Time-resolved measurement of electron energy distribution function in secondary streamer in atmospheric-pressure air using Thomson scattering

Abstract The temporal evolution of the electron energy distribution function (EEDF) in a 2 Hz pulsed positive secondary streamer in atmospheric-pressure air was measured using the laser Thomson scattering (LTS) method. The rotational Raman scattering signal, which was superimposed on the LTS signal, changed because of the fast gas heating in the streamer. The photo-detachment of electrons from negative ions resulted in stronger LTS signal intensities and slower decay rates than expected based on the actual electron density. However, this did not affect the EEDF measurements, because the photo-detached electrons rapidly relaxed thermally. The non-Maxwellian EEDF corresponding to a high reduced electric field E / N ≈ 130 Td was observed in the initial phase of the secondary streamer ( t = 20 ns ), which subsequently decreased to E / N ≈ 80 − 100 Td and remained nearly constant throughout the later phase ( t = 40 − 100 ns ). These findings indicate that the production rates of several reactive species are higher in the initial phase of the secondary streamer, rather than in the later phase. This paper also presents important considerations for the application of the LTS method to atmospheric-pressure plasmas, particularly under conditions in which the air density is significantly higher than the electron density.