Abstract
Using the reduced-description particle-in-cell (RPIC) method, we study the coupling of backward stimulated Raman scattering (BSRS) and backward stimulated Brillouin scattering (BSBS) in regimes where the reflectivity involves the nonlinear behavior of particles trapped in the daughter plasma waves. The temporal envelope of a Langmuir wave (LW) obeys a Schrödinger equation where the potential is the periodic electron density fluctuation resulting from an ion-acoustic wave (IAW). The BSRS-driven LWs in this case have a Bloch wave structure and a modified dispersion due to the BSBS-driven spatially periodic IAW, which includes frequency band gaps at kLW∼kIAW/2∼k0 (kLW, kIAW, and k0 are the wave number of the LW, IAW, and incident pump electromagnetic wave, respectively). This band structure and the associated Bloch wave harmonic components are distinctly observed in RPIC calculations of the electron density fluctuation spectra and this structure may be observable in Thomson scatter. Bloch wave components grow up in the LW spectrum, and are not the result of isolated BSRS. Self-Thomson scattered light from these Bloch wave components can have forward scattering components. The distortion of the LW dispersion curve implies that the usual relationship connecting the frequency shift of the BSRS-scattered light and the density of origin of this light may become inaccurate. The modified LW frequency results in a time-dependent frequency shift that increases as the IAW grows, detunes the BSRS frequency matching condition, and reduces BSRS growth. A dependence of the BSRS reflectivity on the IAW Landau damping results because this damping determines the levels of IAWs. The time-dependent reflectivity in our simulations is characterized by bursts of sub-picosecond pulses of BSRS alternating with multi-ps pulses of BSBS, and BSRS is observed to decline precipitously as soon as SBS begins to grow from low levels. In strong BSBS regimes, the Bloch wave effects in BSRS are strong and temporal anti-correlation with BSRS is due to pump depletion in addition to frequency detuning. In most cases studied, BSBS suppressed the time-averaged reflectivity of BSRS compared to the levels obtained with fixed ions (and therefore no BSBS). The strong spatial modulation of the Bloch Langmuir waves appears to weaken electron trapping and thereby lowers the inflated reflectivity levels of BSRS.
Highlights
Using the reduced-description particle-in-cell (RPIC) method, we study the coupling of backward stimulated Raman scattering (BSRS) and backward stimulated Brillouin scattering (BSBS) in regimes where the reflectivity involves the nonlinear behavior of particles trapped in the daughter plasma waves
The BSRS-driven Langmuir wave (LW) in this case have a Bloch wave structure and a modified dispersion due to the BSBS-driven spatially periodic ion-acoustic wave (IAW), which includes frequency band gaps at kLW $ kIAW=2 $ k0. This band structure and the associated Bloch wave harmonic components are distinctly observed in RPIC calculations of the electron density fluctuation spectra and this structure may be observable in Thomson scatter
Recent efforts to understand backward stimulated Raman scattering (BSRS) saturation mechanisms for the conditions found in plasmas with high Langmuir wave (LW) Landau damping relevant to the National Ignition Facility (NIF) conditions have focused on electron trapping1–3 and the resulting phenomena of BSRS reflectivity inflation above linear convective estimates
Summary
Recent efforts to understand backward stimulated Raman scattering (BSRS) saturation mechanisms for the conditions found in plasmas with high Langmuir wave (LW) Landau damping relevant to the National Ignition Facility (NIF) conditions have focused on electron trapping and the resulting phenomena of BSRS reflectivity inflation above linear convective estimates. Past efforts to understand the BSRS-BSBS interaction focused on regimes where the primary nonlinear saturation mechanisms were the Langmuir decay instability (LDI—a parametric three-wave process in which a LW decays into a counter-propagating LW and an IAW) and associated strong Langmuir turbulence effects This regime of nonlinear saturation of BSRS was shown to be valid only in weak Landau damping regimes for the LW.. LW-IAW coupling in plasmas studied here is a transient effect on the IAW time scale, resulting in a time-dependent Bloch wave structure and imposing a positive frequency shift, which can, in principle, reduce BSRS reflectivity by phase detuning. This detuning competes with pump depletion (due to strong BSBS) in weakening BSRS.
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