Influence of specularly reflected laser light on uniformity of implosion of indirect-drive fusion capsule

The influence of laser light components specularly reflected by the inner wall of an x-ray confining cavity was experimentally investigated by means of time-resolved x-ray imaging and shock wave measurements. The experiments were made for two different cavity shapes (circular and decagonal cylinders) to investigate the geometrical effect of the reflected light. The results were compared with calculations involving a simple illumination code. Time-resolved x-ray images for a gold-coated surrogate sphere at the cavity centre were replicated by the calculations, showing that the reflected light shines strongly on the sphere at the beginning of the laser pulse. It was also found that, for the experimental conditions adopted in this study, about one third of the drive pressure of a shock wave propagating through a planar sample placed at the cavity centre is due to the reflected light. The estimated intensity of the reflected light is high enough to impose noticeable imprinting onto a fusion capsule set in the vacuum cavity conventionally used for indirect-drive implosions.

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