Laser beam smoothing by exploitation of light parameters

Abstract

To achieve the high density compression required for high grain thermonuclear ignition with direct drive laser fusion, implosion of the target fuel must be highly symmetric. Nonuniformities in the radiation pressure cannot exceed 1 or 2%, thus very uniform irradiation must be obtained in the target plane. Theoretical studies have shown that a high level of irradiation uniformity can be achieved by overlapping several dozen laser beams onto the target. To achieve this level of uniformity requires accurate pointing, focusing, timing, and energy balance of the individual beams, but the most challenging requirement is that each of the laser beams has a uniform focal intensity distribution. Intensity variations around a smooth envelope must be less than ~ ±10% for each beam of a multibeam laser system. The effect of beam structure would be to implode some portions of the target faster than others and to seed the Rayleigh-Taylor hydrodynamic instability, resulting in reduced spherical convergence of the target, mixing between the shell and fuel, and substantially degraded neutron yield. Considerable effort had been expended at LLE to determine the present quality of the OMEGA laser beams, and to determine what improvements are necessary to meet the high density milestones set for both the OMEGA and OMEGA Upgrade laser systems.