Nonuniformity in Direct-Drive Implosions Caused by Polarization Smoothing

Although direct-drive inertial confinement fusion targets on OMEGA are illuminated by 60 laser beams with the intention of producing a spherical, 1-D implosion, scattered light is observed to be very asymmetric in contrast to most predictions. In an attempt to optimize irradiation uniformity, each beam is split into two orthogonal polarizations using a distributed polarization rotator (DPR). This is meant to provide instantaneous smoothing by producing two copies of each beam’s speckle pattern that are offset 90 mm on target, effectively doubling the number of beams. While this ensures polarization diversity within the center of each beam, areas near the edge remain linearly polarized because of the far-field shift. For the first time, we have quantified the scattered-light intensity and polarization from individual beams and identified the DPR’s as the source of enhanced nonuniformity. The asymmetry is caused by the polarization dependence of cross-beam energy transfer (CBET)—a process known to strongly impact the coupling of laser energy to the implosion—since beams exchange more or less energy depending on the alignment of their polarizations in the linear edge regions. Agreement with the scattered-light data is significantly improved using a 3-D CBET code that tracks the individual polarizations of each sub-beam. The code predicts there are significant low-mode asymmetries in laser absorption that accompany the nonuniform scattered light, which are expected to degrade implosion performance.[1]. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856.

[1] D. H. Edgell et al., “Nonuniform Absorption and Scattered Light in Direct-Drive Implosions Driven by Polarization Smoothing,” Physical Review Letters (accepted for publication).