Mitigation of multibeam stimulated Raman scattering with polychromatic light

Abstract

A theoretical analysis is presented for the stimulated Raman scattering (SRS) driven by multiple beams with certain frequency differences or bandwidths. The concomitant electrostatic modes are excited by the scattering lights when the pump beams share a common electron plasma wave to drive SRS. This coupling can be suppressed when the frequency difference of the two beams is three times larger than the SRS growth rate of a single beam, where the beams develop their scattering lights independently. To mitigate SRS further under the multibeam irradiation, polychromatic pumps are introduced to reduce the SRS saturation level, where the weakly coupled common modes are heavily damped in hot plasmas. The sidescattering driven by polychromatic light in inhomogeneous plasmas is significantly reduced due to the wavevector mismatch of daughter waves. Particle-in-cell simulations confirm our theoretical analysis and demonstrate that the strength of multibeam SRS and the hot electron production in inhomogeneous plasmas can be controlled at a low level by polychromatic light with a few percentage bandwidth ( ≳ 3 % ), even in the non-linear regime.

Polychromatic drivers for inertial fusion energy

“…( 4), we know that the saturation level of electron plasma wave is reduced by the frequency difference δω 0 = 0. Therefore, the common Langmuir wave shared by the monochromatic lasers at n e /n c ≤ cos 4 θ v /4 is divided into multiple weakly coupled modes by the polychromatic lights with a broad spectrum ∆ω 0 3%ω 0 [40,41]. The scattering lights of the polychromatic beams are also detuned from each other when the frequency difference satisfies δω 0 ∼ 10 −2 ω 0 .…”

“…It should be noted that cos(φ) = 1 is always satisfied for the s-polarized (the electric field of light is perpendicular to the plane) incident beams. Therefore, the beams are all s-polarized in our 2D simulations for the stimulated scattering instabilities, which normally lead to relatively larger growth rates than p-polarized (the electric field of light is parallel to the plane) ones [41]. The transverse width of the plasmas is 44λ, and the resolutions are dx/λ = dy/λ = 0.0286 with 25 particles per cell.…”

“…5(e) and 5(f). The submodes of the electrostatic fields developed by different color beamlets are weakly coupled in the phase space [41]. Therefore, the total saturation level is reduced by the wavenumber detuning.…”

Bandwidth effects in stimulated Brillouin scattering driven by partially incoherent light

“…Mitigation of laser-plasma instabilities through increasing the bandwidth of the driving laser beam(s) has been investigated by several groups [43,[52][53][54][55][56][57]. For a parametric instability with "coherent" growth rate γ 0 and an incoherent pump laser with ∆ω 0 γ 0 , Pesme et al [43] use an "incoherent" growth rate γ inc = 4γ 2 0 /∆ω 0 .…”

“…In theoretical studies [43,52,53], a bandwidth of ∆ω 0 > 10γ 0 or ∆ω 0 /ω 0 ∼ 5% is often employed. In experimental studies [54][55][56][57], ∆ω 0 /ω 0 is typically much smaller, ∆ω 0 /ω 0 < 1%, probably dictated by the properties of the intrinsic bandwidth of the laser gain medium.…”