The multi-avalanche nature of streamer formation in inhomogeneous fields

The streamer formation in a positive point-plane gap both in air and nitrogen has been studied. The primary electrons were created by laser radiation. This gave the opportunity to initiate streamers in a wide range of voltages including the range of a steady corona in air as well as to determine the formative time lags. A streamer starts when the space charge density reaches a critical value in a spatially localized region. In a divergent field the streamer formation is preceded by the accumulation of space charge, i.e. it has a multi-avalanche nature. A model for calculation of formative time lags is presented. For a virgin gap the model describes fairly the accumulation process. For the voltage range of steady corona in air only the qualitative description of the mechanism is given.