Formation of a thermal lens in the exit window of a laser due to passage of a giant laser pulse

An analysis is made of the dynamics of formation of thermal lens in the exit window of a large-diameter (≤ 1 m) laser due to the passage of a high-power laser pulse of λ = 1 nsec to 10 μsec duration. Expressions are obtained for the optical power of a thermal lens and for the radiation intensity corresponding to the appearance of critical optical distortions in the lens. Numerical estimates, obtained for various exit-window materials in the wavelength range λ = 1.06–10.6 μ, show that optical distortions should be slight for pulses of about 1 nsec duration. When τ is increased, these distortions increase strongly. Catastrophic optical distortions in the exit window may be expected when the threshold of avalanche electron multiplication is exceeded.