Spectroscopic measurements of streamer filaments in electric breakdown in a dielectric liquid

Emission spectroscopy has been utilized to provide information about the electron density and temperature in streamers and breakdown arcs in transformer oil. Recorded spectra include strongly broadened hydrogen Balmer- lines and vibration/rotation band profiles of the molecule. The origin of the observed broadening of hydrogen lines is discussed and it is concluded that it arises mainly from collisions with charged particles, so-called dynamic Stark broadening. By assuming that the broadening is due solely to dynamic Stark broadening, electron densities between and were obtained for the rear of positive streamer filaments during the later stages of propagation. For negative streamers we obtained an upper limit of and for breakdown arcs electron densities up to . The temperature information in the emission profiles and the intensity ratio of the hydrogen Balmer lines are discussed. Rough estimations of the temperature are presented both for positive and for negative streamers.