title: Near-field nonuniformities in angularly multiplexed KrF fusion lasers with induced spatial incoherence.
abstract: 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 a 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. We develop a novel algorithm for calculating the time-integrated intensities; compare simulations and measurements of the near-field speckle in the Nike KrF laser; and explore options, such as aberration reduction and optical relaying, for controlling the problem in future angularly multiplexed KrF drivers.
authors.affiliation: Division of Plasma Physics, U.S. Naval Research Laboratory, 4555 Overlook Avenue, SW, Washington, D.C. 20375-9998, USA. lehmberg@this.nrl.navy.mil
authors.forename: Robert H
authors.initials: RH
authors.lastname: Lehmberg
authors.forename: Yung
authors.initials: Y
authors.lastname: Chan