Tatsuzo Hosokawa

First Negative Corona Pulses in 70% N2+30%SF6 Mixture

Waveforms of the first negative corona current pulses in a 70% N2+30% SF6 mixture at pressures of 13.33, 26.66 and 40 kPa have been studied as a function of overvoltage ranging from 0 to 100%. Near the corona threshold, the pulses, taking the form of a simple hump, are governed by the secondary electron emission due to a positive-ion impact. At moderate overvoltage, the formation of a double-peaked pulse was observed. This phenomenon can be explained in terms of the effect of photoemission and secondary electron emission. At higher overvoltages, peculiar current waveforms were observed. This phenomenon was explained by the association of the development of a cathode-directed streamer and its interaction with a cathode surface.

Nanosecond‐pulse breakdown of gas‐insulated gaps

Breakdown characteristics (50% probability of flashover voltage versus pressure‐length characteristics) by impulse voltages with half‐peak widths, 30 and 74 ns, respectively, were investigated at uniform field gaps in the atmospheric gases such as air, N2, and O2. Our investigation shows that under impulse voltages with short durations such as 30 and 74 ns, which seem to have an influence on streamer formation, Paschen’s law is valid at gap lengths l=1 and 2 cm, and streamer criteria are satisfied within an electron developing distance, 0.048–0.18 cm, from the cathode.

Similarities between the initial phase of a transient nonuniform glow discharge in nitrogen and the negative corona Trichel pulse formation in an electronegative gas

Measurements have been made of the initial stage of breakdown in a short negative point‐to‐plane gap in nitrogen at pressures of 50–750 Torr and at various overvoltages. The transient glow discharge regime was found to be preceded by a peaked current signal of conspicuous similarity to the Trichel pulse rise and its initial decay in an electronegative gas.

Breakdown phenomena of gas‐insulated gaps with nanosecond pulses

Breakdown phenomena of gas‐insulated gaps with nanosecond pulses were examined under the spot illumination of the ultraviolet rays on the cathode surface in the atmospheric gases such as air, N2, and O2. Our investigation reveals that the 50% probability of flashover voltage obtained under the spot illumination of the ultraviolet rays shows very close values under the entire illumination on the cathode surface, and the observed time lag of flashover may be explained by the conventional avalanche‐to‐streamer development theory.

Positive‐streamer‐like phenomena in point‐plane corona gaps: Trichel pulses and high‐pressure cathode sheath instabilities

Current pulse shapes have been measured in negative and positive corona discharges mostly in dry air. Negative corona Trichel pulses and pulses corresponding to the primary streamer/cathode contact in a positive point‐plane gap were found to exhibit remarkable similarities, indicating a positive‐streamer‐like mechanism for the Trichel pulse development. High‐frequency instabilities of the high‐pressure filamentary glow discharge, which can be responsible for the arcing from freshly polished cathodes, were observed and attributed to local positive‐streamer‐like breakdowns of the cathode sheath.

Initial Phases of Negative Point-to-Plane Breakdown in N2 and N2+10%CH4: Verification of Morrow's Theory

Measurements have been made of initial phases of breakdown in a short negative point-to-plane gap in nitrogen and in a N2+10%CH4 mixture at pressures of 25~100 Torr in order to verify Morrows theory of stepped pulses in negative corona discharges. This theory explains the step observed on the current growth (Trichel pulse) wavefront in terms of independent photon and ion secondary processes at the cathode. The comparison of current growth waveforms in N2 and in a N2+10%CH4 mixture, where photoemission is suppressed, shows good qualitative agreement with Morrows theory.

The formation of a positive corona in air

An investigation has been made of the photon pulses and the associated current pulses in the pre-onset stage of a positive point corona in air at atmospheric pressure. The transition from burst pulses to streamer pulses is discussed.

Prebreakdown Phenomena and Formation Processes of Low-Pressure Glow Discharges in Ar, Ar/N2 and Ar/O2 Mixtures

In this research, both the prebreakdown phenomena and the transient processes from the prebreakdown phenomena to the glow discharge in low-pressure Ar gas, Ar/N2 and Ar/O2 mixtures were investigated by electrical and optical methods. The prebreakdown phenomenon in the Ar/N2 mixture was similar to that of Ar gas, and a pulseless component has been observed. The prebreakdown phenomenon in the Ar/O2 mixture differed from those in the Ar gas and Ar/N2 mixture, and formed a pulse discharge. Thus, the prebreakdown phenomenon in the Ar gas has been changed to pulse discharge by mixing a small amount of O2 gas which played an important role in the formation of pulse discharge.

Breakdown mechanism of a laser triggered spark gap in a uniform field gap

Breakdown mechanism of a laser triggered spark gap was studied in a uniform field gap. The laser beam was focused on the electrode with positive and negative polarities, and vaporized metal plasma produced by the laser irradiation was used as a trigger. Our investigation shows that the breakdown characteristics in the uniform field are essentially different from those of a needle‐to‐plane gap, and also shows different characteristics under positive and negative polarities.

Waveforms of Prebreakdown Primary Streamers in a Short Positive Point-Plane Gap in Air

Current pulses of single and repetitive prebreakdown streamers have been measured in a positive point-plane gap in dry air at pressures of 3.33–26.7 kPa and at various overvoltages. The pulses of the single streamers exhibited a complex double-peaked form and, in contrast to the commonly held belief, differed significantly from those of the repetitive streamers. To explain the complex waveforms observed, a qualitative physical picture of the processes occurring during the streamer-cathode contact is outlined.