In-situ spectroscopic measurements of streamer properties (electron density and temperature) in dielectric liquids

Spatially and temporally resolved spectroscopic measurements of light emitted from positive streamers in transformer oil have been performed. Analysis of the measurements yields electron densities and indications of the atomic excitation temperatures in the streamers. The emission spectrum from streamers in a DC needle-plane gap was recorded using an optical fibre and a spectrograph with a gated intensified diode array as detector. The measurements highlight how the streamer plasma changes in time from having an electron density of less than 10¹⁶ cm^-3 increasing during the last microsecond before breakdown in parts of the streamer up to 10¹⁹ cm^-3. The results can be interpreted as a phase change occurring prior to breakdown with a change in the radial structure of the streamer plasma. This in turn could perhaps be evidence of a transition to a pre-phase of a leader or a “fast event”. The electron density was determined from the dynamic Stark broadening of the hydrogen Balmer-alpha emission line. The results obtained is not severely influenced by electric fields from space charges in or around the streamers