Figure 2

Time evolution of the ionization front in an (a)–(f) air-filled, (g)–(l) CO\(_{2}\)-filled point-plane gap. Panels have been labelled with the moment in time the image was recorded, while the symbol printed on the needle electrode indicates the polarity of the applied voltage (top rows are positive, bottom rows are negative). Showing dU/dt = 50 kV/ns only.