Dynamic contraction of the positive column of a self-sustained glow discharge in nitrogen/air flow

In this paper the dynamic contraction of the current channel inside a quasineutral positive column of a self-sustained glow discharge in nitrogen and air in a rectangular duct with convection cooling is studied. A set of time-dependent two-dimensional equations for the nonequilibrium weakly-ionized air plasma is formulated, and then solved numerically. The formation and decay of negative ions in air is taken into account. Transition from diffusive to contracted state that occurs in the fluxes of weakly ionized nonhomogeneous plasma is analyzed. It is shown that in nitrogen the contraction occurs in the so-called "hard" or hysteresis mode while in the air the transition to the contracted state depends on the pressure. Under relatively high pressure the hysteresis does not occur and the transition takes place in so called "soft" mode. When the pressure reduces, and thus the role played by electron attachment diminishes, the transition occurs in the hysteresis mode. It is shown that the critical current increases when the pressure drops. Therefore contracted and diffusive states in the current that flows along density gradient of the background gas could coexist in the discharge.