Effect of vapor plasma on the coupling of laser radiation with aluminum targets

The effect of vapor plasma on thermal and impulse coupling of laser radiation with aluminum targets is studied to understand and explain experimental data showing anomalously high coupling to 10.6 μm laser radiation. Heating of vapor by inverse bremsstrahlung absorption of laser radiation, subsequent reradiation in the u.v. and deep u.v. by ionized species, and vapor layer growth are modeled. A computer code has been developed to solve the governing equations. Major conclusions include the following: (a) vapor plasma radiative transport can be an important mechanism for laser/target coupling, (b) aluminum vapor (density × thickness) ≈ 10¹⁷ cm^-2 (corresponding to about 0.01 μm of target material) can result in thermal coupling coefficients of 20% or more, and (c) too much vapor reduces the net flux at the target.