Accession Number : ADA185005

Title :   Theory of Induced Spatial Incoherence.

Descriptive Note : Interim rept.,

Corporate Author : NAVAL RESEARCH LAB WASHINGTON DC

Personal Author(s) : Lehmberg, R H ; Schmitt, A J ; Bodner, S E

PDF Url : ADA185005

Report Date : 16 Sep 1987

Pagination or Media Count : 60

Abstract : This paper describes theoretical and experimental investigations of induced spatial incoherence (ISI), a technique for achieving the smooth and controllable target beam profiles required for direct-drive laser fusion. Analytic and numerical calculations show that nonuniformities due to interference among the beamlets are smoothed by both thermal diffusion and temporal averaging. Under laser-plasma conditions of interest to ICF, average ablation pressure nonuniformities about 1% should be readily attainable. A partial ISI scheme, which allows widely-spaced beamlets to remain mutually-coherent is examined with the resulting high spatial frequency interference structure can be effectively smoothed by thermal diffusion alone. A perturbation analysis shows that the average target profile I(x) remains relatively insensitive to laser beam aberration when the scalelength of that aberration is larger than the initial beamlet width. This aberration will tend to broaden and smooth I(x), rather than introduce any small-scale structure. The broadening is largely controllable because it depends only upon spatial averages of the aberrated quantities over the entire laser aperture; the uncontrollable perturbations can be reduced to about 1% in practical cases.

Descriptors :   *LASER INDUCED FUSION, BEAM FORMING, LASER PUMPING, DIGITAL SIMULATION, PELLETS, IMPLOSIONS, FOCUSING, APERTURES, SYMMETRY, ABLATION, HIGH GAIN

Subject Categories : Fusion Devices(thermonuclear)
      Lasers and Masers

Distribution Statement : APPROVED FOR PUBLIC RELEASE