Pulsed positive corona streamer propagation and branching

The propagation and branching of pulsed positive corona streamers in a short gap is observed with high resolution in space and time. The appearance of the pre-breakdown phenomena can be controlled by the electrode configuration, the gas composition and the impedance of the pulsed power circuit. In a point-wire gap the positive corona shows much more branching than in the parallel plane gap with a protrusion. In air, the branching is more pronounced than in argon. The pulsed power circuit appears to operate in two modes, either as an inductive circuit creating a lower number of thick streamers or as a resistive circuit giving a higher number of thin streamers. A possible cause for branching is electrostatic repulsion of two parts of the streamer head. The electric field at the streamer head is limited, the maximum values found are ~170 kV cm/sup -1/ in air and ~100 kV cm/sup -1/ in argon. At these maximum field strengths, the electrons have 5-10 eV energy, so the ionization is dominated by two-step processes. Differences between argon and ambient air in the field strength at which streamers propagate are ascribed to the difference in de-excitation processes in noble and molecular gases. The fact that the pulsed power circuit can control the streamer structure is important for applications, but this effect must also be taken into account in fundamental studies of streamer propagation and branchin

Analysis of a Cu-Ne hollow cathode glow discharge at intermediate currents

\u3cp\u3eIn a copper-neon hollow cathode glow discharge the authors have measured several parameters. At pressures between 250 and 1000 Pa and cathode current densities between 0.01 and 0.1 A cm\u3csup\u3e-2\u3c/sup\u3e the authors obtained gas temperatures between 900 and 1400K with use of Doppler broadened line profiles. The electron transport temperature was obtained from the time dependent optogalvanic effect to be 0.2 to 1.2 eV. Electron densities were derived from Stark broadening of line profiles recorded with saturated absorption spectroscopy; values were between 1 and 9*10\u3csup\u3e19\u3c/sup\u3e m\u3csup\u3e-3\u3c/sup\u3e. Copper ground state densities have been found with the absorption of resonance radiation and were between 0.2 and 6*10\u3csup\u3e19\u3c/sup\u3e m\u3csup\u3e-3\u3c/sup\u3e. Densities of several excited states, including the neon metastables, were measured with the absorption of dye laser radiation; metastable density was of the order of 10\u3csup\u3e18\u3c/sup\u3e m\u3csup\u3e-3\u3c/sup\u3e. They are compared with values obtained from a collisional radiative model. It turns out that ionisation and excitation of neon is caused by beam electrons, ionisation of copper occurs by charge transfer and loss of ions is caused by ambipolar diffusion only. These processes are incorporated in a transport model which has been solved using a sputtering condition at the cathode boundary. In this sputtering condition thermal diffusion of copper atoms and the sticking coefficients of copper atoms and ions are included. The results of the model are in good agreement with the measurements.\u3c/p\u3

Experiments on how photo- and background ionization affect positive streamers: oxygen concentration, repetition and radioactivity

Positive streamers in air and other oxygen-nitrogen mixtures are generally believed to propagate against the electron drift direction due to photo-ionization. Photo-ionization is the non-local ionization of O2-molecules by UV radiation from excited N2-molecules. This facilitates the streamer propagation by creating free electrons ahead of it. The relative importance of photo-ionization depends on the O2/N2 ratio. Another possible source of free electrons in front of the positive streamer is background ionization. This can be ionization left by previous discharges or by other processes such as cosmic rays or (natural) radio-activity. We study the effects of both photo- and background-ionization on propagation and morphology of positive streamers by changing gas composition and repetition frequency. One particular gas composition is pure nitrogen with a small amount of radio-active 85Kr added to increase background ionization

Streamers in different gases including planetary atmospheres and the role of photo-ionization

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Streamer branching in a short gap: the influence of the power supply

The formation of streamers in a 25-mm gap in air is studied with an intensified charge coupled device camera with high resolution in space and time. Strong branching is observed and streamers reach the cathode in an area that is much wider than the gap length. Switching the high voltage with either a spark gap or a semiconductor stack has a large influence on the branching: much more discharge channels appear when using the semiconductor switch. Pictures taken with 0.8-ns resolution show that the streamers propagate with 3 ms/mm near the electrodes and with 0.5 mm/ns in the middle of the ga