Yuki Inada

Highly sensitive Shack–Hartmann sensor for two-dimensional electron density imaging over extinguishing arc discharges

Highly sensitive Shack–Hartmann-type laser wavefront sensors incorporating meniscus microlens arrays with a long focal length of 238 or 467 mm were developed for imaging two-dimensional electron density distributions over extinguishing atmospheric arc discharges. The use of the novel microlens arrays also had the advantage of realizing the spatial synchronization of the measuring system. The highly sensitive Shack–Hartmann sensors were successfully used for the single-shot imaging of two-dimensional electron density distributions over the extinguishing arc discharges with currents of several amperes and were proven to have improved the measurement sensitivity by two orders of magnitude from 1023 to 1021 m−3. Spatiotemporal evolution of the electron density images showed that just before arc extinction the electron densities at outer radial positions with respect to the interelectrode gap were higher than those on the geometrical axis defined by the electrodes.

Simultaneous imaging of two-dimensional electron density and air-flow distribution over air-blast decaying arc

Sensitive Shack–Hartmann type laser wavefront sensors were applied to simultaneous imaging of two-dimensional electron density and air-flow distributions over decaying arc channels under air blasting with several pressures. Our experimental results showed that higher blasting pressures facilitated the rapid reduction of arc diameters and an increase in the electron densities around the gap centre due not only to the thermal pinch effect but also to air-flow disturbances, although there were no significant effects of the air blasting on the arc conductance.

Intense-Mode Vacuum Arc Characterization by Using 2-D Electron and Vapor Density Image

Shack-Hartmann type laser wavefront sensors were applied to simultaneous single-shot imaging of 2-D electron and copper-vapor density distributions over intense-mode vacuum arc discharges with a pulsed current waveform of 800 A in peak and 24 μs in damping time constant. A parametric analysis of the ion current based on our experimental results suggested that the intense-mode arcs included highly energetic ions moving from cathodes to anodes with a high velocity of 104 m/s and their proportion was larger than or comparable to slow ions with 10-1 m/s drifted from anodes to cathodes by the electric fields. Furthermore, hydrodynamic calculation demonstrated that the copper vapor in the vacuum plasmas dissipated almost instantaneously in a time scale of ~1 μs and it was not residual metal medium but fresh one continuously supplied from the electrodes.

Electrode material dependence of two-dimensional electron and vapour density distribution over vacuum arc discharge

Electrode material dependence of intense-mode vacuum arc behaviour was systematically investigated by using the Shack-Hartmann method capable of simultaneously visualising two-dimensional electron and metal vapour density distributions from single-shot recordings. The electrode materials studied included Cu, CuCr (Cu75Cr25 wt. %), WC, and AgWC (Ag40WC60 wt. %). A comparison between the Cu and CuCr electrodes showed that the metal vapour densities for the CuCr decreased in an even shorter time scale than for the Cu. In the case of the WC electrodes, the widths of the electron density distributions became narrower as the arc current decreased although the electron densities hardly decreased in the decaying process of the arc current. The density measurements conducted at the late stage of the vacuum arcs demonstrated that the metal vapour densities around the anode were maintained at the highest value for the AgWC among the electrode materials in this study.

Instability phenomenon of high-frequency vacuum arc discharge associated with anode activity

Instability phenomena of high-frequency vacuum arcs with an ∼100-kHz current waveform and active anodes were observed by using high-speed photography and optical emission spectroscopy and analyzed with a conventional theoretical model. Our experimental and theoretical studies demonstrated that the instability mechanism for the transient vacuum arcs was completely different from that for the well-known quasi-stationary diffused arcs, where a cathode process predominantly contributes to the instability phenomena. In the case of the high-frequency vacuum arcs, the instability phenomena were mainly induced by a collaborative anode process operated by anode evaporation and anode voltage drop.

Simultaneous measurement of two-dimensional electron and vapour density distribution over vacuum arc

Shack-Hartmann type laser wavefront sensors were applied to simultaneous imaging of two-dimensional electron and copper vapour density distributions over decaying vacuum arc discharges with a pulsed current waveform. Our experimental results showed that the proportions of the electron densities in the total particle densities became higher with increasing time, while the absolute values of the electron and copper vapour densities decreased over the measurement time.

Temperature of hydrogen radio frequency plasma under dechlorination process of polychlorinated biphenyls

It has been reported that RF (radio frequency) hydrogen plasmas promote the dechlorination process of PCBs (polychlorinated biphenyls) under irradiation of MW (microwave). A relative emission intensity spectroscope system was used for single-shot imaging of two-dimensional temperature distributions of RF hydrogen plasmas generated in chemical solutions with several mixing ratios of isopropyl alcohol (IPA) and insulation oil under MW irradiation. Our experimental results showed that the plasma generation frequencies for the oil-contaminating solutions were higher than that for the pure IPA solution. In addition, the plasma temperature in the compound liquids including both oil and IPA was higher than that in the pure IPA and oil solutions. A combination of the plasma temperature measurements and plasma composition analysis indicated that the hydrogen radicals generated in a chemical solution containing the equal volumes of IPA and oil were almost the same amounts of H and H+, while those produced in the ot...

Two-dimensional electron density visualization over plasma flare in vacuum breakdown process

Spatiotemporal evolution of plasma flares in a vacuum breakdown process was characterized by various optical techniques including Shack-Hartmann type laser wavefront sensors for two-dimensional ele...

Fundamental study on re-ignition process for CO 2 -blast arcs in a model circuit breaker using synthetic tests highly controlled by power semiconductors

This paper reports fundamental measurement results on re-ignition process in synthetic tests for CO2 arcs in a gas blast nozzle. The synthetic test system used comprises a DC current source for an arc ignition, a half cycle AC current source, a DC current source for artificial current zero point and an impulse-like voltage source for application of quasi-transient recovery voltage (quasi-TRV). The electric current and voltage were switched by power-semiconductor-switches to guarantee high time-accuracy of the arc current injection and voltage application. After current zero point, quasi-TRV with a peak of 7.5 kV was applied between the electrodes with a precise specified delay time td to judge arc re-ignition. The results indicated that td = 25 μs involves 60% probability of arc re-ignition. The re-ignition was inferred to originate in thermal mode from joule heating by a small current injection to the residual arc during the quasi-TRV application.

Fundamental studies on switching arcs — Experimental and numerical approaches

Both experimental and numerical approaches have been done on switching arc plasmas for fundamental studies. In the experimental approaches, power-semiconductor switching has been used to control intentional current injections and voltage applications to switching arcs with a high accuracy in time. The systematic experiments provided the interruption probability property for different gas kinds and gas flow rates, as well as the dielectric recovery properties between the electrodes. Time evolutions in electron density were derived by Laser Thomson Scattering (LTS) and Shack-Hartmann (SH) method for different gases. On the other hand, for this nozzle space, numerical models were developed to simulate arc dynamic behaviors with and without local thermodynamic equilibrium (LTE) assumptions for different gases. It was found that the time evolution in electron density derived by the chemically non-equilibrium model is in good agreement with the experimental results by LTS.