The role of corona and space charges during femtosecond laser pulse filament guided high voltage discharges in air

Summary form only given. The plasma column left behind by ultrashort laser pulse filamentation is utilized to guide high voltage (HV) discharges in air. Many experiments have been carried out where the filament plasma is placed between two HV electrodes and a discharge is guided across the large gap (for example Ref [1]). Researchers still believe that one application could be potentially guiding lightning in the atmosphere, like a lightning rod, due to the longitudinal extent of the plasma ranging over several 10's of meters [2]. However lab experiments can only demonstrate discharges over a few meters. The reason for this is still not very clear due to the overall physics of the process not being fully understood. Here, we present experimental results in air on the role of space charges and corona during the filament guided HV discharge. Our conclusion is that the main driver of the breakdown is an enhancement of the corona between the electrodes and that the electric fields available limit the discharge distance achievable.Filamentation is a non-linear process where non-linear selffocusing (due to the Kerr effect), diffraction, and plasma defocusing create a dynamic balance [3]. The laser pulse propagates in a small beam diameter longer than the usual Rayleigh length. The plasma left behind the filament has a density on the order of 1016 cm-3 and a low conductivity. Initially it was believed that the plasma could directly guide HV discharges. However, a delay between the actual discharge and the filament placement is observed, ruling out the direct wire-like guiding mechanism. Previously, the reason for this delay was not completely understood. Our experimental results show that an enhancement of the formation of corona at the HV probes can be observed. From there, the dynamics rely on the electric field as a driver for developing leaders across the gap until a conducting connection is formed between the electrodes. These results show that the discharge distance is limited by the electric field and therefore limits the usefulness for the applications originally intended.