ILLUMINATION UNIFORMITY OF SPHERICAL TARGETS USING KILOJOULE-SCALE LASERS WITH OPTICAL SMOOTHING

Illumination uniformity in laser-pellet interactions is investigated for small kilojoule-range laser systems that are capable of symmetrically illuminating spherical targets. We address a variety of options that might be used to control the illumination uniformity on such targets, including the number of beams, pulse energy, laser wavelength, temporal pulse shape, and beam spatial profile shape. We consider only laser beams that are optically smoothed using methods such as induced spatial incoherence or random phase plates. The parameters that are most important for providing good uniformity are identified. We find that the most important parameters governing the uniformity are the number of beams and the shape of the beam profile. Configurations with a small number of beams (e.g., six) are much more sensitive to the beam profile; using techniques such as splitting the focal spots provides an additional degree of freedom that can significantly increase the illumination uniformity. Additionally, we show that the hydrodynamics of the plasma is important in determining the uniformity, although it is not so easily controlled. Quantitatively, we find nonuniformities (measured by peak-to-valley ablation pressure differences) can be less than 10% for six-beam kilojoule-level 1/4 -μm laser wavelength systems, and of the order of a few percent for comparable 24-beam systems.