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

Abstract

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.

Optical emission spectroscopy of electrical discharge machining plasma

“…(1) cannot be used. The temperature value of 0.7 eV is a low electron temperature, but consistent with previous studies on EDM plasmas [8,9] and other similar plasmas [13,16,17]. The plasma is very dense, and thus electrons rapidly lose energy by numerous collisions.…”

“…If then the plasma column expands, density and pressure decrease. Like temperature measurements, these density measurements are consistent with EDM simulation [8] and works on similar plasmas [13,16,17]. On the other hand, the plasma pressure found is lower than the pressure calculated with the simulation [8].…”

“…The type of light emission evolution measured in Fig. 8 is characteristic of a fast filamentary positive streamer [17,20]. Before the breakdown, the filamentary streamer grows rapidly and emits a weak light.…”

“…Studies of pre-breakdown phenomena in dielectric liquids show that different types of streamer exist [17,20]. Their structures, velocities and current depend on many parameters such as the discharge voltage and current, the electrode shape and polarity, the gap distance, the chemical and physical properties of the dielectric and its contamination, for example.…”

Prebreakdown and breakdown phenomena under uniform field in liquid nitrogen

“…[I]. Rather few work was devoted to the study of streamers in LN2 [2,3,4,5]. Streamers were studied using divergent field (point-plane gaps).…”