Obtaining new information about different forms of self-sustained dc discharges that can be realized in pin-to-plane electrode geometry in ambient air is the goal of this paper. Experimental and numerical calculation data uncovering the physics of the temporal and spatial evolution of the negative corona and glow discharge (GD), with increase in current up to the transition to the spark, are presented. Special attention is paid to the properties of diffusive GD at atmospheric pressure, which is a necessary stage (steady-state or transient) preceding the spark and determining the threshold conditions of sparking.

The results of numerical calculations on a steady-state constricted discharge in N2 flow at atmospheric pressure are presented. Basic elementary processes responsible for sustaining the constricted discharge at low and high currents are found. It is shown that the charged particle generation in both regimes is controlled predominantly by an associative ionization . However, metastable states are created in these regimes by different processes. In low-current discharge N2(A) and N2(a′) metastables are created due to mutual collisions of the vibrationally excited molecules, and their collision frequency is determined by the vibration energy distribution function. In high-current discharge these metastables are excited by energetic electrons, and inelastic collision frequency is determined by the electron energy distribution function. The charged particle dynamic balance in the high-current constricted discharge in atmospheric pressure N2 is non-local and sustained by ionization and ambipolar diffusion like that in a low-current diffusive discharge in a tube at low pressure. It was demonstrated that blowing of the discharge by longitudinal gas flow leads to a more pronounced constriction.

The results of a numerical study on the spatio-temporal behavior of transient microdischarges (MDs) in a steady-state dielectric barrier discharge (DBD) excited by a sinusoidal voltage are presented. MDs have a spatial 'memory'-every subsequent MD appears at exactly the same location occupied by the MD at the preceding half-period (HP). In the majority of cases each MD appears at its location only once during every HP. For such a case, the memory effect is not attributed to the residual surface charge deposited by the preceding MD but determined by the residual MD plasma column shunting the gap right up to the beginning of the next HP. In contrast to good memory in space, each individual MD has a large scatter with time in its appearance within every HP, i.e. there is no 'memory' concerning the phase of an applied voltage. This MD jittering within the period is attributed to the stochastic nature of partial surface breakdowns around the bases of the MD plasma column. Numerical calculations show that surface breakdown provides an MD current splash at every HP. Hence, in the steady-state DBD, the volume plasma is responsible for the existence of MD spatial 'memory' (i.e. where the MD appears), and the deposited surface charge is responsible for MD jittering in time (i.e. when the MD appears).

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