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The prebreakdown phenomena and the formation process of the glow discharge in a low-pressure Ar gas were investigated under a uniform field gap. Prebreakdown phenomena were observed for 0.5 Torr cm⩽pd⩽2 Torr cm (where p is pressure, d the gap distance) in Ar gas under conditions of a slowly increasing voltage. It was observed that the prebreakdown phenomena formed pulse discharges up to the transition to the glow discharge. The amplitudes of the photon and current pulses due to the pulse discharge increased with time, and then decreased as soon as the transition to a steady glow discharge occurred. When the overvoltage or external series resistance was increased, the pulse amplitudes increased with the applied voltage and decreased with the resistance. The characteristics of the prebreakdown phenomena were changed by the shape of the electrodes. The formation mechanism of the glow discharge can be qualitatively explained by that of the streamer in a high-pressure discharge. The transient glow discharge wa...

Current wave forms of initial stages of breakdown and corona formation in a short negative point-to-plane gap have been measured with a nanosecond time resolution in air, O2, and N2 at pressures 13.33–100 kPa, and at various overvoltages. The experiments revealed the existence of a stepped form of negative corona Trichel pulses in O2 at atmospheric pressure. To test existing models for the negative corona pulse formation, effects of changing cathode secondary electron emission were studied using a brass cathode coated by CuI and graphite. It is concluded that a negative corona (Trichel) pulse is associated with the ignition of a cathode-directed streamer in the immediate vicinity of the cathode and the subsequent formation of a glow-discharge-type cathode region at the streamer arrival to the cathode. The implications of these results to negative corona applications are discussed.

One of the failures related to gas-insulated switchgear is caused by the poor contact between conductors. This causes breakdown between the contact and the wall of chamber, initiated by the small discharge due to the poor contact. In this paper, the modeled experimental apparatus for the contact portion of the gas-insulated switchgear assumed a rod-to-plane gap with small arc current. The influence of the arc current on the breakdown into the chamber was investigated experimentally, and the breakdown mechanism between the contacts and the wall was studied. It was clarified that the breakdown voltage between the contact and the wall decreased by increasing the current, and the small arc current in the contact portion played an important role in the breakdown between the contact and the wall. The reduction ratio of the breakdown voltage increased with increasing gap length of the small gap. The breakdown voltage was different depending on the duration of the small arc current.

Breakdown phenomena of gas‐insulated gaps with nanosecond pulses were examined by using an image converter camera. The physical appearance, i.e., luminosity of a gap, was investigated on streak photographs taken by the image converter camera. These investigations show that the breakdown mechanism of gas‐insulated gaps with nanosecond pulses can basically be explained by a conventional avalanche‐to‐streamer development theory.

One of the failures related to gas-insulated switchgear is caused by the poor contact between conductors. This causes breakdown between the contact and the wall of chamber, initiated by the small discharge due to the poor contact. In this paper, the modeled experimental apparatus for the contact portion of the gas-insulated switchgear assumed a rod-to-plane gap with small arc current. The influence of the arc current on the breakdown into the chamber was investigated experimentally, and the breakdown mechanism between the contacts and the wall was studied. It was clarified that the breakdown voltage between the contact and the wall decreased by increasing the current, and the small arc current in the contact portion played an important role in the breakdown between the contact and the wall. The reduction ratio of the breakdown voltage increased with increasing gap length of the small gap. The breakdown voltage was different depending on the duration of the small arc current.

Current wave forms of the first Trichel pulses and initial stages of negative point-to-plane breakdown in carbon monoxide have been measured at a pressure of 13.33 kPa. The results include a report of the intricate pulse wave form with a step on the pulse leading edge and a current hump on the pulse trailing part, which indicates that the physical mechanism for the Trichel pulse phenomenon is more complex than the generally accepted description in terms of Morrow's theory [Phys. Rev. A 32, 1799 (1985); 32, 3821 (1985)]. A tentative explanation of the observed pulse wave form based on the presumed existence of a cathode-directed ionizing wave is outlined.

The prebreakdown phenomena and the resulting transient processes from the prebreakdown phenomena to the glow discharge in low-pressure N2, O2 gases, and N2/O2 mixtures were investigated. When the applied voltage was a lower overvoltage ratio less than about 2%, nonprebreakdown phenomenon was observed, and the overvoltage ratio was independent of the gas materials. This phenomenon, however, was observed at the overvoltage ratio of higher than about 3%. The prebreakdown phenomenon in N2 gas differed from those in O2 gas, and formed pulseless components. On the other hand, those in O2 gas formed pulsed components. The prebreakdown phenomenon in N2 gas changed to a pulsed discharge by mixing it with a small amount of more than 0.3% O2 gas. These transient processes from the prebreakdown phenomena to the glow discharges were also observed by a high-speed video camera. As a result, the formation processes of the glow discharge became clear.

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.

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.

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.

The electron temperature in a 3-mm ID narrow-discharge-tube noble-gas positive-column plasma was measured. The experiments were conducted for the gas pressure from 0.1 to 10 Torr and the discharge current from 10 to 40 mA. The results show that except for helium gas, the electron temperature in a narrow-discharge-tube positive column does not depend monotonically on gas pressure, whereas significant discharge current dependence has been observed.

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.

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.

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.

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.

Measurements have been made of the initial stage of breakdown in a short negative point-to-plane gap in helium at pressures of 50-600 Torr. The transient glow discharge establishment 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. The results are interpreted as a possible indication for the unification of these breakdown-phenomenon mechanisms on the conceptual basis of the primary cathode-directed streamer.

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.

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 Morrow's 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 Morrow's theory.

A large-current hydrogen ion source has been developed by using a sheet plasma. The cathode of the discharge tube developed by the authors was lanthanum hexaboride (so called LaB6), which provided a stable plasma current of up to 30 A. The sheet plasma was formed by applying a pair of flat permanent magnets to a discharge plasma column. The electron density and the electron temperature in the sheet plasma were measured by a probe method. It was observed that the values of the electron temperature and the electron density distributed uniformly along the width direction and those values were about 3.8 eV and 1×1010 cm-3, respectively. On the other hand, the half width of the sheet plasma was about 14 mm in the thickness direction. It was clear that the arc current and the gas evacuation capability were important factors used to obtain the high density sheet plasma.

Waveforms of the first negative corona current pulses in a 70% N2+30% SF6mixture 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.

Measurements have been made of the initial stage of breakdown in a short negative point-to-plane gap in argon at pressures of 50-600 Torr. The transient glow discharge formation was found to be preceded by a peaked current signal of conspicuous similarity to the Trichel pulse rise and its initail decay in an electron-attaching gas. The results are compared with Morrow's theory of the stepped Trichel pulse.