Laser-Absorption Nonuniformities Resulting from Beam Smoothing and Cross-Beam Energy Transfer in Direct-Drive Implosions on OMEGA

To reduce laser speckle, OMEGA employs both polarization smoothing using distributed polarization rotators (DPRs) and smoothing by spectral dispersion (SSD) in direct-drive–ignition experiments. Both smoothing methods utilize angular dispersion to separate components of each beam. The DPRs split each beam into two orthogonal polarization components with slightly different angular dispersions, resulting in nearly linearly polarized regions on the edges of each beam. SSD splits each beam into a bandwidth of frequencies with different angular dispersions such that the beam frequency varies across the beam profile. The magnitude of cross-beam energy transfer between beams depends on their polarization and wavelength; therefore, the energy exchange varies significantly across each beam profile. Since the DPRs and SSD orientations on OMEGA's laser beams are not symmetric, the laser intensity varies uniquely over each beam profile. The total nonuniformity is spread over many spherical harmonic modes, including the lowest mode numbers.