Uniformity of energy deposition for laser driven fusion

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Abstract
Laser driven fusion requires a high degree of uniformity in laser energy deposition in order to achieve the high density compressions required for sustaining a thermonuclear burn. The characteristic nonuniformities produced by laser irradiation, with multiple overlapping beams, are examined for a variety of laser‐target configurations. Conditions are found for which the rms variation in uniformity is less than 1%. The analysis is facilitated by separating the contributions from (1) the geometrical effects related to the number and orientation of the laser beams and (2) the details of ray trajectories for the overlapping beams. Emphasis is placed on the wavelength of the nonuniformities in addition to their magnitudes, as the shorter wavelength nonuniformities are more easily smoothed by thermal condution within the target. It is demonstrated how the geometrical symmetry of the laser system effectively eliminates the longer wavelengths, and how shorter wavelength nonuniformities can be ‘‘tuned out’’ by varying parameters such as the focal position and the radial intensity profile of the beam. The distance required for adequate thermal smoothing of the irradiation nonuniformities is found to be 2 to 3 times smaller than previously estimated due mainly to the relatively small spatial wavelength of the nonuniformities. This is a consequence of the geometrical symmetry of the laser system and is relatively insensitive to the details of overlapping beams. The results are particularly important for irradiation with short wavelength laser light (e.g., 0.35 μm), as the small smoothing distances anticipated for moderate laser intensities are found to produce adequate attenuation of the calculated nonuniformities.