode observed by high speed camera was studied in the previous work [Akiyama et al., IEEE Trans. Plasma Sci. 42, 3215 (2014)]. Our observation showed that the pattern of positive discharge was a bush like streamer in both tap water and pure water. The fractal analysis by box-counting method was used to estimate the streamer discharge characteristics. The conclusion showed that (1) the tap water (D = 1.7686) has obviously higher value than pure water (D = 1.6491). (2) In temporal evolution process, the discharge development can be quantified by means of fractal dimensions.

Two-dimensional electron density measurement of pulsed positive primary streamer discharge in atmospheric-pressure air

Elucidating the electron density of streamer discharges propagating in atmospheric-pressure air is critical for achieving a systematic understanding of the production mechanisms of reactive species. Using Shack–Hartmann-type laser wavefront sensors with a temporal resolution of 2 ns, we carried out single-shot two-dimensional electron density measurements for positive primary streamers generated in a 13 mm air gap between pin-to-plate electrodes. The electron density over the positive primary streamers decayed from 1015 to during the propagation. The decay time constant of the electron density in the primary streamer channels was estimated to be ∼2 ns. The distribution widths of the electron density were in good agreement with those of the light emission, typically ranging from 0.8 to 1.5 mm.

Anti-tumor immune response induced by nanosecond pulsed streamer discharge in mice

Plasma is known to activate immune cells in vitro; however, its effect on cancer immunotherapy is not well understood in vivo. In this study, we report B16–F10 tumor growth suppression at a non-irradiated site on a mouse leg after a nanosecond pulsed streamer discharge was applied to the tumor on the other leg. The tumor growth suppression at non-irradiated remote sites was observed from the day next to that of plasma irradiation: the rapid abscopal effect suggests innate immune response activation. Additionally, the production of inflammatory cytokines from splenocytes was enhanced after plasma irradiation. This suggests the activation of adaptive immune response specific to B16–F10 melanoma by plasma irradiation.

Investigation of the Influence of Droplets to Streamer Discharge in Water Treatment by Pulsed Discharge in Air Spraying Water Droplets

We are studying the treatment method that injects the droplets of waste water into pulsed discharge plasma generating by a pulsed power technology as an advanced water treatment technique. The active species, such as ozone and OH radical that have high oxidizing power, are generated in the place injecting the droplets by the discharge, and can be effectively used to decompose the organic compounds that are difficult to decompose. However, there has been few investigation of interaction between the pulse streamer discharge and the droplet in a configuration of a treatment reactor. In addition, a propagation route of pulse streamer discharge has been few investigated in the space putting the droplets in the configuration of the treatment reactor. In this paper, the propagation route of the pulse streamer discharge in the space with the droplets of water has been investigated at the configuration using the wire electrode. In addition, the influence of the droplets to the discharge has been investigated. From results, it was shown that the pulse streamer discharge propagated on the surface of the droplet. Primary streamers mainly occurred at the positions of the droplets. The propagation lengths of secondary streamer in air with the droplets were shorter than that without the droplets. The existence of the droplets prevents the propagation of the secondary streamer.

Pulsed positive streamer discharges in air at high temperatures

Atmospheric-pressure air pulsed positive streamer discharges are generated in a 13 mm point-plane gap in the temperature range of 293 K–1136 K, and the effect of temperature on the streamer discharges is studied. When the temperature is increased, the product of applied voltage and temperature VT proportional to the reduced electric field can be used as a primary parameter that determines some discharge parameters regardless of temperature. For a given VT, the transferred charge per pulse, streamer diameter, product of discharge energy and temperature, and length of secondary streamer are almost constant regardless of T, whereas the streamer velocity decreases with increasing T and the decay rate of the discharge current is proportional to 1/T. The N2(C) emission intensity is approximately determined by the discharge energy independent of T. These results are useful to predict the streamer discharge and its reactive species production when the ambient temperature is increased.

Applicability of pulsed power technique for the degradation of methylene blue

Present study evaluated the efficiency of pulsed power technique (PPT) for the degradation of methylene blue (MB) under different operating and environmental conditions. Various reactive oxygen species (ROS) such as hydroxyl radicals, hydrogen peroxide, ozone and superoxide radical were quantified. The maximum concentrations generated were found to be 63mg/L, 28mg/L, 1mg/L and 18mg/L for hydroxyl radicals, hydrogen peroxide, ozone and superoxide radical, respectively for 12min of pulsed streamer discharge at 23kV applied voltage and 25Hz frequency. It was observed that magnitude of applied voltage, frequency (pulses/sec), pH, alkalinity and natural organic matter (NOM) present in the water had a significant effect on hydroxyl radical and hydrogen peroxide generation. MB degradation efficiency and ROS formation increased with applied voltage and frequency. An initial MB concentration of 50mg/L was completely degraded within 10min at 23kV input voltage and 25Hz frequency. Acidic pH favoured ROS formation whereas high alkalinity and NOM reduced the MB degradation efficiency and ROS formation. Treatment time for the 100% degradation of 10mg/L of MB was increased from 6min to 8min in presence of 150mg/L of alkalinity. The MB degradation efficiency was further decreased to 95.6% when alkalinity was increased to 300mg/L for 10min streamer discharge. Mineralization of dye during treatment was confirmed by total organic carbon and inorganic ions analyses and IR spectra. Energy efficiency of ROS formation in the reactor was also evaluated.