Characteristics of plasma decay of a femtosecond laser pulse generated filament in nitrogen and oxygen mixture*

Understanding air plasma is of great interests in various applications such as directed energy system, and air-breathing plasma propulsion. In this work, we investigate the plasma decay characteristics in a femtosecond laser-generated plasma filament in a gas mixture of nitrogen and oxygen. The gas mixture is generated in a glass cell using two mass flow rate controllers, one for N₂ and one for O₂. The plasma decay is monitored by microwave Rayleigh scattering. At the atmospheric pressure, the addition of oxygen increases the initial peak density of the generated filament and slows down early initial plasma decay. With increased oxygen composition, the decay rate accelerates on the tens of nanoseconds scale, the decay time-constant decreasing from τ∼17 ns with no oxygen to  τ∼10 ns with 20% oxygen. Using a self-consistent multi-dimensional plasma kinetic model [1], we reproduce the experimentally observed dynamics and discuss the plasma behavior in detail.

[1] S. Pokharel and A. A. Tropina, J. Appl. Phys. 134, 223301 (2023).