Laser Beam Smoothing Caused by the Small-Spatial-Scale B-Integral

78 LLE Review, Volume 82 Introduction Target irradiation uniformity is an important aspect of the direct-drive approach to inertial confinement fusion (ICF),1,2 where the capsule is directly irradiated by a symmetrically arranged cluster of high-intensity, ultraviolet (UV) laser beams. Nonuniformity in laser irradiation seeds the Rayleigh–Taylor hydrodynamic instability, which consequently degrades target performance.3,4 Various techniques are employed on the OMEGA5 laser to improve the on-target irradiation uniformity to reduce laser imprint: two-dimensional smoothing by spectral dispersion (2-D SSD),6–8 distributed phase plates (DPP’s),9,10 polarization smoothing (DPR’s),5,11,12 and multiple-beam overlap. A complete understanding of the laser focal-spot dynamics is essential to ICF performance, and it provides valuable feedback as a laser diagnostic tool. In Ref. 13, the smoothing rate of 2-D SSD on OMEGA was investigated both experimentally and numerically. Excellent agreement between the experimental results and the corresponding simulations was found for all 2-D SSD cases and for low-energy shots without applied frequency modulation (FM) (i.e., without SSD). Laser beam smoothing of high-power glass lasers caused by small-spatial-scale and whole-beam Bintegral effects with DPP’s and no applied FM is examined in this article.