3. Filament-guided electric discharge

Let's consider another laboratory experiment, where the voltage across the electrodes is gradually increased. When the resulting field between the electrodes reaches 28 kV/cm, a spontaneous electric discharge occurs in the air. This usually occurs where peak effects are greatest. On the other hand, at about 30% below the spontaneous discharge threshold, an electrical discharge can be triggered and guided by a laser filament that makes contact (figure 3 ).

An interesting feature of these guided discharges is their reproducibility. The time between the passage of the laser pulse and the discharge fluctuates very little from one shot to the next, unlike in more conventional laser-induced discharges where no filaments are used. Box 1 explains the mechanism...