Extended lifetime of high density plasma filament generated by a dual femtosecond-nanosecond laser pulse in air

A substantially extended lifetime of a high-density plasma channel generated in the wake of an intense femtosecond pulse propagating in air is experimentally demonstrated. Free electron density above 10¹⁵ cm^-3 in the formed plasma filament is measured to be sustained for more than 30 ns. This high-density plasma lifetime prolongation of more than one order of magnitude is achieved by properly timed irradiation of the filament with a relatively low-intensity nanosecond laser pulse, in comparison with a filament without such irradiation. The experimental results are in good agreement with our theoretical model that follows the evolution of the temperature and density of various molecules, atoms, and ion species. The results point to the possibility of generating extremely long time duration, stable high-density plasma filaments in air.