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Plasmas on liquids have provided significant applications in material, environmental, and biological sciences. The mechanisms of these chemical reactions in liquids have been primarily described by the plasma–liquid interactions and convection by an electrohydrodynamic flow. Although shock waves play a significant role in radical formation, agitation and cell destruction, not much information is available on underwater shock waves induced by the surface discharge on water. In this study, an underwater shock wave generated by the pulsed surface discharge on water using the laser shadowgraph method has been demonstrated. The results reveal that the shock wave generated by the discharge on water was transmitted into the water. The mean velocity of the shock wave reached 1.7 km s−1. The results indicate that the surface discharge accelerates the reaction in the water by the combined action of the underwater shock wave and the plasma reaction at the air–water interface. The results are expected to aid in the understanding the mechanisms of existing applications, such as decomposition, synthesis, and sterilization., Journal of Physics D: Applied Physics, 55(11), art.no. 11520; 2021

Pulsed surface discharge on water has been given attention as a water purification technology and material synthesis method. The contribution of this study is to clarify the plasma state of a positive pulsed surface discharge on water. Although the pulse width of the current dramatically decreased with an applied voltage ranging from 10 to 20 kV under a conductivity of water of 1 mS cm-1 and a water depth of 2 mm, the plasma state in the vicinity of the needle electrode was a local thermodynamic equilibrium (LTE) state. Validation of the LTE was performed by the McWhirter criterion. On the other hand, the propagating discharge along the water surface was supposed to be in non-LTE state. Consequently, the pulsed surface discharge on water was in mixed states of LTE and non-LTE., Japanese Journal of Applied Physics, 59, art.no.SHHA07; 2020

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., Journal of Physics D: Applied Physics, 53(40), art.no. 40LT01; 2020

Propagation of a surface discharge on water is evaluated using simulation results of the voltage/current waveforms and discharge contact area. Voltage/current waveforms are calculated using an exponential function which assumes the resistance of water decreases with increasing discharge contact area. A conductive disk having a given potential is used to model the discharge contact area and the temporal variation of the radius of the disk is discussed as the discharge propagation. The calculation in the current field is replaced by a calculation of the electrostatic field using the similarity between current and electrostatic fields. The calculation of the electrostatic field is conducted by a charge simulation method. The discharge phenomena are classified into two stages which are the breakdown of needle-to-water gap and the surface discharge propagation on water. The electrostatic field calculation at needle-to-water gap is performed to determine the initial discharge contact area. Expansion of the contact area in the creepage direction, is evaluated by the electric field calculation at the edge of the conductive disk. The expansion terminates when the field becomes lower than 26 kV/cm. The velocity of the expansion increases with the applied voltage. The maximum contact radius increases with decreasing conductivity of water under the same applied voltage. The tendency of simulation results of the expansion of the contact area are consistent with the previous observation results of the discharge propagation., IEEE Transactions on Dielectrics and Electrical Insulation, 26(2), pp.439-446; 2019

Solid insulated switchgear (SIS) has been developed as a substitution for sulfur hexafluoride gas insulated switchgears in the medium voltage class.Its main circuit including a vacuum interrupter is coated with epoxy resin.An embedded electrode was employed in order to enhance the surface insulation performance for further miniaturization and higher stress design. In the previous studies, the surface insulation performance was examined by using a model electrode system which modified the electrode system of the moving part of SIS and the estimation method of the surface breakdown voltage was proposed.In addition, the simulation of the surface breakdown characteristics of a model electrode system was conducted and a design method was proposed. In the present study, the insulation performance of the solid part was taken into account and the insulation design was conducted using the proposed design method. The 'equivalent insulation distance' was introduced and the partial discharge inception field strength was also taken into account.The insulation design was done for 22, 33 and 77 kV systems and the values of the design parameters were provided. In addition,it was found that two different types of the design concepts were available., IEEE Transactions on Dielectrics and Electrical Insulation, 26(1), pp.56-63; 2019

This paper describes the current dependence of dry-band discharge. Model electrodes that simulated the dry-band was introduced. The dependence was examined on the voltage and current waveforms, emission spectra and their intensities, rotational temperature, etc. As a result, two different discharge types, continuous and intermittent discharges, were observed in a half cycle of applied ac voltage. They were characterized by continuous current and light emission, and by pulsative current and intermittent light emission, respectively. The duration of the continuous discharge decreased with the value of series resistance, while that of the intermittent discharge increased. The rotational temperatures of the continuous discharge and the intermittent discharge were about 6000 K and 1500 K, respectively. The spectral emission intensity related to the electrolyte during the continuous discharge was higher compared with the intermittent discharge, and decreased with the value of series resistance. The spectral emission intensity related to nitrogen molecule during the intermittent discharge was almost constant irrespective of the value of series resistance. The characteristics of the continuous and intermittent discharges corresponded to those of the terminated partial arc and spark (scintillation), respectively., IEEE Transactions on Dielectrics and Electrical Insulation, 25(1), pp.154-161; 2018

The authors propose a new method for evaluating the contact area of a surface discharge on water by analyzing the resistance of water solution. This method supplements optical observations of light emissions and the Schlieren method. The contact area was modeled as a disk electrode, and the theoretical resistance of water was calculated under the assumption of a mutual similarity between the current and electrostatic fields. A simulated electrostatic field was calculated using a common charge-simulation method. The experimental values of the resistance of water were obtained by dividing the measured voltage drop at water by the measured current. The contact area evaluation was performed by comparing the experimental and theoretical resistance values. The contact area increases with increasing the applied voltage and is practically independent of water depth., IEEE Transactions on Plasma Science, 46(6), pp. 2079-2084; 2018

Solid insulated switchgear (SIS) has been developed as a substitution for medium voltage SF6 gas insulated switchgear. Its main circuits including a vacuum interrupter are coated with epoxy resin and a solid/gas insulation system is formed. For further miniaturization and higher stress design, the authors investigated the surface insulation performance of the solid/gas insulation system containing an embedded electrode. In the previous study, the authors investigated the surface breakdown characteristics. The authors also investigated the surface discharge propagation characteristics and proposed an estimation method of the surface breakdown voltage. In the present study, the authors investigated the surface breakdown characteristics numerically. The calculation was done based on the estimation method proposed previously. The divided voltage ratio, the radius of the embedded electrode, the enhancement factor of the surface insulation strength and the peak value of the surface breakdown voltage are examined. As a result, the characteristics of the parameters and the criteria for the optimum insulation design were clarified., IEEE Transactions on Dielectrics and Electrical Insulation, 24(5), pp.3055-3062; 2017

Complexity of branching pattern and OH production of water surface discharges was investigated by comparing nanosecond water surface discharge (NWSD) and microsecond water surface discharges (MWSDs) that were defined as NWSD and MWSD, respectively. Experimental and analyzed results between NWSD and MWSD under comparable maximum discharge length l d are summarized as follows: 1) NWSD showed the greatest complexity of branching pattern by fractal analysis; 2) electron density of NWSD was approximately two times greater than MWSD and both orders were 10 -17 cm -3 ; 3) emission intensity of OH (A-X) from MWSD was greater than NWSD; 4) rotational temperature of NWSD was almost constant around 1000 K irrespective of l d and rotational temperature of MWSD increased with increasing l d ranging from 2000 to 4000 K. It was found that the complexity of discharge pattern on water may be affected by the field intensity at water/air boundary. OH production was presumed to be caused by thermal dissociation in this experimental condition., IEEE Transactions on Plasma Science, 45(4), pp. 711-717; 2017

This paper reports on the dependence of voltage rise rate on positive streamer branching and shock waves in supercritical carbon dioxide. Voltages with rise rates of 0.21 and 4.5 kV/ns were applied to a needle-To-plane electrode. Prebreakdown phenomena involving streamer growth and shock wave propagation were observed by means of a shadowgraph method. The results show that the spread angle of the streamer at the needle tip under 4.5 kV/ns was nearly twice as large as that under 0.21 kV/ns. The discharge may initiate without density reduction due to electrostrictive force. While shock wave Mach number was little affected by the voltage rise rate, it increased by increasing the negative voltage peak. Consequently, the velocity of shock wave is presumed to be influenced by the population of the vibrational state of carbon dioxide., IEEE Transactions on Plasma Science, 44(12), pp.3189-3195; 2016

Solid insulated switchgear (SIS) has been developed as a substitution for medium voltage sulfur hexafluoride (SF6) gas insulated switchgears. Its main circuit including a vacuum interrupter is coated with epoxy resin. Therefore, a solid/gas insulation system is composed. For further miniaturization and higher stress design of SIS, improvement of the insulation performance is required. In the solid/gas insulation system, the breakdown strength along the surface is lower than that of solid. Then, the authors investigated the surface insulation performance of the solid/gas insulation system containing an embedded electrode. In the present study, the sizes of the embedded electrode and the solid insulation material were varied and the partial discharge inception voltage and the surface breakdown voltage were measured. As a result, it was found that the surface breakdown voltage varied considerably with the radius of the embedded electrode. From the result, it was deduced that the surface insulation performance could be improved by choosing the size of the embedded electrode appropriately. Moreover, the effects of the thickness of the insulation material and of the surface distance on the surface insulation performance were clarified., IEEE Transactions on Dielectrics and Electrical Insulation, 23(2), pp.787-794; 2016

Solid insulated switchgear (SIS) has been developed as a substitution for medium voltage SF6 gas insulated switchgear. Its main circuits including a vacuum interrupter are coated with epoxy resin, therefore, a solid/gas insulation system is formed. For further miniaturization and higher stress design of SIS, the authors investigated on the surface insulation of the solid/gas insulation system containing an embedded electrode. In the present study, the surface breakdown voltage was estimated based on the surface discharge propagation characteristics. From the measurement of the surface flashover voltage on a dielectric against the surface distance, the relation between the surface flashover voltage and the surface distance was found to be expressed with an equation derived from the empirical equation presented by Toepler. Then, the surface breakdown voltage was estimated based on the obtained equation. The estimated values were compared with the experimental data obtained varying a parameter of the radius of the embedded electrode, thickness of the epoxy resin plate or the radius of the plate. As a result, a good correlation was obtained. Therefore, the estimation method of the surface breakdown voltage on the solid/gas insulation with embedded electrode was established., IEEE Transactions on Dielectrics and Electrical Insulation, 23(5), pp.3026-3033; 2016