Impact of ion collisions on backscattering competition under the Langdon effect
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摘要: 背向受激拉曼散射与背向受激布里渊散射是激光聚变中广泛存在的两种激光等离子体不稳定性。为了深入理解两者之间的竞争过程，通过求解包含超高斯电子分布函数的五波耦合方程，并考虑了离子-离子碰撞对离子极化率的修正，分析了CH等离子体中Langdon效应和离子碰撞对两种不稳定性竞争关系和反射率的影响，研究结果表明，Langdon效应既可以改变背向受激拉曼散射与背向受激布里渊散射的色散关系，也可以改变电子等离子体波和离子声波的阻尼，而离子-离子碰撞的修正则主要是改变背向受激布里渊散射的色散关系和离子声波的阻尼，两者都可以使背向受激拉曼散射在密度相对更低的条件下在与背向受激布里渊散射的竞争中占据优势。
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关键词:
- 激光等离子体不稳定性 /
- 受激拉曼散射 /
- 受激布里渊散射 /
- Langdon效应 /
- 离子碰撞
Abstract:Background Backward stimulated Raman scattering (SRS) and backward stimulated Brillouin scattering (SBS) are two major laser-plasma instabilities that influence the laser-target energy coupling efficiency in inertial confinement fusion (ICF). Hot electrons excited by SRS can preheat the fuel. Their nonlinear competition determines the effectiveness of laser-plasma coupling and thus the performance of laser-driven fusion. In realistic laser fusion conditions, the electron distribution often deviates from Maxwellian due to strong laser heating, leading to nonthermal effects such as the Langdon effect. Additionally, ion-ion collisions in multispecies plasmas like CH can alter the damping and dispersion of ion acoustic waves.Purpose This study aims to investigate the impact of the Langdon effect and ion-ion collisions on the competition between SRS and SBS in CH plasma, particularly focusing on their respective reflectivities under varying plasma conditions.Methods Five-wave coupling equations describing the nonlinear interactions among the pump laser, scattered light, Langmuir wave, and ion acoustic wave were numerically solved. A super-Gaussian electron distribution function was employed to incorporate the Langdon effect, while ion-ion collision effects were included through modifications to the ion susceptibility. The dispersion relations and damping characteristics of both electron plasma waves (EPWs) and ion acoustic waves (IAWs) were analyzed in detail.Results The results reveal that the Langdon effect notably reduces Landau damping of EPWs and modifies the dispersion relation of SRS, enhancing its growth rate. Simultaneously, ion-ion collisions increase IAW damping and shift the SBS dispersion curve, weakening its instability. These combined effects lead to a dominance of SRS over SBS at lower electron densities, altering the overall backscattering reflectivity spectrum in laser fusion plasma.Conclusions Both the Langdon effect and ion-ion collisions play crucial roles in reshaping the nonlinear dynamics of SRS and SBS. Their influence must be considered in predictive models of laser-plasma interactions. These findings provide insight into optimizing plasma parameters for improved control of backscatter instabilities in inertial confinement fusion experiments. -
图 1 不同超高斯指数下，受激布里渊散射（SBS）的空间增长率随散射波长偏移的变化关系
Figure 1. The spatial growth rate of SBS versus the scattered wavelength shift at different super-Gaussian exponents
图 3
$\chi _{\text{i}}^I$ 和$|1 + {\chi _{\text{i}}}{|^2}$ 随相速度${v_{ph}}$ 的下降速率Figure 3. The dropping rates of
$\chi _{\text{i}}^I$ and$|1 + {\chi _{\text{i}}}{|^2}$ versus${v_{ph}}$ 
图 4 SRS与SBS背向散射率随电子数密度的变化
Figure 4. Variation of backward scattering rates of SRS and SBS with electron density
表 1 参数设置
Table 1. The representative cases for CH (Ion ratio is 1:1) plasmas
Case ne/nc Te/(keV) Ti/(keV) Collision a 0.1 3000 200 no b 0.1 3000 1200 no c 0.1 3000 200 yes d 0.1 3000 1200 yes 
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