Laser breakdown in air at ultrahigh laser pulse repetition rates

Some specific features of interaction of intense femtosecond laser pulses with air at ultrahigh pulse repetition rates have been experimentally studied. Data on the dynamics of plasma cloud expansion and the plasma electron density on time intervals no longer than 10 ns are obtained by femtosecond interferometry. These data are interpreted in terms of the most likely mechanisms of ionised gas recombination. The effect of ultrahigh-frequency laser radiation on a medium was modelled by double-pulse irradiation with a short delay Δt between the pulses: from 1 ps to 11 ns. A nonmonotonic dependence of the degree of air ionisation by the second pulse on the delay time Δt is found; possible mechanisms of these dependences are discussed in terms of the processes of femtosecond radiation absorption in the residual plasma.