Laser plasma instabilities and their suppression strategies
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摘要: 受激拉曼散射、受激布里渊散射等激光等离子体不稳定性（LPI）是激光等离子体物理领域最重要的研究课题之一。特别是在激光驱动的惯性约束聚变中，LPI会造成相当份额的激光能量损失，破坏辐射对称性，产生的超热电子还会预热靶丸，进而影响压缩效率和聚变能量增益。近期，在美国国家点火装置上开展的实验表明对LPI物理过程的充分理解和有效控制对成功实现ICF点火至关重要。我们对近期LPI方面的一系列研究进展进行了简单介绍与讨论。首先，回顾了描述LPI过程的三波耦合理论，由此得出了LPI在线性阶段的增长率。接着讨论了一些复杂情景下的LPI物理过程，譬如LPI的非线性发展阶段、级联LPI、多光束LPI以及LPI间的非线性耦合。最后，着重介绍了一系列抑制LPI的技术方案，包括束匀滑技术、光束时域整形、宽带激光、偏振旋转激光以及外加磁场等。
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关键词:
- 激光等离子体不稳定性 /
- 惯性约束聚变 /
- 受激拉曼散射 /
- 受激布里渊散射 /
- 宽带激光
Abstract: The issue of laser plasma instabilities (LPIs) including stimulated Raman scattering, stimulated Brillouin scattering and so on is one of the most fascinating subjects in laser plasma physics. In particular, LPIs may cause significant laser energy loss and produce hot electrons to preheat fusion targets, which affect target compression and fusion energy gain in laser-driven inertial confinement fusion. Recent experiments carried out on the National Ignition Facility, the largest laser facility in the world for laser fusion, indicate that the understanding and the control of LPIs are essential to the realization of laser fusion. In this paper, we present a review on recent studies of LPIs. Firstly, we retrospect the classical theoretical model of LPIs, which offers a good estimation of growth rate in the linear development stage. Then, we discuss some progresses on the understanding of LPIs in more complex and real scenarios, such as LPI development in the nonlinear regions, cascaded LPIs, multi-beam LPIs, and nonlinear couplings between LPIs. Following the exploration of LPI physics, we emphasize on the strategies for the control of LPIs, including beam smoothing techniques, temporal profile shaping, broadband laser, laser polarization rotation, external magnetic field and so on. -
图 5 （a）三个不同的STUD脉冲的样本，其尖峰的宽度和高度的占比度为20%，50%和80%，涨落为10%，以使它们的乘积对于每个尖峰（开，关）都是恒定的[27]；（b）对于随机位相板和不同参数的STUD脉冲束SRS反射率与平均入射光强度和线性增益的关系[28]
Figure 5. （a） Samples of three different STUD pulses are shown with 20%，50% and 80% duty cycle and 10% jitter in the widths and heights of the spikes such that their product is constant for each spike （on，off） pair[27].（b） SRS reflectivity vs average incident intensity （left） and linear gain （right） for random phase plate （RPP） and a variety of STUD-pulse beams[28]
图 6 （a）解耦的宽带激光束由许多子束组成，例如100个子束，每两个相邻频率子束之间的频差大于0.1%；（b）不同带宽激光束的背散射光随时间的演化[33]
Figure 6. （a） A decoupled broadband laser beam is composed of many beamlets such as 100 beamlets with frequency difference larger than 0.1% between every two adjacent-frequency beamlets. （b） Temporal profiles of the backscattering light found for the incident light with different bandwidths under the same energy[33]
图 8 （a）（d）（g）当旋转频率分别为
$\varOmega /2{\text{π}} =0 {\text{，}}\varOmega /2{\text{π}} =1{\rm{THz}}$ ，$\varOmega /2{\text{π}} =5{\rm{THz}}$ 时散射光的瞬时反射率随时间的变化图，黑线电表反射率在z方向的分量，红线代表z和y方向的总反射率；（b），（e），（h）是与（a），（d），（g）对应的等离子体波随时间变化图；（c），（f），（i）是与（a），（d），（g）对应的电子动量分布随时间变化图。所有模拟的入射激光强度为$2\times {10}^{15}\;{\rm{W}}/{{\rm{cm}}}^{2}$ [39]Figure 8. （a），（d） and （g） are the normalized instantaneous reflectivities of the cases
$\varOmega /2{\text{π}} =0$ ，$\varOmega /2{\text{π}} =1{\rm{THz}}$ and$\varOmega /2{\text{π}} =5{\rm{THz}}$ . Black and red lines respectively represent the reflectivity component in the z direction and the total of the y and z directions. （b）， （e） and （h） are the time versus space of the plasma waves（Ex）for the cases corresponding to （a），（d） and （g）.（c）， （f） and （i） are the instantaneous electron energy distributions for the same Ω as （a），（d） and （g）. All the figures are under the pump intensity of$2\times {10}^{15}\;{\rm{W}}/{{\rm{cm}}}^{2}$ [39].
图 9 （a）交替偏振入射光情况下的强度分布，蓝色实线是在一个方向上偏振的光的强度分布，而红色虚线是在另一方向上偏振的光，两个偏振方向垂直，交替周期为400 T；（b）STUD脉冲与偏振方向交替性变化的SBS散射率[40]
Figure 9. （a） Intensity profile for the alternating polarization incident light case. The blue solid line is the intensity profile of light polarized in one direction，and the red dashed line is the light polarized in the other direction，The two light polarization directions are perpendicular to each otherand the alternating period is 400 T；（b） Scattering level of SBS with STUD and alternating polarization pulses[40]
图 11 四种等离子体中频率范围[0.9ω0，0.999ω0]中的SBS反射率：（a）H2；（b）HeH；（c）CH和（d）AuB等离子体。频率为ω0的弱反射光从背向散射光中滤除。对于（a）H2，种子光强度为Is＝1
$ \times 1{0}^{-6}{I}_{0} $ ，而对于（b）～（d），种子光强度为Is＝1$ \times 1{0}^{-4}\;{I}_{0} $ [43]Figure 11. The SBS reflectivity in the frequency scope [0.9ω0，0.999ω0] for the four plasmas：（a） H2，（b） HeH，（c） CH，and （d） AuB plasmas. The weak reflected light with a frequency of ω0 is filtered from the backward scattering light. For（a）H2， the seed light intensity is Is＝1
$ \times 1{0}^{-6}{I}_{0} $ ，while for （b）～（d），Is＝1$ \times 1{0}^{-4}\;{I}_{0} $ [43]
图 12 （a）等离子体尺度L为1200λ，（b）等离子体尺度为400λ。对于大尺度的等离子体散射率随着激光强度的增加而增加，随着磁场强度的增加而降低。当磁场强度足够强时，可以完全降低SRS散射率到能够接受的水平[45]
Figure 12. Relectivities of Raman scattering for different magnetic fields and incident lights. Values of laser amplitude a are shown with legends. （a） seed level 10−9，plasma length L＝1200λ0；（b） seed level 10−9，and plasma length L＝400λ0[45]
图 13 （a）PIC模拟中，磁化等离子体（
$0.01，{T}_{{\rm{e}}}={T}_{{\rm{i}}}=1 {\rm{eV}}$ ）中的背向散射光的能量光谱分布。其中磁场${{B}}_{0}$ 的模拟范围从10 T到10 kT，与散射光之间的夹角为$ 75° $ ；（b）磁化等离子体在${{{\varOmega }}_{\rm{e}}}{{/}}{{\rm{\omega }}_{\rm{0}}}{\rm{ = 0}}{\rm{.75}}\;({{{B}}_{\rm{0}}}{\rm{ = 8100}}\;{\rm{T)}}$ 时随角度变化的背向散射光谱，表现出MLF散射对角度的依赖性，其中虚线代表理论分析预测[46]Figure 13. （a） Spectral energy distribution of light backscattered from magnetized plasma （
$N=0.01，{T}_{{\rm{e}}}={T}_{{\rm{i}}}=1{\rm{eV}}$ ） in PIC simulations. The magnetic field B0 is varied between 100 T and 10 kT at$\theta =75$ °.（b） Backscattering spectrum from a magnetized electron-proton plasma as a function of angle at${{{\varOmega }}_{\rm{e}}}{{/}}{{\rm{\omega }}_{\rm{0}}}{\rm{ = 0}}{\rm{.75}}\;({{{B}}_{\rm{0}}}{\rm{ = 8100}}\;{\rm{T)}}$ ，illustrating dependence of MLF scattering on angle. Dashed lines show analytic predictions[46]表 1 不同机制的激光等离子体不稳定性
Table 1. Different mechanisms of laser plasma instabilities
instability decay process Stimulated Raman Scattering (SRS） EM → EM ＋ EPW Stimulated Brillouin Scattering (SBS) EM → EM ＋ IAW Two Plasmon Decay (TPD) EM → EPW ＋ EPW ion acoustic decay instability EM → EPW ＋ IAW Langmuir decay instability EPW → EPW ＋ IAW electromagnetic wave decay instability EPW → EM ＋ IAW two ion acoustic decay instability IAW → IAW ＋ IAW 
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Rochester: Laboratory for Laser Energetics（LLE）, University of Rochester, 2018. -
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