Production of extended plasma channels in atmospheric air by amplitude-modulated UV radiation of GARPUN-MTW Ti : sapphire – KrF laser. Part 2. Accumulation of plasma electrons and electric discharge control

Abstract

The problem of the production of extended () plasma channels is studied in atmospheric air by amplitude-modulated laser pulses of UV radiation, which are a superposition of a subpicosecond USP train amplified in a regenerative KrF amplifier with an unstable confocal resonator and a quasi-stationary lasing pulse. The USPs possess a high () peak power and efficiently ionise oxygen molecules due to multiphoton ionisation, and the quasi-stationary lasing pulse, which has a relatively long duration (), maintains the electron density at a level by suppressing electron attachment to oxygen. Experiments in laser triggering of high-voltage electric discharges suggest that the use of combined pulses results in a significant lowering of the breakdown threshold and enables controlling the discharge trajectory with a higher efficiency in comparison with smooth pulses. It was shown that controlled breakdowns may develop with a delay of tens of microseconds relative to the laser pulse, which is many orders of magnitude greater than the lifetime of free electrons in the laser-induced plasma. We propose a mechanism for this breakdown, which involves speeding-up of the avalanche ionisation of the air by negative molecular oxygen ions with a low electron binding energy () and a long lifetime (), which are produced upon cessation of the laser pulse.