Fig. 1. (a) Plasma channel paths revealed by the plasma spots. (b) A detailed view of the plasma spots distribution after long exposures. The laser pulses propagate from right to left.

Fig. 2. Schematic of the conductivity measurements and the measured current waveforms at different laser energies. The inset shows the currents within 20 ns.

Fig. 3. (a) Breakdown voltages of spontaneous discharges and laser-induced discharges at different gap lengths. (b) Spontaneous discharge and induced discharges by pulses of 100 mJ in energy, where the laser pulses propagate from right to left and all the discharge lengths are 18 cm.