弱磁化等离子体中受激拉曼散射和受激布里渊散射抑制研究

doi:10.19596/j.cnki.1001-246x.8569

摘要/Abstract

摘要：

本文针对典型激光聚变等离子体参数条件, 利用动理学粒子模拟程序研究横向磁场和激光带宽在抑制受激拉曼散射(SRS)和受激布里渊散射(SBS)中的作用。模拟发现横向磁场对非均匀等离子体中SRS的非线性自共振增强有显著抑制作用, 分析认为横向磁场作用于SRS激发的电子等离子体波(EPW)势阱中的俘获电子, 使它们在横向上加速, 对EPW造成非线性阻尼, 同时减小EPW的非线性频移量, 从而缩窄非均匀等离子体中SRS的自共振空间, 极大降低SRS反射率。在此基础上利用横向磁场抑制SRS的特性, 以及SBS增长对激光带宽的敏感性, 提出了利用横向磁场和宽带激光将SRS和SBS同时抑制在低反射率水平的方案。在采用数十特斯拉横向磁场和实验中易于达到的千分之一量级的激光带宽时以及慢性约束聚变(ICF)相关参数下, SBS和SRS的反射率都得到了有效抑制。

关键词: 受激拉曼散射, 受激布里渊散射, 非线性动理学效应, 磁化等离子体, 自共振

Abstract:

Finding ways to suppress stimulated Raman scattering(SRS) and stimulated Brillouin scattering(SBS) is an important subject in inertial confinement fusion(ICF). Under the typical laser fusion plasma conditions, the effects of transverse magnetic field and laser bandwidth on the suppression of SRS and SBS are studied by utilizing the particle-in-cell(PIC) program. It is found that the transverse magnetic field can significantly inhibit the autoresonance of SRS in the non-uniform plasma. The transverse magnetic field accelerates the electrons trapped in the electron plasma wave(EPW) excited by SRS, which causes the nonlinear damping and reduces the nonlinear frequency shift of the EPW, thus narrowing the autoresonance space and significantly reducing the SRS reflectivity. Based on the characteristics that SRS can be suppressed by a transverse magnetic field and the SBS growth is sensitive to the laser bandwidth, a scheme is proposed that utilizing the transverse magnetic field and the broadband laser to suppress SRS and SBS simultaneously. When the transverse magnetic field is on the order of 10 T and the bandwidth is on the order of 0.001, both of which are easy to achieve in experiments, the total reflectivity of SRS and SBS can be effectively suppressed in the ICF-related plasmas.

Key words: stimulated Raman scattering, stimulated Brillouin scattering, nonlinear kinetic effect, magnetized plasma, autoresonance

周远志, 郑春阳, 刘占军, 曹莉华, 程瑞锦, 贺贤土. 弱磁化等离子体中受激拉曼散射和受激布里渊散射抑制研究[J]. 计算物理, 2023, 40(2): 136-146.

Yuanzhi ZHOU, Chunyang ZHENG, Zhanjun LIU, Lihua CAO, Ruijin CHENG, Xiantu HE. Suppression of Stimulated Raman Scattering and Stimulated Brillouin Scattering in Weakly Magnetized Plasmas[J]. Chinese Journal of Computational Physics, 2023, 40(2): 136-146.

图/表 9

图1 当入射激光I0 = 5 × 1015 W ·cm-2时，不同外加磁场下EPWs在时空中的演化

Fig.1 Snapshot of the spatial evolution of EPWs in different Bext when I0 = 5 × 1015 W ·cm-2

图2 (a) 在左边界测量得到的SRS反射率的包络线，模拟中的激光强度为I0=5×1015 W ·cm-2。绿色虚线和黑色点线分别代表了初始的种子光水平和理论预测的动理学增益后的反射水平。T0是泵浦光的周期。(b)和(c)在左边界测量得到的散射光的频率分布，测量的是磁场条件为Bext=0或Bext=90 T时的情况。

Fig.2 (a) Reflectivity of the scattered light at the left boundary when I0 = 5 × 1015 W ·cm-2. The green dash-dot line and black dotted line represent the initial seed level and the level expected by kinetic amplification respectively. T0 is the period of pump light. Frequency distributions of the scattered light waves at the left boundary when (b) Bext=0 or (c) Bext=90 T.

图3 不同外加磁场和不同入射光条件下时间平均的SRS反射率(种子光强度为泵浦光的1/500，模拟时间为3.5 ps。)

Fig.3 Time averaged SRS reflectivity under different magnetic fields and different pump lights (The seed light intensity is 1/500 of the pump light, and the simulation time is 3.5 ps.)

图4 在粒子模拟中追踪得到的电子运动轨迹(ωL/ωc=87.4，ωB/ωc=15.7, vpL/c=0.15。)

Fig.4 Orbit of an electron in the particle-in-cell (PIC) (ωL/ωc=87.4, ωB/ωc=15.7, vpL/c=0.15.)

图5 在t=600 T0时刻统计得到的电子在px-py空间上的分布函数

Fig.5 The electron distribution in px-py space at t=600 T0

表1 SBS和SRS的主要参数

Table 1 Main parameters of SBS and SRS

散射机制	kλ_De	ω/ω₀	γ₀/ω₀	ν_LD	G
SBS	0.367	0.997	5.63 × 10^-4	-0.104ω_a	3.65
SRS	0.313	0.276	2.25 × 10^-3	-0.0144ω_L	4.46

图6 (a) 无外加磁场时，反射率随入射激光带宽的变化; (b) 单色光入射时，反射率随横向磁场强度的变化

Fig.6 (a) The reflectivity changes with the bandwidth of the pump light when Bext= 0; (b) The reflectivity changes with the intensity of transverse magnetic field when the incident light is monochromatic

图7 固定带宽为2 × 10-3ω0时，SRS和SBS的反射率随横向磁场强度的变化

Fig.7 When the fixed bandwidth is 2 × 10-3ω0, the reflectivity of SRS and SBS changes with the intensity of transverse magnetic field

表2 不同方案下的激光透射率

Table 2 Transmittivity in different schemes

无抑制方案	2 × 10^-3带宽	20 T横向磁场	带宽+磁场
0.788	0.837	0.850	0.941

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