Modeling of fast ionization waves in pure nitrogen at moderate pressure

The fast ionization wave (FIW) discharges in pure nitrogen in a capillary tube at 27 mbar, initiated by positive polarity, high-voltage nanosecond pulses, are numerically studied by coupled two-dimensional plasma fluid modeling. The 2D fluid code based on local mean energy approximation is validated and used, an extended three-exponential Helmholtz photoionization model is proposed for pure nitrogen. The development and structure of the nitrogen FIW is analyzed, the electric field and current are calculated compared with experimental measurements. The evolution of radial distribution of electrons and N2(C 3Π u ) during the FIW development stage and in the afterglow are analyzed, the radial profiles of electron density show a 'hollow' structure in the FIW development stage, the temporal-spatial evolution of N2(C 3Π u ) is dominated by the competition between the pooling reaction of N2 (A3Σu+) and the quenching by electrons. The role of photoionization on the nitrogen FIW radial morphology is discussed, the equivalent background electron density of photoionization in nitrogen discharge is suggested to be (4-6) × 1013 m-3. Spatial distribution of specific energy deposition (SED), fast gas heating (FGH) energy and temperature rise are obtained, heating efficiency varies with electric field E/N and SED and tends to be 10% at high SED. The dominating reactions responsible for FGH and their fractional contribution in space and time are analyzed, in the near axis region, pooling reactions of N2 (A3 Σu+) and N2(B 3Π g ) contribute up to 80% FGH energy, electron impact dissociation of molecular nitrogen contributes about 10%, while e-N2+ recombination and quenching of N(2 D) atoms by N2 molecules contribute rest.