Optimizing Laser Spots with Precise Near Field Phase Control and Phase Plate Design

Fusion power plants will demand new laser and target designs that will likely include more precise control of the near field (NF) wavefront of the beam (via adaptive optics etc.) coupled with phase plate designs producing accurately shaped laser spots in the quasi-far field (QFF). In particular, phase plates that create the needed intensity profile in the QFF, and away from the best focus of the beam, show promise to illuminate targets with better control of the intensity profile than present spot shaping techniques that produce nearly exponential probability distributions of intensity 'speckles' on the target with spatial extents close to the diffraction limit of the optics (~fλ) [1]. The sensitivity of the QFF focal spot intensity profile produced by such phase plates to imperfections in the NF wavefront will be analyzed, to show how the optical design can minimize sensitivity to NF errors of specific spatial sizes and allow both the magnitude of the QFF intensity distortions to be reduced, and their spatial size increased, compared to fully developed speckle distributions. Planned experiments studying SBS driven gas optics designed to compress the pulses of excimer lasers will also be analyzed, and it will be shown any remaining spectra of NF errors (as from turbulence in the gas cell) is likely to affect both the magnitude and spatial size of the QFF intensity distortions, influencing both plasma instabilities and the effectiveness of spot smoothing with dispersed bandwidth. [1] J. Goodman, Wiley (2000) Sec. 7.5