Near-Field Nonuniformities in Angularly-Multiplexed KrF Fusion Lasers with Induced Spatial Incoherence; Journal article

摘要

Induced Spatial Incoherence (ISI) has been proposed for KrF laser drivers to achieve the high degree of spatial beam uniformity required for direct-drive inertial confinement fusion. Although ISI provides ultrasmooth illumination at the far-field of the laser, where the target is located, it can still allow the beams in the quasi near-field to develop time-averaged spatial structure. This speckle, which arises primarily from random phase aberration, builds up as the laser beams propagate away from the pupil plane located at the final amplifier stage; it is distinct from any structure imposed by gain nonuniformities in the amplifiers. Because of the spatial incoherence, the speckle is significantly smaller than that experienced by coherent beams; nevertheless, it remains a damage issue, especially for the long beam delay paths required in angularly-multiplexed KrF lasers. This paper develops a novel algorithm for calculating the time-integrated intensities, compares simulations and measurements of the near-field speckle in the Nike KrF laser, and explores options, such as aberration reduction and optical relaying, for controlling the problem in future angularly-multiplexed KrF drivers.