High Spatial Resolution Measurement of Electric Field in Positive Secondary Streamer Discharge under Atmospheric-Pressure Air*

 Electric field measurement of non-equilibrium atmospheric-pressure plasma is crucially important for systematic understanding of the production mechanisms of reactive species. However, the electric field measurement has never been achieved for a streamer discharge under atmospheric-pressure air, due to its irreproducible, complex branching structure. Further, low spatial resolution of conventional electric-field sensors has prevented precise determination. Here, the electric field measurement was first achieved for the streamer discharge by the development of two original apparatuses: a multi-anode plasma generator capable of reproducibly realizing a non-branching, straight streamer discharge and an electric-field induced second harmonic (E-FISH) generation sensor with one-order-superior spatial resolution. The precise electric field measurement demonstrated that in an initial phase of the secondary streamer discharge, the electric field inside the discharge was 100 Td, which was comparable to previous simulation results. On the other hand, the electric field in the later phase showed a decrease with increasing time and finally, it reached as low as 70 Td. Such a gradual decrease has never been predicted in the previous simulation studies. The unpredicted time evolution of the electric field suggested that many reactive species were generated selectively in the initial phase, e.g., N₂(C), which is the main source of optical emission from the secondary streamer discharge.