Surface Modification of ICF Target Capsules by Pulsed Laser Ablation

Topographical modifications of spherical surfaces are imprinted on National Ignition Facility target capsules by extending the capabilities of a recently developed full-surface (4π) laser ablation and mapping apparatus. The laser ablation method combines the precision, energy density, and long reach of a focused laser beam to preimpose sinusoidal modulations on the outside surface of high-density carbon capsules and the inside surface of glow discharge polymer capsules. Sinusoidal modulations described in this paper have submicron to tens of microns vertical scale and wavelengths as small as 30 μm and as large as 200 μm. The modulated patterns are created by rastering a focused laser fired at discrete capsule surface locations for a specified number of pulses. The computer program developed to create these raster patterns uses inputs such as the laser beam intensity profile, the material removal function, the starting surface figure, and the desired surface figure. The patterns are optimized to minimize surface roughness. In this paper, simulated surfaces are compared with actual ablated surfaces measured using confocal microscopy.