The role of incidence angle in the laser ablation of a planar target

Abstract

The effect of the laser ray incidence angle on the mass ablation rate and ablation pressure of planar inertial confinement fusion (ICF) targets is explored. In this work, a modified version of the textbook model of laser ablation [Manheimer et al., Phys. Fluids 25, 1644 (1982)] is used to demonstrate that the mass ablation rate and ablation pressure scale with the 4/3 and 2/3 power of the cosine of the laser ray incidence angle relative to the target normal. Using an idealized planar model of ablation, it is demonstrated that constant irradiance is insufficient to produce similar velocities of a target when comparing cases driven at different incidence angles. Additionally, the effect of the incidence angle on the absorption fraction is demonstrated and it is shown that a longer duration is needed to achieve maximum absorption for greater angles. This further affects the mass ablation rate and ablation pressure in the first nanosecond of ablation. These results, when extrapolated, may provide insight into the variation of drive conditions encountered due to incidence in polar direct drive ICF.