Yasushi Minamitani

Industrial Applications of Pulsed Power Technology

A review of mainly the past two years is undertaken of the industrial applications of pulsed power. Repetitively operated pulsed power generators with a moderate peak power have been developed for industrial applications. These generators are reliable and have low maintenance. Development of the pulsed power generators helps promote industrial applications of pulsed power for such things as food processing, medical treatment, water treatment, exhaust gas treatment, ozone generation, engine ignition, ion implantation and others. Here, industrial applications of pulsed power are classified by application for biological effects, for pulsed streamer discharges in gases, for pulsed discharges in liquid or liquid- mixture, and for material processing.

Electrical breakdown of water in microgaps

Experimental and modeling studies on electrical breakdown in water in submillimeter gaps between pin and plane electrodes have been performed. Prebreakdown, breakdown and recovery of the water gaps were studied experimentally by using optical and electrical diagnostics with a temporal resolution on the order of one nanosecond. By using Mach–Zehnder interferometry, the electric field distribution in the prebreakdown phase was determined by means of the Kerr effect. Electric fields values in excess of the computed electric fields, which reach >4 MV cm−1 for applied electrical pulses of 20 ns duration, were recorded at the tip of the pin electrode, an effect which can be explained by a reduced permittivity of water at high electric fields. Breakdown of the gaps, streamer-to-arc transition, was recorded by means of high-speed electrical diagnostics, and through high-speed photography. It was shown, through simulations, that breakdown is initiated by field emission at the interface of preexisting microbubbles. Impact ionization within the micro-bubbles gas then contributes to plasma development. Experiments using pulse–probe methods and Schlieren diagnostics allowed us to follow the development of the disturbance caused by the breakdown over a time of more than milliseconds and to determine the recovery time of a water switch. In order to trigger water switches a trigger electrode with a triple point has been utilized. The results of this research have found application in the construction of compact pulse power generators for bioelectric applications.

Decomposition of Dye in Water Solution by Pulsed Power Discharge in a Water Droplet Spray

A method for decomposing dye by spraying water solution of dye into nonequilibrium plasma in gas phase was investigated using a pulsed-corona discharge reactor. The corona reactor consists of a discharge wire and a cylindrical electrode. The water solution of indigo carmine with a concentration of 20 mg/L was sprayed into the reactor from a showerhead. The sprayed water solution was circulated into the reactor. The dependence of the discharging power injected into the reactor on the decomposition rates of chromogenic and unsaturated bonds in indigo carmine was investigated. Indigo blue was decolored by 1-min circulation with the discharging power of 8.6 W. The nuclear magnetic resonance spectrometry showed that indigo carmine was decomposed completely by 60-min treatment. The chromogenic and unsaturated bonds were almost decomposed at 9 and 360 J/mg, respectively. As the discharging power increased, the decomposition rates of both bonds in indigo carmine increased. The decomposition rates of the chromogenic bond showed equal values in equal discharging power regardless of the difference of discharge repetition rate and charging voltage of the capacitor. In contrast, the higher repetitive pulses were more effective for the decomposition of the unsaturated bond rather than the magnitude of charging voltage in equal discharging power.

Comparison of catalytic activity of aluminum oxide and silica gel for decomposition of volatile organic compounds (VOCs) in a plasmacatalytic Reactor

By reducing the distance between the dielectric end-fittings of a coaxial pulsed corona reactor to values on the order of millimeters, the efficiency of volatile organic compound (VOC) decontamination of atmospheric pressure air could be increased by a factor of seven over that obtained in reactors with electrode lengths in the centimeter range. The increased efficiency is attributed to the increasing effect of surface streamers with increased electron density compared to streamers in the gas space. Packing the discharge gap of the reactor with catalytically active silica gel pellets further increased the energy efficiency of VOCs decontamination by more than 50% over that obtained in the absence of any packing. Silica gel was shown to remove VOC by adsorption until it became saturated. The adsorption provides a tool for handling surplus VOCs under fluctuating input conditions. Destruction of preadsorbed organics on silica gel was also demonstrated. Aluminum oxide pellets showed no catalytic effect on destruction of VOCs under these conditions.

The Effect of a Water-Droplet Spray and Gas Discharge in Water Treatment by Pulsed Power

This paper presents the effect of a water-droplet spray and gas-phase discharge in a method of water treatment by spraying wastewater into nonthermal plasma. Three water-treatment methods by pulsed-power discharge were investigated. These are treatment methods by gas-phase discharge in the water-droplet spray, by ozone gas injected in water, and by ozone gas injected in the water-droplet spray. The comparison of gas-phase discharge in the water droplets with ozone gas injected in the water-droplet spray shows the effect of free radicals except ozone. The comparison of ozone gas injected in the water with ozone gas injected in the water-droplet spray shows the effect of the water-droplet spray. Brilliant-blue FCF solution was used as wastewater in the treatment. The decolorization rate of the treatment by spraying the water droplets into the gas-phase discharge reached 80% at 4 min. The decolorization time of the treatment by spraying water droplets into an atmosphere of ozone concentration of 250 and 100 ppm took 12 and 20 min to reach the decolorization rate of 80%, respectively. These results show that other radicals including OH radical and excluding ozone mainly contribute to decompose brilliant-blue FCF in the former treatment. The decolorization rate of the treatment by injecting ozone gas of concentration of 100 ppm into the water solution was 55% in time elapse of 40 min. The result shows that spraying water droplets into reacting fields in the water treatment results in faster decolorization time.

Streamers in water filled wire-cylinder and packed-bed Reactors

Streamers in a coaxial reactor filled with tap water have been investigated. Up to six primary plasma channels originated simultaneously from a common point at the center electrode, from where they branched out radially, driven by the electric field. Points of origin were distributed over the wire with an average separation of 5 mm. The streamers propagated at above 60 km/s velocity near the center electrode and slowed to about 30 km/s when they reached the middle of the gap. The streamers accelerated again as they approached the outer electrode. Packing the reactor with silica gel beads generated more homogeneous plasma.

The Effect of Spraying of Water Droplets and Location of Water Droplets on the Water Treatment by Pulsed Discharge in Air

In this paper, some characteristics of a method for decomposing organic compounds by spraying a water solution of organic compounds as droplets into pulsed streamer discharges in air are presented. For the characteristics, the effect of spraying the water droplets compared with water film flowing in a chamber wall, and locations of water droplets into the discharge area on water treatment by the pulsed discharge in air was investigated. In the experiments, a coaxial electrode was used. A solution of organic dye, indigo carmine, was used for a sample. Experimental methods of the effect of spraying the water droplets were to sample each solution gotten through the discharge area as water droplets and water film. Experimental methods of the effect of locations of the water droplets were to spray the water droplets near the wire electrode or the cylindrical electrode. The time spent for decolorizing the water droplets, including indigo carmine, was 0.57 times shorter than that flowing along the inner wall of a reactor into the discharge. This result shows that spraying as water droplets into the discharge in air is effective for faster treatment of water. The time spent for decolorizing the water droplets, including indigo carmine, at the location near the cylindrical electrode of the reactor was 1.5 times faster than that at the location near the wire electrode of the reactor. This result shows that the location near the cylindrical electrode of the reactor has many radicals that are effective for degradation of organic compounds.

Influence of a Circuit Parameter for Plasma Water Treatment by an Inductive Energy Storage Circuit Using Semiconductor Opening Switch

In this paper, we investigate a water treatment method that sprays waste water droplets into a pulsed discharge space. For this method, it is important to apply pulsed voltages with a short pulse width and a fast rise to the electrode to realize high energy efficiency. An inductive energy storage (IES) circuit using a semiconductor opening switch (SOS) outputs pulsed voltages similar to the above-mentioned voltage. We report the characteristics of the water treatment using the IES circuit. The capacitance and inductance in the IES circuit are varied. An increase in the inductance in the secondary circuit of the pulse transformer, results in the formation of pulsed voltages with a longer pulse width; thus, the ratio of the thermal loss to the discharge energy becomes high. However, the energy transfer efficiency improves. Additionally, we vary the maximum current of the SOS, keeping the pulse width constant, by adjusting the capacitance and the inductance. As a result, when the current in the SOS increased, pulsed voltages with a higher peak and a shaper rise are obtained. Further, the pulse width of the voltage and the energy transfer efficiency are not affected by the current in the SOS. Hence, the energy density of the discharge is higher. The increase in the secondary inductance and the forward current increased the energy efficiency of the water treatment system owing to the increase in the energy transfer efficiency and the discharge energy density.

Decomposition of Sodium Acetate by Pulsed Discharge in Water Droplet Spray

The characteristics and the effects of oxygen on the decomposition of sodium acetate have been investigated by pulsed discharge in water droplet spray in O₂ and N₂ used as carrier gases in the reactor. Two experimental conditions of gas mixing ratios of O₂/N₂ = 10/90 and O₂/N₂ = 50/50 were performed to evaluate the effect of oxygen. In both conditions, acetate was decomposed by pulsed discharge in water droplet spray, and formate and the other products were detected as intermediate products. Formate was gradually decomposed with increased treatment time. Acetate decomposition rates after 10 h treatment in the gas mixing ratios of O₂/N₂ = 10/90 and O₂/N₂ = 50/50 were 39% and 80%, respectively, and TOC decomposition rates were 48% and 78%, respectively. It was found that it is possible to decompose sodium acetate. These results indicate that organic compounds which have a carbon-carbon single bond can be completely decomposed by this technology. Decomposition efficiencies in the gas mixing ratios of O₂/N₂ = 10/90 and O₂/N₂ = 50/50 were 43 μg/kJ and 97 μg/kJ, respectively. Decomposition efficiency in the gas mixing ratio of O₂/N₂ = 50/50 is about 2.2 times higher than that of the gas mixing ratio of O₂/N₂ = 10/90 . These obtained results show that oxygen plays important roles in enhancing the decomposition of organic compounds.

Optical measurements of the electric field of pulsed streamer discharges in water

The electric field of a streamer tip of a pulsed streamer discharge in water has been investigated by an optical measuring method. Crossed polarizers were used for the measurement of the electric field using the Kerr effect. Pulsed power with a 300 ns pulse width and 53 kV peak voltages was applied to a pin electrode with a Blumlein circuit. The electric field is obtained from the intensity of the laser beam passing through the second polarizer. The beam intensity was recorded by a highspeed camera with an exposure time of 100 ns. The electric field of the streamer tip, which could not propagate any further, was 320 kV/cm.