Tamiya Fujiwara

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.

Ion acceleration in a solenoid-free plasma expanded by permanent magnets

Ion acceleration is achieved in a low-pressure solenoid-free plasma expanded by permanent magnet arrays. Although a permanent magnet normally forms cusp magnetic fields which prevents plasma diffusion and double layer formation, by employing double concentric arrays of permanent magnets, a constant field area, and a diverging magnetic field can be generated near the outlet of the plasma source. In the source, a rapid potential drop with 4cm thickness from 50V to 20V is generated at the diverging field area for 0.35mTorr and a supersonic ion beam accelerated through the potential drop is observed in the diffusion chamber. The beam energy can be increased up to over 40eV with a decrease in gas pressure.

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.

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.

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.

Ion extraction from carbon shunting arc plasma

The ion current characteristics of the shunting arc discharge are described in this letter. A carbon rod with 2 mm diameter and 40 mm length was employed for arc generation. The combination of the shunting arc and the negative pulse voltage applied to target is promising for plasma-based ion implantation and deposition for metallic or semimetallic three-dimensional materials. The delay time, which is defined as the time between the start of the arc current and applying the pulse voltage, was varied. The ions are extracted from the shunting arc plasma by applying a pulsed bias voltage to a target set nearby the arc source. The arc current lasts 40 μs, of which peak is 1.7 kA. The extracted target current has a sharp peak at the initial stage, followed by a stationary state. The stationary current decreases with increasing the delay time and increases with increasing the bias voltage. Under the assumption of a collisionless ion sheath, the plasma density was estimated. At the boundary between the ion sheath...

Energy Efficiency of Corona Discharge Reactor Driven by Inductive Energy Storage System Pulsed Power Generator

Characteristics of a pulse corona reactor driven by an inductive energy storage (IES) pulsed power generator are described in this paper with focusing on the influence of streamer-to-glow transition on NO removal efficiency. A pulsed high voltage with a short rise time of under 30 ns is employed to generate streamer discharges homogeneously in whole the discharge region. Fast recovery diodes are used as semiconductor opening switch (SOS) to shorten the rise time. The various resistors are employed as dummy load to clarify a suitable circuit parameter such as the capacitance of a primary energy storage capacitor and/or the inductance of a secondary energy storage inductor. The energy transfer efficiency of the pulsed power generator has a maximum value of 50% at 714 Omega dummy load resistance. A co-axial cylinder type discharge chamber was used as the corona discharge plasma reactor driven by the IES pulsed power generator. The pulsed power generator supplies 30 kV pulse with 300 pps repetition rate. The co-axial cylinder plasma reactor consists of 1 mm diameter tungsten wire and 19 mm i.d. copper tube with 30 cm length. NO removal from the simulated diesel engine exhaust gas (N2:O2=9:1, Initial NO concentration=200 ppm) increased with input energy into the reactor. The energy efficiency for NO removal was obtained to be 25 g/kWh at 30 % removal in gas flow rate of 2 L/min. However, the energy efficiency decreased to 5 g/kWh with increasing capacitance of the primary capacitor from several hundreds pF to several nF. This decrease was caused by a streamer-to-glow transition. The efficiency was affected by oxygen concentration in the gas mixture.

Observation of weakly and strongly diverging ion beams in a magnetically expanding plasma

The spatial distribution of an ion beam created in a magnetically expanding plasma using permanent magnets is experimentally investigated for 0.35 and 1 mTorr, where the magnetic-field strength is about 100 G in the plasma source and is decreasing into a few gauss in the diffusion chamber. The beam profile for 0.35 mTorr is weakly divergent. On the other hand, the strongly diverging beam is detected for 1 mTorr. The results are discussed from the viewpoint of the plasma-potential structures and imply the beam divergence caused by the radial electric fields in the ion acceleration region and the diffusion chamber.