Study on Townsend first ionization coefficient in a streamer filament of the pulsed electric discharge in water

Streamers in water usually consist of several weakly ionized gaseous filaments. It is important to understand the ionization and excitation processes occurring in a streamer filament. In this paper, we studied the Townsend first ionization coefficient alpha in a streamer filament. The emission images of streamers in water were acquired by using an ultra-high speed camera system. Based on the emission images, it was found that the luminance of a streamer filament decreased exponentially as the distance increased from the anode tip. The Townsend first ionization coefficient in the streamer filament in water was then estimated by the emission profile method. It was found that the Townsend first ionization coefficient in a streamer filament was 5.3 cm(-1) on average and did not depend on the water conductivity or the applied voltage of interest in the present paper. The Townsend first ionization coefficient estimated by the emission profile method is in agreement with that deduced from the electron density profile of the streamer filament in water, indicating that the emission profile method for measuring the Townsend first ionization coefficient can be applicable to the streamers in water. We also estimated the reduced Townsend first ionization coefficient alpha/N in a streamer filament in water and found that the obtained value for alpha/N was four orders of magnitude lower than those obtained from the glow discharge in low pressure water vapor, but at least ten orders of magnitude greater than those predicted by the empirical formulas.