Time-dependent modeling on initiation of pre-ionized laser filament in atmospheric air under an external field far below conventional threshold

Abstract

In this paper, atmospheric laser filament initiation is simulated under an external field 5.7 kV/cm, which is far below the conventional breakdown threshold (25–30 kV/cm). The time-dependent model is based on a self-consistent solution to a coupled electron energy distribution function, vibrational kinetics, gas thermal balance, as well as detailed air chemistry. The model is validated with the experiment results on plasma density decay both with and without an external field. The slow decay of plasma density is ascribed to the decreased electron recombination and increased detachment rate under an external field. The initiation process is facilitated by the increase in the electron temperature, gas heating, and efficient vibrational excitation under the external field. We have varied the detachment rate between oxygen negative ions and nitrogen vibrational states to study its role on the initiation process, and found that larger detachment rates lead to shorter breakdown delay times. If the detachment rate is taken to be 3 × 10−11 cm3 s−1 as an upper limit, the breakdown threshold and delay time are consistent with the previous experiment study at least semiquantitatively. This work is beneficial for studying the initiation of a preformed laser filament under an external field as well as potential applications in lightning initiation and guiding below the conventional gas breakdown threshold.