Kiyoto Nishijima

Numerical Analysis of Electrical Breakdown Induced by Laser Irradiation in N2/O2 Gas Mixture

This paper presents a theoretical investigation on the electrical breakdown process of air induced by intense pulsed laser beams. We develop a computer simulation model including both the Boltzmann equation and the rate equations. The calculations are performed for different laser wavelengths of 308 nm (XeCl), 694 nm (ruby), 1.06 µ m (YAG) and 10.6 µ m ( CO2). The result on the laser wavelengths is qualitatively consistent with the experimental result. The laser-induced breakdown process is discussed in detail.

Sparkover characteristics of air-CO 2 mixed gas in nonuniform field gap

The atmospheric carbon dioxide concentration has increased since widespread industrialization began. Moreover, there are currently places where the carbon dioxide concentration in the air is high, for example, inside tunnels and forest fire area. Hence, it is important to research the effect of the CO2 concentration on the insulation characteristics of air. This study examined the sparkover characteristics of air-CO2 mixed gases. Two electrodes were used: a rod 5 mm in diameter and a plane electrode 400 mm in diameter. The gap length was 62.5 mm. Synthetic air (nitrogen: 79%, oxygen: 21%) was used in the sparkover tests, and the sparkover voltage was measured in several air-CO2 mixtures. High DC and AC voltages were applied to the electrodes in the sparkover voltage measurement tests. Streamers were examined under several conditions using an image intensifier. The results show that the sparkover voltage depends on the CO2 concentration. Specifically, when the CO2 level increased 0.01% from 0.75% to 0.76%, the sparkover voltage fell rapidly by approximately 20 kV (from 54.3 kV to 33.7 kV) under a positive DC voltage. Similarly, the sparkover voltage also depends on the CO2 concentration under a negative DC voltage and a high AC voltage.

Kinetic Model of Gas Heating of Laser-Produced Plasmas by CO2 Laser in Atmospheric Air

A kinetic model was developed for theoretical investigations on laser-induced breakdown and gas heating in an N2/O2 gas mixture. This model calculates particle densities, electron energy, electron rate coefficient, vibrational temperature, gas temperature, and the other plasma parameters. The gas temperature was calculated from the potential differences in all chemical reactions. The analyses were performed for an N2/O2 mixture of 1 atm irradiated with a O2 laser. The results show that the electron density of 1023 m-3 is the threshold of electrical breakdown and gas heating, and that the electron-ion recombination and the collisional de-excitation of vibrationally excited N2(X1Σg+) have a strong effect on the gas heating.

Formation of Nonbranched Positive Streamer With High-Frequency Impulse Voltage in Atmospheric Air

The transition phenomenon of branched positive streamer to single filamentary discharge (SFD) with increase of the discharge-frequency was studied using a fast intensified charge coupled-device camera and an image converter camera. In addition, the gas temperature in SFD was measured by spectroscopic measurement. As a result, it was found that the transition to SFD progresses with localized gas heating along discharge channel. Furthermore, it was revealed that SFD is a nonbranched positive streamer.

Influence of gas heating and product molecules on discharge characteristics in dry-air-fed ozonizer

Influences of gas heating and product molecules on discharge characteristics in dry-air-fed ozonizer was investigated. Especially, the dependence of gas temperature in streamer channel on occurrence frequency of streamer discharge was investigated in detail. In the experiment, repetitive positive or negative impulse streamer was generated respectively between needle-plane electrodes in synthetic air at atmospheric pressure. The repetition rate of the applied pulse voltage was controlled from 0.1 to 7.0 kilo-pulse/sec (kpps). The temperature of streamer channel was estimated from the emission spectrum of N2 2nd positive system band (0-0). As a result, the gas temperature increased with increasing the pulse repetition rate in both polarities. In the case of positive polarity, the gas temperature increased from 341 to 383 K in response to the increase of repetition rate (0.1 to 6.0 kpps). Eventually, sparkover occurred at 7.0 kpps due to the increase of reduced electric field (E/N) caused by gas heating. The product ozone concentration was saturated with increasing the pulse repetition.

Localized Residual Heat and Formation of Nonbranched Positive Streamer With Highly Repetitive Streamer Discharge

The mechanism of a mode change from a branch-type to a nonbranch positive streamer with increase in pulse repetition rate was investigated from the point of view of background gas heating. In this experiment, a repetitive streamer was generated between a needle and a plane electrode. The repetition rate of impulse positive streamer discharge was changed from 500 to 10 500 pulse/s. The branching of a streamer decreased with the increase in pulse repetition rate and the streamer shape completely changed to a nonbranched type at 10 000 pulse/s. The gas temperature in the repetitive streamer was quantitatively measured at different pulse repetition rates by a spectroscopic method using N2 second positive system band (0, 0). In addition, the gas density change between the electrodes due to gas heating caused by the repetitive streamers was observed using the laser schlieren method. As a result, it was confirmed that the gas temperature in repetitive streamers increased with the increase in pulse repetition rate. The gas temperature in a streamer increased up to 420 K by the accumulation of background gas heating at 10 000 pulse/s. Furthermore, residual gas heat in the shape of a single filament due to a highly repetitive streamer and its dissipation process were clearly visualized.

Qualitative gas temperature distribution in positive DC glow corona using spectral image processing in atmospheric air

An experimental method of determining a qualitative two-dimensional image of the gas temperature in stationary atmospheric nonthermal plasma by spectral image processing was presented. In the experiment, a steady-state glow corona discharge was generated by applying a positive DC voltage to a rod-plane electrode in synthetic air. The changes in the gas temperature distribution due to the amplitude of applied voltage and the ambient gas pressure were investigated. Spectral images of a positive DC glow corona were taken using a gated ICCD camera with ultranarrow band-pass filters, corresponding to the head and tail of a N2 second positive system band (0–2). The qualitative gas temperature was obtained from the emission intensity ratio between the head and tail of the N2 second positive system band (0–2). From the results, we confirmed that the gas temperature and its distribution of a positive DC glow corona increased with increasing applied voltage. In particular, just before the sparkover voltage, a distinctly high temperature region was formed in the positive DC glow at the tip of the rod electrode. In addition, the gas temperature decreased and its distribution spread diffusely with decreasing ambient gas pressure.

Gas Heating and Streamer-to-Leader Transition of Impulse Surface Discharge on Quartz Glass in Atmospheric Air

Flashover on the surface of dielectric materials often causes extensive damage to the insulation system, resulting in reduced withstand voltage and system downtime as the damaged component is located and replaced. Therefore, understanding the flashover mechanism on dielectrics is of great significance to reduce incidences. In this work, the change in gas temperature in an impulse surface discharge during streamer-to-leader transition was measured in atmospheric air by the spectroscopic method. In the experiment, a quartz glass having a conductive indium tin oxide-coated back-surface was used as an insulation material having a back electrode. As a result, the increase in gas temperature in a discharge channel was confirmed when the discharge phase transformed into a leader.

The occurrence of streamer corona and space charge field measurement in positive DC glow corona discharge with laser-triggering method

In the condition of positive polarity DC glow corona discharge for rod-plane and wire-plane gap arrangement in the atmospheric air, when this ion drift space is irradiated by pulsed-laser, streamer corona will be induced with high probability. The occurrence of the streamer corona and its propagation process were observed with high-speed camera in detail. From above results, we can see that electron-burst which is produced from negative ions by laser photo-detachment process, drifts to glow corona area and streamer corona is induced, and we knew also that streamer corona is consisted by primary streamer and secondary streamer. Further, different position of the discharge axis is irradiated by laser, we obtain the time lag of the induced streamer corona. By this way, it is verified that the measurement of the space charge electric field in the ion drift space is possible.

Laser-induced breakdown characteristics of nonuniform dc corona discharge fields

Laser-induced breakdown of nonuniform field gaps in atmospheric air was caused by an XeCl excimer laser. This laser beam was focused on the center of a rod-to-plane, needle-to-plane or hemisphere-to-plane gap with dc voltage. The laser energy dependency of breakdown voltage was investigated for positive and negative applied voltages and gap lengths of 15 to 25 mm. It was found that the positive breakdown voltage was higher than the negative one in the hemisphere-to-plane gaps or the rod-to-plane gaps with low voltages, where there is little or no space charge. However, the effect of polarity is reversed in the rod-to-plane gaps with large spacings and the needle-to-plane gaps. On the basis of the dependence of the discharge mode on this polarity effect, it can be seen that it was caused by the difference in development of positive and negative streamers in the corona discharge. >