High-speed videos were recorded using a Vision Research Phantom VEO 410L 1MP camera. In all experiments, the camera was located on the side of the cell, i.e., with the line of sight perpendicular to the line between the electrodes (see Fig. in the SI) For imaging the ignition process and the stable vapor layer, a frame rate of 1000 fps, an exposure time of 50 µs and a resolution of 1280 × 800 pixels were chosen. Both the ignition process and the vapor layer were imaged during the same experiment, where a small ring light was used for background illumination. For imaging the individual discharges, a frame rate of 57 000 fps, an exposure time of 17 µs, and a resolution of 256 × 256 pixels were chosen.