Main characteristics of femtosecond-laser induced plasma enhanced by a nanosecond heating pulse.*

A femtosecond-laser induced plasma, enhanced by a nanosecond laser pulse, constitutes a powerful dual-pulse configuration enabling precise control over plasma generation. Applications of such laser-induced plasmas include waveguiding, remote sensing, ignition, and directed energy deposition. This study employs multi-temperature non-equilibrium plasma model to investigate fundamental characteristics of plasma produced by the dual-laser pulse approach.  The developed multi-temperature plasma kinetics model for nitrogen-oxygen is validated and extended to incorporate laser coupling mechanisms such as inverse Bremsstrahlung absorption, avalanche ionization, and photo-detachment induced by the heating pulse. Associative ionization and photo-detachment are identified as significant processes that facilitate coupling between the heating laser pulse and femtosecond plasma filament. Temporal delays between the femtosecond and nanosecond heating pulses are examined to optimize plasma enhancement below optical breakdown.