SPATIALLY AND TEMPORALLY RESOLVED ELECTRON-DENSITY MEASUREMENTS IN STREAMERS IN DIELECTRIC LIQUIDS

Spatially and temporally resolved spectroscopic measurements of light emitted from positive streamers in transformer oil are presented. Anal yses of the measurements performed with a DC needle-plane gap yield el ectron densities and indications of the atomic excitation temperatures in the streamers. The hydrogen emission reveals an electron density b elow 10(16) cm(-3) during the main part of the streamer propagation ti me (80-90%). Later the light is also characterized by emission from a high-density plasma with electron densities in the range 10(18)-10(19) cm(-3). The electron density during this time increases approximately linearly with distance from the initiation point and a density factor of four higher has been measured at the streamer tip than at the root . Measurements with high spectral resolution detect both high and low electron densities simultaneously. A tentative model of the interior o f the streamer plasma, spatially resolved, is presented.