Die Entwicklung der Elektronenlawine in den Plasmakanal, untersucht mit Bildverst�rker und Wischverschlu�

Abstract

The application of high gain image intensifier and image converter streak shutter techniques to investigations of faintly luminous early stages of gaseous discharges discloses in some detail the development of electron avalanches into anode-and cathode-directed streamers as soon as the critical amplification (n~ l0 s) is attained in the gap. The velocities of both of the streamer branches were measured quantitatively as to the dependence on pressure (p)*, field (E/p) and gas (Nitrogen and various mixtures), typical values being 0.5 ... 1 -10 a cm/sec. We find that streamer propagation is suitably described in a coordinate-system drifting with the electrons (driftvelocity v_ ~ 1 -10 7 em/sec) and that the way it depends on the discharge parameters suggests that ionization by electron impact (in the eigen-spacecharge distorted field) and photoionization are the determining effects. --The cathode-directed streamers show a considerable intensification and acceleration as soon as they are encountered by cathodic secondary avalanches, which however are obviously not essential for the initiation of the process in itself. --The streak images give also an impression of persistingexcitation and ionization processes in the region of the streamer-paths and of the transition into the beginning of the bright spark, correlated with several ionizing fronts observed moving towards anode or cathode with velocities even > 10 a cm/sec.(The paper includes also details of the gaseous near UV-radiation which was utilized for registration with image intensifier or converter, especially with respect to the decaytime of the excited states.)The results obtained are essentially a verification and to some extend completion of predictions made on the basis of early cloud chamber investigations.

Positive- and Negative-Pulsed Streamer Discharges Generated by a 100-ns Pulsed-Power in Atmospheric Air

“…The enhanced and accelerated streamer stage was observed in both directions; the significantly fast and steep ionization waves were found to start at the cathode as well as at the anode on arrival of the respective streamers. The measured velocities were up to 10 mm/ns [8], [9]. In addition, the emission from positive pulsed streamer discharges in air at atmospheric pressure was observed with an intensified charge-coupled device camera with a high-speed gate in previous work in which the streamer propagation process and its propagation velocity in a coaxial electrode were reported [11].…”

Field-enhanced propagation of corona streamers

“…The negative streamer advances in a diffusive and selfchoking manner, making propagation into the lower-field region away from the point source relatively difficult. Wagner [1966] found that fields of 3 X 10 • v m -• or more are necessary for long-range propagation of negative streamers formed in the middle of a uniform-field gap. However, the details of the positive streamer results provide us with some important insight regarding the physics controlling their propagation.…”

An Experimental Test of the Streamer Breakdown Criterion

“…At voltages close to the minimum breakdown potential the spark channel has been shown to develop from the constriction of the diffuse glow discharge formed from the superposition of many generations of electron avalanches (Cavenor and Meyer 1969). At large overvoltages, however, single avalanches have been observed to develop into luminous conducting filaments, called streamers, which completely bridge the electrode gap within the transit time of the initial avalanche (Wagner 1966).The criterion proposed for the occurrence of this streamer mode of development is that the charge carrier number in the initial avalanche should attain a value of some lO9 ions and electrons and that the electrons be concentrated within the diffusion radius of the avalanche. Under such conditions it is suggested that enhancement of the electric field by the space charge, in the vicinity of the avalanche, is sufficient to allow gas ionizing radiation to extend the ionized region simultaneously towards both electrodes (Raether 1964).…”

“…At voltages close to the minimum breakdown potential the spark channel has been shown to develop from the constriction of the diffuse glow discharge formed from the superposition of many generations of electron avalanches (Cavenor and Meyer 1969). At large overvoltages, however, single avalanches have been observed to develop into luminous conducting filaments, called streamers, which completely bridge the electrode gap within the transit time of the initial avalanche (Wagner 1966).…”