Spectroscopic investigation of impulse corona discharges

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Abstract
The spectra of corona discharges in air at atmospheric pressure, in a 06 m point-plane gap under positive impulse voltage, have been measured in the near-ultraviolet and visible regions. Two types of corona discharges have been investigated: the first corona which develops from the electrode tip; and the leader coronas which develop successively from the leader tip. The spectrum of the first corona appears to be due almost exclusively to molecular nitrogen (second positive system), while that of the leader coronas also contains some weaker radiation of molecular positive ions of nitrogen (first negative system) and oxygen (first and second negative system), together with some of the brighter lines of atomic oxygen. The radiation of molecular oxygen and atomic nitrogen does not appear with a measurable intensity. A comparison of the relative intensities of the N2 second positive and N2+ first negative bands with low-pressure data indicates that under high-pressure conditions the repartition of population in the various vibrational levels is affected by secondary deactivation processes (electronic and vibrational quenching). By taking into account such processes, the relative intensities of a number of these bands have been theoretically calculated for different electron energy distributions. A comparison between the present experimental and theoretical results shows that a Druyvesteyn energy distribution is more likely for the electrons in the avalanches developing in front of the streamer tip. Furthermore, this comparison makes possible the evaluation of the electron mean energy and the average field in these avalanches: they both turn out to be larger in the leader coronas than in the first corona. This difference in the electron mean energy is responsible for the different spectral characteristics of the first and leader coronas.