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Your search keyword '"Yamashita, Takahiko"' showing total 41 results
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41 results on '"Yamashita, Takahiko"'

3. Anomalous plasma temperature at supercritical phase of pressurized CO2 after pulsed breakdown followed by large short-circuit current

Author

Furusato, Tomohiro, Ashizuka, Naokazu, Kamagahara, Takeshi, Matsuda, Yoshinobu, Yamashita, Takahiko, Sasaki, Mitsuru, Kiyan, Tsuyoshi, and Inada, Yuki

Subjects
optical emission spectroscopy, pulsed arc discharge, pulsed power, breakdown, supercritical fluids
Abstract

The relation between breakdown characteristics and plasma temperature of a pulsed arc discharge in highly pressurized CO2 was investigated up to a supercritical phase. A transient arc discharge was generated by applying nanosecond pulsed voltage with a rising rate of 0.7 kV/ns to a point-to-plane gap of 1 mm. The breakdown voltage, arc current, and consumption energy increased with the CO2 density in the gas phase. However, they were constant at the CO2 densities in the supercritical phase. The plasma temperature determined from the blackbody radiation ranged from 8000 to 12000 K and its dependence with respect to the CO2 density was all similar to the breakdown voltage, arc current, and consumption energy characteristics except under the critical density of CO2 (469 kg/m3). Although the breakdown voltage, arc current, and consumption energy in supercritical phase were also constant against the CO2 density, the plasma temperature demonstrated a local decrease around the critical density. The anomaly of the plasma temperature is consistent with the calculated result of the isochoric specific heat of CO2 which has a local maximum around the critical density., IEEE Transactions on Dielectrics and Electrical Insulation, 25(5), pp.1807-1813; 2018

Published
2018

4. Observation of Shock Waves in Supercritical CO2 Generated by Nanosecond Pulsed Arc Discharge Plasma

Author

FURUSATO, Tomohiro, INADA, Yuki, MATSUDA, Yoshinobu, and YAMASHITA, Takahiko

Subjects
Shock Wave, Pulsed Power, Discharge Plasma, Supercritical CO2, Shadowgraph method
Abstract

2019年度衝撃波シンポジウム (2020年3月4日-6日. 神戸大学 海事科学研究科（深江キャンパス)）神戸市, 兵庫県, Symposium on Shock Waves in Japan, FY2019 (March 3-4, 2020. Kobe University), Kobe, Hyogo Japan, 2019年度衝撃波シンポジウムは、新型コロナウイルスの感染拡大防止のため開催中止, 資料番号: SA6000148052, レポート番号: 1D3-3[若]

Published
2020

5. Shape of contacting surface between an electrolytic solution and local discharge on it

Author

Matsuo, Hisao, Yamashita, Takahiko, and Fujishima, Tomoyuki

Subjects
Electric insulators -- Research, Business, Electronics, Electronics and electrical industries
Abstract

To develop a propagation model of a local discharge on a wet polluted insulator, the authors have examined the propagation characteristics and the propagation conditions of the local discharge on an electrolytic solution simulating a wet polluted insulator surface. To calculate the relation between the propagation length of the local discharge and the time required for the propagation, an electric field distribution near the tip of the local discharge must be known. In this paper, the authors have examined a voltage distribution in an electrolytic solution to obtain the electric field. As a result, the shape of the contacting surface between the local discharge and the solution is obtained under impulse voltage. A calculation of the voltage distribution is performed with the shape. A comparison between calculated values and measured values of the distribution is given. Index Terms--Local discharge, Wet polluted insulator, Electrolytic solution, Voltage distribution.

Published
2003

8. Propagation velocity and photoemission intensity of a local discharge on an electrolytic surface

Author

Matsuo, Hisao, Fujishima, Tomoyuki, and Yamashita, Takahiko

Subjects
Electrolytes -- Research, Electric insulators -- Research, Electric discharges -- Research, Photoemission -- Research, Business, Electronics, Electronics and electrical industries
Abstract

An electrolytic solution surface was used to simulate the surface of wet polluted insulators. A local discharge propagates on the solution surface when an impulse voltage is applied to the surface. The intensity of photoemission from the tip of the local discharge, the propagation velocity of the local discharge, and the discharge current were measured simultaneously. It was found that the propagation velocity increases with the photoemission intensity and the resistivity of the solution, and the influence of the air pressure and the applied voltage waveform on the velocity is very small. Furthermore, it was deduced that the ratio of the current through the local discharge tip to the total discharge current is not influenced by the local discharge length and the applied voltage waveform.

Published
1996

12. Shock-wave propagation in supercritical CO2 induced by nanosecond-pulsed arc plasma.

Author

Furusato, Tomohiro, Inada, Yuki, Sasaki, Mitsuru, Matsuda, Yoshinobu, and Yamashita, Takahiko

Subjects
PLASMA torch, SPECIFIC heat capacity, MACH number, SHOCK waves, VACUUM arcs, SUPERCRITICAL carbon dioxide
Abstract

Shock waves generated by arc plasma in supercritical (SC) CO2 have the potential to create novel reaction fields. However, there have been few studies of shock-wave characteristics in SC-CO2. This study provides the results of visualization of shock-wave propagation in SC-CO2 generated by nanosecond-pulsed arc plasma. A propagating cylindrical shock wave originating from a discharge channel was observed using time-resolved shadowgraph imaging. The shock wave separated from the cylindrical dark-region induced by pulsed arc plasma within 64 nanoseconds. The Mach numbers of the shock waves were investigated against the medium density of the CO2 ranging from a high-pressure gas phase to the SC phase. The Mach number reached a local maximum at the critical CO2 density level. The anomaly in the Mach number can be explained by the local maximum of the specific heat capacity ratio of the CO2 at close to the critical condition of SC-CO2. [ABSTRACT FROM AUTHOR]

Published
2020
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39. Electric Potential near a Tip of a Local Discharge on a Conductive Film.

Author

Matsuo, Hisao, Yamashita, Takahiko, and Fujishima, Tomoyuki

Subjects
*ELECTRIC insulators & insulation, *INSULATING materials, *THIN films, *ELECTRIC potential, *ELECTRIC currents, *ELECTRICAL conductors, *ELECTRIC waves
Abstract

To develop a propagation model of a local discharge on a wet polluted insulator, the authors have examined characteristics of the local discharge on an electrolytic solution simulating the wet polluted insulator surface. The local discharge was propagated on the solution under the impulse voltage. A shape of a contacting surface between the local discharge and the solution has been already obtained by the authors. By using the contacting surface, an electric potential near a tip of the local discharge on a conductive film of 1 mm in thickness was calculated. Furthermore, the potential was measured with a probe. The calculated values and the measured values of the electric potential agreed well regardless of the propagation length of the local discharge, the width of the conductive film, the resistivity of the conductive film, the peak value of the applied voltage and the applied voltage waveform. [ABSTRACT FROM AUTHOR]

Published
2006
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