Saturation of cross-beam energy transfer for multispeckled laser beams involving both ion and electron dynamics

Abstract

The nonlinear saturation of crossed-beam energy transfer (CBET) for multispeckled laser beams crossing at arbitrary angles is examined using vector particle-in-cell simulations. CBET is found to saturate on fast (∼10s of picosecond) time scales involving ion trapping and excitation of oblique forward stimulated Raman scattering (FSRS). Ion trapping reduces wave damping and speckle interaction increases wave coherence length, together enhancing energy transfer; ion acoustic wave (IAW) breakup in the direction transverse to the wavenumber increases wave damping and contributes to CBET saturation. The seed beam can become unstable to oblique FSRS, which leads to beam deflection at a large angle and a frequency downshift (by the plasma frequency). FSRS saturates on fast ∼picosecond time scales by electron plasma wave self-focusing, leading to enhanced side-loss hot electrons with energy exceeding 300 keV. This may contribute to fuel preheat but FSRS can be mitigated by the presence of a density gradient. Such growth of FSRS contributes to the saturation of CBET. Scaling simulations show that CBET, as well as FSRS and hot electrons, increases with beam average intensity, beam diameter, and crossing area, but that CBET is limited by the excitation of FSRS and IAW breakups in addition to pump depletion. FSRS deflects the seed beam energy by greater than 40% of the incident beam energy and puts a few percent of the incident beam energy into hot electrons. FSRS limits the efficacy of CBET for symmetry tuning at late stages in the implosion and may account for a large portion of the “missing energy” in implosions that use gas-filled hohlraums.