Streamers in liquid nitrogen: characterization and spectroscopic determination of gaseous filament temperature and electron density

Streamers initiated in point-plane electrode geometry under impulse voltage are studied. Characterization up to 30 kV in both polarities shows two streamer types. In negative polarity, only slow `bush-like' streamers with a velocity of about 100 m s-1 are observed. In positive polarity, both `bush-like' and filamentary streamers appear. Positive `bush-like' streamers have similar velocity as in negative polarity, whereas filamentary streamers have a propagation velocity up to 30 km s-1. This high velocity is correlated to large transient current and light emission. Spectroscopic study of the light emitted by filaments shows mainly the second positive system of nitrogen. This system allows us to estimate the gas temperature inside a filament by making a comparison with a numerically simulated spectrum. A temperature close to 500 K is found. Dilution of hydrogen gas in liquid nitrogen allows us to determine also the electron density by Stark broadening of the Balmer Hα line. An electron density of about 1.1018 cm-3 is found.