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Optical smoothing broadens cross beam energy transfer resonance

Y. Lalaire, C. Ruyer, A. Debayle, G. Bouchard, R. Capdessus, A. Fusaro, P. Loiseau, L. Masse, P. E. Masson-Laborde, D. Bénisti

Abstract

We use the theoretical framework introduced in the companion paper to provide simple formulas as regards the resonance conditions for CBET with smoothed laser beams.Our analytical CBET model with optical smoothing shows that these fusion-critical lasers produce a significantly broader resonance than conventional plane wave models predict. In particular, temporal smoothing, as used in many high energy laser facilities, and flow components normal to the CBET ion acoustic waves, significantly modify the power transfer between smoothed beams. Our model predicts that the energy transfer rate out of resonance is substantially higher with optical smoothing than without, a result that has profound implications for optimizing predicting and interpreting future fusion experiments. We provide a simple criterion which pinpoints the laser and plasma parameters for which laser smoothing impacts CBET. These findings pave the way for experimental investigations in high-energy-density physics and fusion energy.

Optical smoothing broadens cross beam energy transfer resonance

Abstract

We use the theoretical framework introduced in the companion paper to provide simple formulas as regards the resonance conditions for CBET with smoothed laser beams.Our analytical CBET model with optical smoothing shows that these fusion-critical lasers produce a significantly broader resonance than conventional plane wave models predict. In particular, temporal smoothing, as used in many high energy laser facilities, and flow components normal to the CBET ion acoustic waves, significantly modify the power transfer between smoothed beams. Our model predicts that the energy transfer rate out of resonance is substantially higher with optical smoothing than without, a result that has profound implications for optimizing predicting and interpreting future fusion experiments. We provide a simple criterion which pinpoints the laser and plasma parameters for which laser smoothing impacts CBET. These findings pave the way for experimental investigations in high-energy-density physics and fusion energy.
Paper Structure (3 equations, 3 figures)

This paper contains 3 equations, 3 figures.

Figures (3)

  • Figure 1: Power exchange predictions (lines, Eq. \ref{['eq:dPssd']}) vs plasma conditions: (a,b) drift velocity $v_{dy}/c_s$ with $\omega=0$, (c) frequency shift $\omega/kc_s$ with $\mathbf{v}_d=0$. Simulation parameters: $n_e=0.04n_c$, $T_e=2$ keV, $T_i=1$ keV, $f_\#=8$, $\lambda_0=1\,\rm \mu m$, $I_0=40\,\rm TW/cm^2$ and $\omega_d/ 2\pi = 14.25~\text{GHz}$. PIC simulation results shown as markers lalaire.
  • Figure 2: Threshold [Eq. \ref{['eq:sigma']}] for a plasma flow $\mathbf{v}=v_{dx}\Hat{\mathbf{x}}+v_{dy}\Hat{\mathbf{y}}$ ($v_{dz}=\sigma_{v_{dz}}=0$) for a frequency shift (a) and in the drift case (b). The region where plane wave CBET models are (not) satisfactory is noted "PW" ("PW$\neq$RPP+SSD"). (a) Black lines: $v_{dx}=0$ for various f-number; solid red line: $v_{dx}=c_s$ and $f_\#=8$. The lasers have $\lambda_0=0.35\,\rm \mu m$, we set $c_s=10^{-3}c$ and $\nu_L/kc_s=0.1$.
  • Figure 3: CBET coupling parameter calculated at the entrance hall of a Troll radiative hydrodynamic simulation of the N210808 NIF shot at $6\,\rm ns$ in the plane wave (a) and smoothed beam (b) cases. The material boundaries are the solid grey lines. The $30^o$ and $44^o$ cone edges are superimposed as dashed red and blue solid lines.