Theory of Induced Spatial Incoherence.
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.
Document Details
- Document Type
- Technical Report
- Publication Date
- Sep 16, 1987
- Accession Number
- ADA185005
Entities
People
- Andrew J. Schmitt
- R. H. Lehmberg
- S. E. Bodner
Organizations
- United States Naval Research Laboratory