Ryo Sasamoto

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.

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.

Quantitative visualization of gas temperature distribution in atmospheric DC glow corona by spectral image processing

This study presents an experimental method of determining a two-dimensional image of the gas temperature in atmospheric DC glow corona by spectral imaging. A steady-state glow corona discharge as a partial discharge was generated by applying positive DC voltage to rod-to-plane electrode in synthetic air (N2:79%, O2:21%) at atmospheric pressure. Spectral imaging displays an image, and measures its specific spectra at each pixel in the image. The spectral images were taken by an Intensified CCD camera (ICCD) with ultra-narrow band-pass filters, corresponding to head and tail of nitrogen second positive system band (0-2) (2P(0-2)). The gas temperature was estimated from the emission intensity ratio between the head and tail of 2P(0-2). In this result, the change in the distribution of gas temperature in a positive DC glow discharge due to the amplitude of applied voltage was clearly visualized by spectral image processing.

Influence of barrier thickness on discharge behavior in air gap with GFRP insulator under impulse voltage stress

The barrier effect is well known to increase the breakdown voltage of air gap. While, previous research has suggested that the occurrence of secondary discharge at the underside of barrier accompanied by the development of upper surface corona discharge on the barrier contributes to the reduction of the barrier effect. The properties of the underside discharge are, however, still not well understood. This paper presents the backside discharge phenomenon and the influence of barrier thickness on discharge behavior in air gap with solid insulator, especially pertaining to the occurrence of underside discharge. In the experiment, a GFRP board was used as insulation barrier, positive or negative steep impulse voltage was applied to needle-plane short gap with insulation barrier in atmospheric air, the second discharge occurred at the backside of the barrier in response. The spatiotemporal changes of discharge luminescence at underside of barrier were observed by ICCD camera.

Spatial distribution measurement of plasma palameters using spectral image processing

This paper presents an experimental method to determine qualitative two-dimensional images of gas temperature and mean electron energy in stationary atmospheric non-thermal plasma by using spectral image processing. In this work, steady-state glow corona discharge generated by positive DC voltage was observed. The electrode configuration was rod-to-plane and the ambient gas was synthetic air at atmospheric pressure. The changes of gas temperature distribution and mean electron energy distribution due to amplitude of applied voltage and ambient gas pressure were investigated. Spectral image of glow corona was taken by using a gated ICCD camera with ultra-narrow band-pass filters. The qualitative gas temperature and mean electron energy were derived from emission intensity ratios on obtained spectral images. As a result, we confirmed that the gas temperature and its distribution of positive DC glow corona increased with increasing the applied voltage. Especially, just before the spark-over voltage, a distinctly high-temperature region was formed in positive DC glow at the tip of rod electrode. On the other hand, the increase of mean electron energy at the tip of rod electrode was not as large as increase of applied voltage.