Dynamic contraction of the positive column of a self-sustained glow discharge in molecular gas flow

We study dynamic contraction of a quasineutral positive column of a self-sustained glow discharge in nitrogen and air in a rectangular duct with the convection cooling. A set of time-dependent two-dimensional equations for the nonequilibrium weakly-ionized plasma is formulated, and then solved numerically. Transition from diffusive state to contracted one is analyzed. It is shown that in nitrogen the contraction occurs in the so-called ``hard'' or hysteresis mode while in air the character of the transition depends on pressure. Under relatively high pressure, 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 in air occurs in the hysteresis mode. In such a case the inhomogeneous contraction can occur, when a high density plasma channel starting from the initial perturbation near one electrode propagates to the opposite electrode. The discharge evolution of such kind is computed for a case of self-sustained glow discharge in nitrogen. The results are in agreement with experiment.