Formation of an optical pulsed discharge in a supersonic air flow by radiation of a repetitively pulsed CO2 laser

Abstract

Results of optimisation of repetitively pulsed CO2-laser generation are presented for finding physical conditions of forming stable burning of an optical pulsed discharge (OPD) in a supersonic air flow and for studying the influence of pulse parameters on the energy absorption efficiency of laser radiation in plasma. The optical discharge in a supersonic air flow was formed by radiation of a repetitively pulsed CO2 laser with mechanical Q-switching excited by a discharge with a convective cooling of the working gas. For the first time the influence of radiation pulse parameters on the ignition conditions and stable burning of the OPD in a supersonic air flow was investigated and the efficiency of laser radiation absorption in plasma was studied. The influence of the air flow velocity on stability of plasma production was investigated. It was shown that stable burning of the OPD in a supersonic flow is realised at a high pulse repetition rate where the interval between radiation pulses is shorter than the time of plasma blowing-off. Study of the instantaneous value of the absorption coefficient shows that after a breakdown in a time lapse of 100 — 150 ns, a quasi-stationary 'absorption phase' is formed with the duration of ∼1.5 ms, which exists independently of air flow and radiation pulse repetition rate. This phase of strong absorption is, seemingly, related to evolution of the ionisation wave.