Near-Field Nonuniformities in Angularly-Multiplexed KrF Fusion Lasers with Induced Spatial Incoherence
NAVAL RESEARCH LAB WASHINGTON DC PLASMA PHYSICS DIV
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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.
- Inorganic Chemistry
- Lasers and Masers
- Fusion Devices (Thermonuclear)