When a high energy nanosecond (ns) laser induces breakdown in the air, the plasma density generated in the rarefied atmosphere is much smaller than that at normal pressure. It is associated with a relatively lower absorption coefficient and reduces energy loss of the laser beam at low pressure. In this paper, the general transmission characterizations of a joule level 10 ns 1064 nm focused laser beam are investigated both theoretically and experimentally under different pressures. The evolution of the electron density (n e ), the changes in electron temperature (T e ) and the variation of laser intensity (I) are employed for numerical analyses in the simulation model. For experiments, four optical image transfer systems with focal length (f) of 200 mm are placed in a chamber and employed to focus the laser beam and produce plasmas at the focus. The results suggest that the transmittance increases obviously with the decreasing pressure and the plasma channels on the transmission path can be observed by its self-illumination. The simulation results agree well with the experimental data. The numerical model presents that the maximum n e at the focus can reach 1019 /cm3, which is far below the critical density (n c ). As a result, the laser beam is not completely shielded by the plasmas.

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