Tsuyoshi Kiyan

Electron Temperature and Electron Density of Underwater Pulsed Discharge Plasma Produced by Solid-State Pulsed-Power Generator

A pulsed discharge produced underwater has been an attractive method to treat waste water. For the optimization and realization of the water treatment system utilizing underwater pulsed discharge, modeling analysis could be one of the essential works. However, there is still no simulation work about the underwater pulsed discharge due to the lack of knowledge about its characteristic parameters such as electron temperature, electron density, and so on. In this paper, the temperature and the electron density in a pulsed discharge plasma produced underwater are measured and presented. A magnetic pulse compressor (MPC) was developed and used to create the electrical discharge in water. The developed MPC is all-solid state and is, therefore, a maintenance-free generator. To define the temperature and the electron density in an underwater pulsed discharge plasma, two kinds of spectroscopic measurements, called the line-pair method and Stark broadening, were carried out. According to the experimental results, the temperature and the electron density in the pulsed discharge plasma between point-plane electrodes immersed in water are determined to be 15000 K and 1018/cm3, respectively.

Negative DC Prebreakdown Phenomena and Breakdown-Voltage Characteristics of Pressurized Carbon Dioxide up to Supercritical Conditions

This paper deals with the experimental results on prebreakdown phenomena and breakdown voltage characteristics of a negative dc point-to-plane gap in-compressed carbon dioxide up to the supercritical pressure as the first step to develop a plasma reactor with supercritical carbon dioxide. The gap length and the curvature radius of the point tip were 200 and around 35 mum, respectively. The experimental results show the following: 1) corona discharge preceding complete breakdown is observed more clearly in liquid and supercritical fluid than in gas; 2) the estimated discharge onset voltage according to the streamer theory is in fairly good agreement with the measured breakdown voltage in the gas density region of 0.1-30 kg ldr m-3; 3) the breakdown mechanism in liquid can be classified into two categories: bubble-triggered breakdown at lower pressure and non bubble-triggered breakdown at higher pressure; 4) the breakdown mechanism in supercritical fluid is similar to that in higher pressured liquid; and 5) the density and temperature dependences of breakdown voltage in liquid and supercritical fluid are related closely with the breakdown mechanism.

High-Performance Pulsed-Power Generator Controlled by FPGA

The high reliability, high repetition rate, high performance, and compactness of pulsed-power generators are required for industrial applications. Also, the control of a pulsed-power generator becomes more complicated with increasing functions. An all-solid-state pulsed-power generator can be controlled by using a field-programmable gate array (FPGA). The pulsed-power generator consists of a charger, a magnetic pulse compression circuit, and a controller using the FPGA. The performance characteristics of the pulsed-power generator, such as the variable firing interval from shot to shot and the diagnosis of incorrect operation, are easily achieved by rewriting the programming of the Verilog hardware description language on the FPGA.

Initiation mechanism of a positive streamer in pressurized carbon dioxide up to liquid and supercritical phases with nanosecond pulsed voltages

In this work, the initiation process of an electrical discharge in pressurized carbon dioxide up to the liquid and supercritical phases was investigated using Schlieren and photomultiplier techniques. A pulsed positive voltage with a rise time of about 40?ns and half-width of 150?ns was applied to a point-to-plane gap. The experimental results showed that the discharge started with a primary streamer consisting of a burst primary streamer and a successive primary streamer, and a back discharge followed them. It was predicted from an analysis of the experimental results that the initiation criterion of a burst primary streamer was electron multiplication on the order of 108 independent of the medium conditions. That is, a direct ionization process without bubble formation is supported as an initiation mechanism of nanosecond positive discharges in pressurized carbon dioxide. The medium state in the burst primary streamer channel was a gas or pseudo-gas of 50?200?kg?m?3 density.

Polarity Effect in DC Breakdown Voltage Characteristics of Pressurized Carbon Dioxide up to Supercritical Conditions

This paper deals with the effect of the polarity and gap geometry on DC breakdown voltage characteristics of a point-to-plane gap in supercritical carbon dioxide (SC CO2) that is required to design a plasma reactor. In the experiments, the gap width d and the curvature radius of the point tip r were changed in the range of 80 to 300 mum and 50 to 170 mum respectively, and the CO2 state was controlled within the gas, liquid, and SC phases. The experimental results showed that a remarkable polarity effect appeared under certain gap conditions. As a result, it was found that negative polarity and a higher stressed electrode are desired conditions for the dc plasma reactor since an active corona supplying rich chemical radicals appears in SC CO2 under such conditions.

Temperature and Nitric Oxide Generation in a Pulsed Arc Discharge Plasma

Nitric oxide (NO) is increasingly being used in medical treatments of high blood pressure, acute respiratory distress syndrome and other illnesses related to the lungs. Currently a NO inhalation system consists of a gas cylinder of N2 mixed with a high concentration of NO. This arrangement is potentially risky due to the possibility of an accidental leak of NO from the cylinder. The presence of NO in the air leads to the formation of nitric dioxide (NO2), which is toxic to the lungs. Therefore, an on-site generator of NO would be highly desirable for medical doctors to use with patients with lung disease. To develop the NO inhalation system without a gas cylinder, which would include a high concentration of NO, NAMIHIRA et al have recently reported on the production of NO from room air using a pulsed arc discharge. In the present work, the temperature of the pulsed arc discharge plasma used to generate NO was measured to optimize the discharge condition. The results of the temperature measurements showed the temperature of the pulsed arc discharge plasma reached about 10,000 K immediately after discharge initiation and gradually decreased over tens of microseconds. In addition, it was found that NO was formed in a discharge plasma having temperatures higher than 9,000 K and a smaller input energy into the discharge plasma generates NO more efficiently than a larger one.

Initiation Mechanism of a Negative Nanosecond Pulsed Discharge in Supercritical Carbon Dioxide

This paper deals with the initiation mechanism of a negative nanosecond pulsed discharge in supercritical (SC) carbon dioxide that was examined in detail using Schlieren and photomultiplier techniques. A negative pulsed voltage with a rise time of about 90 ns and half-width of 410 ns was applied to the point electrode. The experimental results show that: 1) The negative primary streamer was of a form quite different than that of a positive one: a bushlike negative streamer and a filamentlike positive streamer; 2) a drastic change in the density dependence of streamer initiation voltage appeared around the subcritical phase in the characteristics for streamer initiation voltage versus medium density; and 3) a shock wave of speed 1-1.5 Mach began growth following a delay of around 50 ns from the streamer initiation. It was predicted from the analysis of the experimental results that the drastic change in the streamer initiation voltage versus medium density characteristics may be due to a transition between the two types of generation mechanisms of the initial electron: electron detachment from negative ions in the gas phase and field electron emission from the point electrode in the SC and liquid phases. The shock wave was presumed to be caused by thermal relaxation of the vibrational energy in carbon dioxide molecules in a decay process of the primary streamer.

Visualization of positive pulsed streamer in supercritical carbon dioxide by Schlieren Method

This paper deals with visualization of positive pulsed streamers and arc discharge under a needle-to-plane gap in supercritical carbon dioxide (SCCO2) by the Schlieren Method. In addition, differences in streamer branches in gas, liquid and supercritical phases of CO2 are evaluated by means of fractal dimension. The experimental results are summarized as follows: 1) a tree-like streamer growing from the tip of needle was successfully visualized. The field strength for streamer initiation and the mean velocity of the streamer are estimated as approximately 9 MV/cm and 60 km/s, respectively. Pre-breakdown phenomena in SCCO2 are divided into two processes: streamer growth and shockwave expansion. 2) Streamer branches in SC phase were found to be more complicated than in other phase of CO2. 3) Bubble formation in the streamer initiation process was not recognized in SCCO2. For dielectric breakdown, the cylindrical shockwave grows along the streamer channel. A trace of light emission is recognized near the center of the arc column with a residual image of the streamer.

Weibull Statistical Analysis of Pulsed Breakdown Voltages in High-Pressure Carbon Dioxide Including Supercritical Phase

Pulsed discharge plasma in supercritical fluids (SCFs) has attracted a great deal of attention in the field of plasma application for chemical processing. Characteristics of electrical breakdown are one of the important concerns for dielectric reliability of a plasma reactor with SCFs. In this paper, the pulsed breakdown voltages of quasi-uniform gaps were measured by changing the CO2 medium from gas to supercritical phase at a given temperature, and Weibull statistical analysis was applied to the measured breakdown voltages. The area effect on breakdown voltage, which is known to give an equivalent effective constant to Weibull shape parameter m, was examined with three sphere-to-sphere gaps. The experimental results indicate that the dispersion in the pulsed breakdown voltage in pressurized carbon dioxide obeys Weibull distribution, and the Weibull shape parameter m depends on the state of the CO2 medium. The dependence of m on the medium density that was obtained from direct analysis of breakdown voltages for a given electrode system indicated a similar tendency to that of the effective constant by area effect analysis.

Positive Pulsed Streamer in Supercritical Carbon Dioxide

A positive pulsed streamer in a needle-to-plane gap in supercritical carbon dioxide was observed by using a laser schlieren method. The treelike streamer is initiated at a needle electrode when the electric field reaches approximately 9 MV/cm and propagates spatially toward a plane electrode. The streamer is accompanied by the cylindrical and spherical shock waves. A low-density region along the streamer channel decays with time, and no bubble formation occurs finally.