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Basing on the simple form of the energy conservation law and taking into account a multiplication of electrons, we show that the Townsend mechanism of electron multiplication in a gas is valid at sufficiently large interelectrode distance even at so large values of an electric field strength, when it is possible to neglect ionization friction. Correspondingly, the runaway electron producing in a gas is determined not by the local criteria accepted presently, but by the ratio of interelectrode distance and the characteristic electron multiplication length. Basing on numerical simulations for nitrogen gas we show that the critical discharge voltage Ucr(pd ), at which the runaway electrons begin prevail, is a function of the product of the interelectrode distance by the gas pressure pd . This function (escape curve of Ucr-pd dependence) separates the area of an effective multiplication of electrons and the area, in which electrons escape discharge gap not having time to be multiplied. The curve Ucr(pd ) has the upper and lower branches. Using Ucr(pd ) we obtain the analog of well-known Paschen curve, which describes additionally the absence of a self-sustained discharge at a high voltages sufficiently rapidly supplied across the electrodes. Escape curves for helium, xenon and nitrogen are presented.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only. |
