Laser beam smoothing caused by the small-spatial-scale B integral

Smoothing, caused by the small-spatial-scale B integral, was measured on the OMEGA laser (a high-power, solid-state laser used for inertial confinement fusion research) without applied bandwidth. The intrinsic nonuniformity of laser irradiation [i.e., irradiation without smoothing by spectral dispersion] was determined from fluence distributions in equivalent-target-plane images of beams with phase plates. These data are compared with simulations that include both small-spatial-scale and whole-beam B integrals. The nonuniformity decreases with increasing average intensity. High-intensity beams can acquire bandwidth as a result of the intensity-dependent phase accumulated in the laser chain. The far-field speckle pattern produced by a phase plate can shift as the near-field phase front changes, which decreases the nonuniformity. The far-field power spectrum is affected mainly in the high spatial frequencies, where it is not expected to mitigate hydrodynamic instabilities