Stepwise Development of a Positive Long Spark in the Air

Abstract

A spark discharge is modelled in a long air gap of atmospheric pressure at a positive voltage with a long front duration, excluding the quasi-continuous development of the leader. It is shown that, with a front steepness of less than 30 kV/μs, an ionization wave moves in the gap, whose speed is 2 orders of magnitude slower than the speed of streamers in the streamer zone of the leader. The electric field behind the front of such a wave along the entire length of the formed primary channel is kept within 20–25 kV/cm, providing an electron density of 10¹¹ cm^–3 for hundreds of microseconds. The state of the gas in the primary channel changes dramatically with the propagation of a disturbing field effect of a nanosecond in duration. The disturbance range to the head of the primary channel initiates the development of a streamer flash from this head with the initial velocity of ~10⁹ cm/s, which leads to a sharp increase in the brightness of the radiation from the channel. The reason for the amplification of radiation is the active production of electronically excited particles. The gas temperature in the channel does not exceed 1000 K. According to characteristics such as the gas temperature, longitudinal electric field, and electron density, the channel in the step phase of the development of a long spark differs fundamentally from the leader’s channel, which is able to exist and develop only in the quasi-continuous phase.