The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory contains a 192-beam 4.2 MJ neodymium glass laser (around 1053 nm or 1w) that is frequency converted to 351nm light or 3w. It was built to access the extreme high energy density conditions needed to support the nation’s nuclear stockpile in the absence of further underground nuclear tests, including studying Inertial Confinement Fusion (ICF) and ignition in the laboratory.
Over the last year, important results have been obtained demonstrated a fusion yield of 1.35MJ with 1.9MJ of laser energy (and 440 TW power) injected in the target, bringing the NIF to the threshold of ignition [2-3]. As the yield curve near ignition is steep, the laser performance team has focused on providing improved power accuracy and precision (better shot-to-shot reproducibility) with a high-fidelity pulse shaping system (HiFiPS), and also on extending the NIF operating power and energy space by 15% to 2.2MJ and 500TW.
Stimulated rotational Raman scattering in air is a powerful parasitic process that degrades high intensity pulses propagated over significant distances. Through this inelastic scattering process, laser photons are converted to higher (Anti-Stokes) or lower (Stokes) energies, according to rotational mode transitions in the nitrogen and oxygen molecules. The full wave-mixing problem involves numerous frequencies including both Stokes and Anti- Stokes processes, multiple rotational line transitions, and multi-harmonic generation, with each generated field acting as a seed for subsequent scattering processes. Multiple numerical models of these processes were integrated into Lawrence Livermore National Laboratory’s in-house nonlinear optical chain propagation software, Virtual Beamline++. The complex spatio-temporal dynamics of single, and multi-frequency stimulated rotational Raman scattering are highlighted and discussed. General limitations of steady-state, dynamic two-level, multi-harmonic, and multi-rotational models are demonstrated and compared.
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