The role of incidence angle in the laser ablation of ICF targets

The effect of the laser incidence angle on the mass ablation rate and ablation pressure of inertial confinement fusion (ICF) targets is explored using an idealized planar model. NIF, in its current configuration, has its 192 laser beams clustered within 50 degrees of the poles of its target chamber. Therefore, non-normal incidence angles are encountered in all direct drive ICF experiments currently being carried out at the NIF. In this work, a modified version of the textbook model of laser ablation [Manheimer et al. Phys. Fluids 25, 1644 (1982)] is used to illustrate that the mass ablation rate and ablation pressure scale with the 4/3 and 2/3 power of the cosine of laser incidence angle. This implies that a uniform shell mass and velocity cannot be simultaneously obtained when beams intercept the target with a variety of incidence angles. However, with the correct intensity variations, uniform dynamic pressure can be achieved approximately. Additionally, the conduction zone length is found to increase with increasing incidence angle, resulting in decreased laser imprint. Predictions of the scaling of imprint efficiency are found to be in good agreement with prior experimental measurements.