Spectroscopic investigation of laser-initiated pre-breakdown and breakdown events in dielectric liquids

Knowledge of streamer parameters such as electron and ion density, temperature, electron temperature, and electric field are crucial for understanding and modelling the pre-breakdown phase in liquid electric breakdown. At present experimental data for these quantities are very limited. However, in this contribution it is shown how optical diagnostic techniques can improve the situation. In this paper, spectroscopic measurements on streamers and breakdown arcs in transformer oil are presented. From the emission spectra, properties such as average temperature, average electron and ion density, and electric field in the streamer can be inferred. Spectra from these phenomena have earlier been examined in strongly in-homogeneous geometries or overvoltage homogeneous gaps; however to our knowledge the analysis proposed and performed in this paper is new.