Multi-beam effects on backscatter and its saturation in experiments with conditions relevant to ignition

To optimize the coupling to indirect drive targets in the National Ignition Campaign (NIC) at the National Ignition Facility E. Moses, Phys. Plasmas 16, 041006 (2009), a model of stimulated scattering produced by multiple laser beams is used. The model has shown that scatter of the 351 nm beams can be significantly enhanced over single beam predictions in ignition relevant targets by the interaction of the multiple crossing beams with a millimeter scale length, 2.5 keV, 0.02-0.05 critical density, plasma. The model uses a suite of simulation capabilities and its key aspects are benchmarked with experiments at smaller laser facilities. The model has also influenced the design of the initial targets used for NIC by showing that both the stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) can be reduced by the reduction of the plasma density in the beam intersection volume that is caused by an increase in the diameter of the laser entrance hole (LEH). In this model, a linear wave response leads to a small gain exponent produced by each crossing quad of beams (∼1 per quad) which amplifies the scattering that originates in the target interior where the individual beams are separated and crosses many or all other beams near the LEH as it exits the target. As a result all 23 crossing quads of beams produce a total gain exponent of several or greater for seeds of light with wavelengths in the range that is expected for scattering from the interior (480 to 580 nm for SRS). This means that in the absence of wave saturation, the overall multi-beam scatter will be significantly larger than the expectations for single beams. The potential for non-linear saturation of the Langmuir waves amplifying SRS light is also analyzed with a two dimensional, vectorized, particle in cell code (2D VPIC) that is benchmarked by amplification experiments in a plasma with normalized parameters similar to ignition targets. The physics of cumulative scattering by multiple crossing beams that simultaneously amplify the same SBS light wave is further demonstrated in experiments that benchmark the linear models for the ion waves amplifying SBS. The expectation from this model and its experimental benchmarks is shown to be consistent with observations of stimulated Raman scatter in the first series of energetic experiments with ignition targets, confirming the importance of the multi-beam scattering model for optimizing coupling.