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在直接驱动激光聚变研究中, 激光辐照靶丸会激励起受激布里渊散射(SBS)和交叉束能量传递(CBET)等过程, 降低激光与靶丸的能量耦合效率以及激光辐照均匀性, 导致靶丸内爆品质下降. 本文使用一套基于光纤收集信号的背向散射诊断系统, 诊断了双锥对撞点火(DCI)实验中波长在351 nm附近的时间分辨背向散射光谱. 通过对比不同激光辐照条件下散射光谱的特征, 结合光谱强度与入射激光能量以及激光偏振态的相关性分析, 确认背向散射信号中包含了分别来自CBET和背向SBS过程的散射成分, 确认镜像激光束之间的偏振夹角对CBET的影响. 实验结果表明, 在当前DCI实验中, 在351 nm附近的背向反射率不高于3%, 显著低于球对称辐照直接驱动中心点火方案的实验结果.
In the research of direct-drive laser fusion, laser irradiation of a target pellet can stimulate various laser plasma instabilities, such as stimulated Brillouin scattering (SBS) and cross-beam energy transfer (CBET), which significantly reduce the energy coupling efficiency between the laser and target pellet as well as the laser irradiation uniformity, leading the implosion quality to degrade. In the double-cone ignition (DCI) scheme of laser fusion scheme, the diagnosis of SBS and CBET is important owing to the different target configurations and oblique incident laser irradiation from the traditional spherically symmetric direct-drive central ignition scheme. In this paper, a simple and reliable backscattering diagnostic system is developed and applied to the diagnosis of the time-resolved backscattering spectrum at wavelength near 351 nm in a DCI experiment on the Shenguang-II upgrade (SG-IIU) facility. We use the system to carry out an experimental study of the SBS process and CBET process in DCI. The backscattering diagnostic system collects the backscattered light signal through the scattered light by reflector mirror via an optical fiber. The signal is dispersed by a spectrometer and then recorded by a streak camera. The signal contains both the laser reference signal from the frequency doubling crystal and the backscattered light. With the help of the reference signal, the diagnostic system can reliably give the energy fraction of backscattered light. The experimental results show that the energy fraction of backscattered light around 351 nm is not higher than 3%, which is significantly lower than the experimental result of the spherically symmetric irradiation direct-drive central ignition scheme. By analyzing the correlation between the backscattered signal and the laser irradiation conditions and combining the results of a set of comparative experiments, we determine that the backscattered signal contains both CBET and SBS. There is a significant difference in the CBET fraction between the backscattered signal of the #5 laser and the backscattered signal of the #7 laser. By combining the polarisation state of the laser beams, we confirm that this phenomenon is related to the polarisation angle between the laser beams. This finding provides a reference for designing subsequent large-scale laser fusion devices. -
Keywords:
- double-cone ignition scheme /
- cross beam energy transfer /
- backward stimulated Brillouin scattering
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Gong T 2005 Ph. D. Dissertation (Hefei: University of Science and Technology of China) (in Chinese)
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图 1 神光Ⅱ升级激光打靶辐照示意 (a) 激光辐照俯视图; (b) 激光辐照侧视图
Fig. 1. Schematic diagram of Shenguang IIU laser irradiation configuration: (a) Top view of laser irradiation; (b) side view of laser irradiation.
图 2 激光偏振示意图 (a) 上四路激光偏振示意图; (b) 沿激光传输方向偏振示意
Fig. 2. Laser polarization diagram: (a) Diagram of the polarisation of the upper four lasers; (b) diagram of the polarisation along the direction of laser transmission.
图 3 背向散射诊断示意 (a) 激光光路光学元件组成与诊断设备; (b) 诊断系统典型结果
Fig. 3. Schematic diagram of backscatter diagnosis: (a) Composition and diagnosis equipment of optical elements of laser optical path; (b) typical results of diagnostic system.
图 4 典型背向时间分辨散射光谱与激光波形 (红色曲线)
Fig. 4. Typical time-resolved backscattering spectrum and laser waveform (red line).
图 5 对比实验的背向散射光谱
Fig. 5. The results of the experiments with different laser conditions.
图 6 SBS能量与各路激光能量关系 (a)与#1路激光能量的关系; (b)与#1+#3路激光能量之和的关系
Fig. 6. Relation between SBS energy and laser energy: (a) Relationship with #1 way laser energy; (b) relationship with the sum of #1+ #3 way laser energy.
图 7 #5, #7路三倍频波段背向散射统计 (a) 三倍频波段背向散射总份额; (b) CBET份额与SBS份额
Fig. 7. Statistics of #5 and #7 Channels: (a) Total triplet-band backscattered energy fraction; (b) CBET and SBS energy fraction.
表 1 #5, #7 路激光偏振情况
Table 1. #5, # 7 laser beam polarization.
诊断窗口 靶型 激光
入射角/(°)偏振方向 与入射面
夹角/(°)与靶面法向
夹角/(°)与镜像光束
偏振夹角/(°)反射光与镜像光束
偏振夹角/(°)#5背散
(#3镜面反射方向)双锥靶 50 7.5 40.6 81.2 15 #7背散
(#1镜面反射方向)双锥靶 50 23 45.2 89.7 46 
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[1] Nuckolls J, Wood L, Thiessen A, et al. 1972 Nature 239 139
Google Scholar
[2] McCrory R L, Regan S P, Loucks S J, et al. 2005 Nucl. Fusion 45 S283
Google Scholar
[3] Lindl J 1995 Phys. Plasmas 2 3933
[4] Craxton R S, Anderson K S, Boehly T R, et al. 2015 Phys. Plasmas 22 110501
Google Scholar
[5] He X T, Li J W, Fan Z F, Wang L F, Liu J, Lan K, Wu J F, Ye W H 2016 Phys. Plasmas 23 82706
Google Scholar
[6] Molvig K, Schmitt M J, Albright B J, et al. 2016 Phys. Rev. Lett. 116 255003
Google Scholar
[7] Betti R, Zhou C D, Anderson K S, Perkins L J, Theobald W, Solodov A A 2007 Phys. Rev. Lett. 98 155001
Google Scholar
[8] Clery D 2022 science. adg 378 1154
[9] Zhang J, Wang W M, Yang X H, et al. 2020 Philos. Trans. A Math. Phys. Eng. Sci. 378 20200015
[10] Kruer W L 1988 The Physics of Laser Plasma Interactions (New York: Addison-Wesley Publishing Company) p88
[11] Kirkwood R K, Moody J D, Kline J, et al. 2013 Plasma Phys. Control. Fusion 55 103001
Google Scholar
[12] Qiu J, Hao L, Cao L, Zou S 2021 Matter Radiat. at Extremes 6 65903
Google Scholar
[13] Neuville C, Tassin V, Pesme D, et al. 2016 Phys. Rev. Lett. 116 235002
Google Scholar
[14] Kritcher A L, Zylstra A B, Callahan D A, et al. 2022 Phys. Rev. E 106 25201
Google Scholar
[15] Froula D H, Igumenshchev I V, et al. 2012 Phys. Rev. Lett. 108 125003
Google Scholar
[16] Williams E A, Cohen B I, Divol L, et al. 2004 Phys. Plasmas 11 231
Google Scholar
[17] Turnbull D, Colaïtis A, Follett R K, et al. 2018 Plasma Phys. Control. Fusion 60 54017
Google Scholar
[18] Michel P, Rozmus W, Williams E A, Divol L, Berger R L, Glenzer S H, Callahan D A 2013 Phys. Plasmas 20 56308
Google Scholar
[19] Randall C J, Thomson J J, Estabrook K G 1979 Phys. Rev. Lett. 43 924
Google Scholar
[20] Kirkwood R K, Michel P, London R, et al. 2011 Phys. Plasmas 18 56311
Google Scholar
[21] Seka W, Edgell D H, Knauer J P, et al. 2008 Phys. Plasmas 15 56312
Google Scholar
[22] Igumenshchev I V, Edgell D H, Goncharov V N, et al. 2010 Phys. Plasmas 17 122708
Google Scholar
[23] Igumenshchev I V, Seka W, Edgell D H, et al. 2012 Phys. Plasmas 19 56314
Google Scholar
[24] Zhu J Q, Zhu J, Li X C, et al. 2018 High Power Laser Sci. Eng. 6 e55
Google Scholar
[25] 张喆, 远晓辉, 张翌航, 刘浩, 方可, 张成龙, 刘正东, 赵旭, 董全力, 刘高扬, 戴羽, 谷昊琛, 李玉同, 郑坚, 仲佳勇, 张杰 2022 71 155201
Google Scholar
Zhang Z, Yuan X H, Zhang Y H, Liu H, Fang K, Zhang C L, Liu Z D, Zhao X, Dong Q L, Liu G Y, Dai Y, Gu H C, Li Y T, Zheng J, Zhong J Y, Zhang J 2022 Acta Phys. Sin. 71 155201
Google Scholar
[26] 乔战峰, 卢兴强, 赵东峰, 朱宝强 2008 中国激光 9 1328
Google Scholar
Qiao Z F, Lu X Q, Zhao D F, Zhu B Q 2008 Chin. J. Lasers 9 1328
Google Scholar
[27] 赵闯, 袁鹏, 李欣焱, 郑坚, DCI 联合研究团队 2023 光学学报 43 1114001
Zhao C, Yuan P, Li X Y, DCI Joint Research Team 2023 Acta Opt. Sin. 43 1114001
[28] 龚韬 2015 博士学位论文 (合肥: 中国科学技术大学)
Gong T 2005 Ph. D. Dissertation (Hefei: University of Science and Technology of China) (in Chinese)
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