Koichi Takaki

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.

Production of atmospheric-pressure glow discharge in nitrogen using needle-array electrode

An atmospheric pressure glow discharge was generated using a needle-array electrode in nitrogen, and the voltage–current characteristics of the glow discharge were obtained in a range from 1 mA to 60 A. A pulsed high voltage with short rise time under 10 ns was employed to generate streamer discharges simultaneously at all needle tips. The large number of streamer discharges prevented the glow-to-arc transition caused by inhomogeneous thermalization. Semiconductor opening switch diodes were employed as an opening switch to shorten the rise time. The glow voltage was almost constant until the discharge current became 0.3 A, whereas the voltage increased with the current higher than 0.3 A. Electron density and temperature in a positive column of the glow discharge at 60 A were obtained to 1.4×1012cm−3 and 1.3 eV from calculation based on nitrogen swarm data.An atmospheric pressure glow discharge was generated using a needle-array electrode in nitrogen, and the voltage–current characteristics of the glow discharge were obtained in a range from 1 mA to 60 A. A pulsed high voltage with short rise time under 10 ns was employed to generate streamer discharges simultaneously at all needle tips. The large number of streamer discharges prevented the glow-to-arc transition caused by inhomogeneous thermalization. Semiconductor opening switch diodes were employed as an opening switch to shorten the rise time. The glow voltage was almost constant until the discharge current became 0.3 A, whereas the voltage increased with the current higher than 0.3 A. Electron density and temperature in a positive column of the glow discharge at 60 A were obtained to 1.4×1012cm−3 and 1.3 eV from calculation based on nitrogen swarm data.

Multipoint barrier discharge process for removal of NO/sub x/ from diesel engine exhaust

We investigated the application of the dielectric barrier discharge process to the removal of NO/sub x/ from exhaust gas of a 20-kVA diesel engine generator. A multipoint-to-plane geometry was used as an electrode for low-voltage operation. A pyrex glass plate with a 2-mm thickness was put on the plane electrode as the dielectric barrier. The reactor consists of six discharge cells, with each cell consisting of five stacked multipoint-to-plane electrodes where the exhaust gas flows with a high flow rate. The exhaust gas was successfully treated at a flow rate of 1.2 m/sup 3//min, In order to produce repetitive high-voltage pulse, we made two switching electronic devices, an insulated gate bipolar transistor (IGBT), and a pulse transformer. This pulse modulator supplied the voltage with peak value of 10 kV and repetition rate of 2.5 kp/s (pulses per second) to the reactor. The energy transfer efficiency of the pulse modulator is 65% at 300 W of the power consumed in the reactor, The NO/sub x/ (NO+NO/sub 2/) in the exhaust gas was reduced by 35 ppm with an electrical efficiency of 32 g/kWh.

Removal of nitric oxide in flue gases by multi-point to plane dielectric barrier discharge

An experimental study on the removal of NO/sub x/ in flue gas has been carried out using plasma chemical reactions in a dielectric barrier discharge. A multipoint-to-plane geometry is used for the electrode used to lower the operating voltage. The effect of the multipoint electrode configuration on the characteristics of a discharge and NO/sub x/ removal has been investigated. Plasma is produced in a narrow gap by a dielectric barrier discharge at low applied voltage with sinusoidal waveform of 23 kV rms. Specific energy to reduce NO is 63 eV. Electric energy consumed in the discharge increases linearly with area of multipoint electrode, and is approximately 1 /spl mu/J/point at 2.7 kV. In regard to the multipoint electrode configuration, the consumed energy can be increased by reducing the angle of the point. However, the energy efficiency of NO removal becomes small if the point angle is small. It also decreases with reducing the number of points per unit area. In regard to treatment of exhaust gas from a diesel engine generator (20 kVA), NO can be almost completely depleted by the multipoint-to-plane barrier discharge for electrical load below 35% of the rated output.

Characteristics of a high-current transient glow discharge in dry air

The transition of a high-current glow to an arc and the energy dissipation during the glow phase have been investigated in dry air by static breakdown. The measurements have been made for uniform-field gaps up to 2 cm at gas pressures of 1-20 Torr. A low-inductance capacitor of 1.89 µF and a discharge circuit with a coaxial system were used to produce a transient glow discharge with a current in excess of 400 A. The experimental results showed that the high-current glow discharge occurred at a cathode current density of 3.2 A cm-2, which is almost two orders larger than the value deduced from the formula for Cu-air normal glow; jn = 240×10-6p2, where jn and p are the current density and gas pressure, respectively. The transition from the glow to an arc starts to develop in the discharge region near the cathode. The energies dissipated in the discharge or in the cathode fall region during the glow phase depend on the gas pressure and/or the electrode separation. However, the dissipated energy density in the cathode fall region during the glow phase is independent of the gas pressure and the electrode separation, and is approximately 0.035 J cm-3 in the present experiment.

Two-dimensional LIF measurements of humidity and OH density resulting from evaporated water from a wet surface in plasma for medical use

In plasma medicine, plasma is applied to a wet surface and is often accompanied by dry-gas flow. The dry-gas flow affects water evaporation from the wet surface and influences production of reactive species derived from water vapor, such as OH radicals. In this study, the effect of the dry-gas flow on two-dimensional distributions of humidity and OH radical density are examined by measuring them using laser-induced fluorescence (LIF). First, humidity is measured when nitrogen flows from a quartz tube of 4 mm inner diameter onto distilled water and agar media from 5 mm distance. NO gas is added to the nitrogen as a tracer and humidity is obtained from the quenching rate of NO molecules measured using LIF. This measurement has a spatial resolution of 0.2 mm3 and a temporal resolution of less than 220 ns. The two-dimensional humidity distribution shows that the dry-gas flow pushes away water vapor evaporating from the wet surface. As a result, a low-humidity region is formed near the quartz tube nozzle and a high-humidity region is formed near the wet surface. The thickness of the low-humidity region reduces with increasing gas flow rate. It is 0.1–0.5 mm for the flow rate of higher than 0.3 l min−1. Next, the OH density is measured when a nanosecond pulsed streamer discharge is applied to a distilled water surface with dry-air flow. The OH density decreases with increasing gas flow rate due to decreased humidity. When the flow rate is lower than 0.1 l min−1, the OH distribution is approximately uniform in the plasma region, while the humidity distribution shows a large gradient. The importance of the thin high-humidity region on the flux of reactive species onto the wet surface is discussed.

Voltage–current characteristics of high-current glow discharges

The voltage–current characteristics of glow discharges in gas mixture (N2:O2=8:2) at a pressure of 10 Torr were obtained with the discharge current up to 150 A. Parallel-plane electrodes with a diameter of 10.7 cm and a discharge chamber with co-axial geometry were used to produce glow discharge with high current. The glow discharge voltage was almost constant until the whole surface of the cathode was covered with glow, i.e., until the discharge current became 3.7 A in our experimental condition (a normal glow discharge mode). The voltage, however, increased with the current when the glow covered over the cathode (an abnormal glow discharge mode). The electron density in positive column of the high-current glow discharge was obtained to be 3×1011 cm−3 from Langmuir probe measurements.

Streamer polarity dependence of NOx removal by dielectric barrier discharge with a multipoint-to-plane geometry

In this paper, we experimentally investigated an effect of streamer polarity on the reduction of NOx by using a barrier discharge plasma reactor with a multipoint-to-plane geometry. A high-frequency sine wave voltage generator and a repetitive pulse generator, which generated narrow and high-voltage pulses were used to remove nitrogen oxide (NOx). By the optical spectrum measurement, it is clarified that electrons with high energy, more than 10 eV, are produced in the discharge region at atmospheric pressure. The intensity ratio of the first negative system of N2+ (391.4 nm, threshold energy 18 eV) to the second positive system of N2 (337.1 nm, threshold energy 11 eV) neither depends on the applied voltage nor the frequency, but it depends on streamer polarity. Further, it is shown that the value is higher in a positive streamer than in a negative streamer. The use of a multipoint-to-plane electrodes caused the streamer polarity interchange every half cycle of the sine wave of the applied voltage, while in a case of the parallel plane electrodes, the positive streamer was only generated in the discharge region. The simulated gas and the exhaust gas from a diesel engine generator were used to confirm the NOx removal performance of the reactor.

Purification of High-Conductivity Water Using Gas–Liquid Phase Discharge Reactor

Water purification by streamer discharge using pulsed-power generator under a high-conductivity water containing pollutants has been investigated. A gas-liquid separated reactor was developed to treat highly conductive solution. A wire electrode was placed in the gas phase and a plane electrode was immersed in the water. A pulsed high voltage generated by six stacked Blumlein lines was applied to the wire electrode to generate streamer discharge in the gas phase, which propagated into the air bubble injected into the water. Indigo carmine solution was employed as a specimen. Natrium chloride was used to adjust the solution conductivity in the range from 10 to 30 000 S/cm. A solution with 30 000- S/cm conductivity was successfully decolorized with energy efficiency of 75 mg/Wh. Some species of gas such as air, oxygen, nitrogen, and argon were injected to clarify dominant reactions of the decolorization. The result showed that the ozone produced by gas-phase discharges mainly contributed to the decolorization of the solution. The decolorization rate depended on the chloride ion supplied from the natrium chloride by scavenging of hydroxyl radical and on the copper ion eluted from the plane electrode by redox reactions such as Fenton reactions.

Improvements in plant growth rate using underwater discharge

The drainage water from plant pots was irradiated by plasma and then recycled to irrigate plants for improving the growth rate by supplying nutrients to plants and inactivating the bacteria in the bed-soil. Brassica rapa var. perviridis (Chinese cabbage; Brassica campestris) plants were cultivated in pots filled with artificial soil, which included the use of chicken droppings as a fertiliser. The water was recycled once per day from a drainage water pool and added to the bed-soil in the pots. A magnetic compression type pulsed power generator was used to produce underwater discharge with repetition rate of 250 pps. The plasma irradiation times were set as 10 and 20 minutes per day over 28 days of cultivation. The experimental results showed that the growth rate increased significantly with plasma irradiation into the drainage water. The growth rate increased with the plasma irradiation time. The nitrogen concentration of the leaves increased as a result of plasma irradiation based on chlorophyll content analysis. The bacteria in the drainage water were inactivated by the plasma irradiation.