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 using an idealized model. Polar direct drive (PDD) on the National Ignition Facility (NIF) requires the repointing of its 192 beams clustered within 50 degrees of the poles to minimize the imparted polar varying payload kinetic energy of the target. Due to this repointing, non-normal incidence angles of the beam centerlines are encountered in any PDD design. The formulation of a PDD scheme that minimizes non-uniformity is a significant challenge that requires an understanding of the induced differences in ablation including those of incidence angle. 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 for the planar case during the beginning portion of the laser ablation. This result is considered with an idealized 1-D two segment planar model using rays at different incidence angles for each segment. Within the model, the segments cannot have equal mass and velocity simultaneously without tailoring the ray's time-dependent intensity for each segment. However, with the correct segment-to-segment time-independent ray intensities, equal velocity and dynamic pressure can be achieved approximately without tailoring. Additionally, an analytic prediction for the conduction zone width as a function of incidence angle is provided. It is predicted that this width increases with incidence angle, resulting in a decrease in laser imprint, an effect which has been previously observed in experiments. These results, when generalized to spherical geometry, may provide insight into the implosion dynamics encountered in PDD.

One-step laser preparation of unidirectional liquid spontaneous transport structures

“…When the laser beam is tilted at incidence (here, the laser incidence angle q is defined as the angle between the laser beam axis and the direction of the normal vector [39]), the ablation region is elliptical (as shown in figure 2(a)), and the relationship between the ellipse dimensions and the radius of the laser beam waist w can be expressed by equation (1) as.…”

Effect of Impurity Atoms on Thermonuclear Yield

“…Current research has determined optimum laser pointing that minimizes CBET for the polar direct-drive geometry. These pointings also maximize laser absorption [118,119] so that more than 90% of the laser energy is coupled into the outer shell. Integrated simulations of these experiments predict 1.4 MJ of thermonuclear yield using NIF.…”