Characteristics of plasma decay in dual pulse energy deposition for air and nitrogen in atmospheric pressure

Understanding air plasma is of great interest in various applications including directed energy systems, air-breathing plasma propulsion, and atmospheric plasma sources. In this work, we investigate plasma decay characteristics in a dual pulse energy deposition scheme, where initial pre-ionization is achieved with a femtosecond laser pulse and subsequent energy addition is made with a secondary nanosecond laser pulse. A plasma filament is generated at atmospheric pressure in a N₂-O₂ gas mixture with varying oxygen concentrations from 0% to 20%. The results show that as O₂ concentration increases, the early initial plasma decay on a nanosecond scale slows down. However, following the initial decay - over the scale of tens of nanoseconds - the decay accelerates, becoming faster than in the O₂-0% case. Theoretical fits accounting for two-body and three-body recombination reveal enhanced two-body process and suppressed three-body process with increasing O₂ concentration. In the dual pulse energy deposition, we find that the addition of O₂ slows electron decay through associative ionization and photo-detachment processes.