Nonlinear evolution of stimulated Raman scattering near 1/4 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mtext>c</mml:mtext> </mml:msub> </mml:mrow> </mml:math> driven by a broadband laser

Abstract The nonlinear evolution and energy transfer process of broadband laser-driven stimulated Raman scattering (SRS) near 1/4 critical density ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mtext>c</mml:mtext> </mml:msub> </mml:mrow> </mml:math> ) have been investigated by particle-in-cell simulations. We find that in this regime, where wave–wave interactions dominate the evolution of SRS, laser reflectivity increases at moderate bandwidth ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi mathvariant="normal">Δ</mml:mi> <mml:mi>ω</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.6</mml:mn> <mml:mi mathvariant="normal">%</mml:mi> </mml:mrow> </mml:math> ) and subsequently decreases as the bandwidth becomes larger. And at moderate bandwidth, the bursts of SRS lead to increased trapping of super-hot electrons ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mo>&gt;</mml:mo> </mml:mrow> </mml:math> 50 keV) by electron plasma wave (EPW). We also find that the EPW undergoes a mode-coupling process: under the density modulations introduced by the ion-acoustic wave ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>k</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>2</mml:mn> <mml:msub> <mml:mi>k</mml:mi> <mml:mn>0</mml:mn> </mml:msub> </mml:mrow> </mml:math> ) generated via Langmuir decay instability (LDI), the energy of the SRS-generated EPW ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>k</mml:mi> <mml:mo>∼</mml:mo> <mml:msub> <mml:mi>k</mml:mi> <mml:mn>0</mml:mn> </mml:msub> </mml:mrow> </mml:math> ) is transferred to a coupled-EPW at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>k</mml:mi> <mml:mo>∼</mml:mo> <mml:mn>3</mml:mn> <mml:msub> <mml:mi>k</mml:mi> <mml:mn>0</mml:mn> </mml:msub> </mml:mrow> </mml:math> , which has a lower phase velocity. The coupled-EPWs trap a significant number of electrons in the 10–50 keV range, resulting in heating of the background plasma. Both moderate and large bandwidths are effective in suppressing the LDI and mode-coupling process, thereby reducing the trapping of electrons in the 10–50 keV range and limiting the increase in plasma temperature.