The stem evolution during the dark period is important for subsequent corona pulses or leader initiation. To this end, a physical model is built to simulate the stem evolution, in which the migration process of space charge and thermodynamic processes of the stem channel (heat conduction, heat convection and thermal diffusion) are both considered. The effect of voltage rise rate, electrode shape, and air pressure on the stem evolution during dark period is investigated. Results show that the space charge migration contributes to the stem elongation on the axial length and be more influential than thermodynamic processes at high voltage rising rates. Moreover, the stem elongation in the axial direction is more evident with a smaller curvature radius of the electrode tip. In addition, as the air pressure decreases, the central temperature of the stem channel rapidly decreases, and the heat dissipation is dominant. This study can help to further understand the characteristics of the dark period and its effect on subsequent discharges.

Long air gap discharge, Positive leader, Stem, Dark period, Numerical simulation