Application of cross-beam energy transfer to control drive symmetry in ICF implosions in low gas fill Hohlraums at the National Ignition Facility
Application of cross-beam energy transfer to control drive symmetry in ICF implosions in low gas fill Hohlraums at the National Ignition Facility
复制标题
应用交叉束能量转移控制国家点火装置低气体填充 Hohlraums 中 ICF 内爆的驱动对称性
DOI：
10.1063/5.0004866
复制
发表时间：
2020
影响因子：
2.2
通讯作者：
O. Hurricane
中科院分区：
文献类型：
--
作者：
L. Pickworth;T. Döppner;D. Hinkel;J. Ralph;B. Bachmann;L. Masse;L. Divol;L. Benedetti;P. Celliers;H. Chen;M. Hohenberger;S. Khan;O. Landen;N. Lemos;B. MacGowan;D. Mariscal;P. Michel;M. Millot;A. Moore;J. Park;M. Schneider;D. Callahan;O. Hurricane
在国家点火装置（NIF）的间接驱动惯性约束聚变实验中，交叉束能量传递（CBET）通过设置束锥内、外波长差Δλ来实现。从历史上看，在高（≥0.9 mg/cm 3 4 He）气体填充的黑腔中，胶囊内爆的热点对称性控制是通过大量的CBET实现的。然而，这些内爆设计遭受飞行中对称摆动，高SRS后向散射的内锥，和显着的热电子产生的威胁DT燃料预热。随后在较大的低（≤0.6 mg/cm 3 4 He）气体填充的空腔中进行的实验表明，通过在Δλ = 0 A的激光驱动过程中改变内锥分数，同时保持反向散射和热电子产生非常低，可以实现圆形内爆。为了能够在给定的空腔尺寸下驱动更大的胶囊，需要用于内爆对称性控制的附加工具。考虑到这一目标，本文提出了一个详细的实验研究，使用CBET在低气体填充黑腔NIF的电流峰值功率能力附近。我们发现，与高充气设计相比，P2 Legendre模式对Δλ变化的灵敏度高出2.5倍。我们将这一观察结果归因于这样一个事实，即后向散射仍然非常低，CBET仍然处于线性状态，正如模拟所示。因此，1A量级的小得多的Δλ足以维持内爆对称性，同时保持激光-空腔耦合高和热电子产生非常低。虽然这项研究使用塑料烧蚀胶囊，我们的研究结果可以推广到其他烧蚀材料，因此，显示出很大的希望，使用波长失谐作为一个强大的杠杆内爆对称性控制在未来的低气体填充设计，需要较小的情况下，胶囊的比例，以增加能量耦合到胶囊。
Cross beam energy transfer (CBET), invoked by setting a wavelength difference, Δλ, between inner and outer beam cones, can be used to increase the drive on the waist in indirectly driven inertial confinement fusion experiments at the National Ignition Facility (NIF). Historically, hot spot symmetry control in capsule implosions in high (≥0.9 mg/cm3 4He) gas fill Hohlraums was enabled by substantial CBET. However, these implosion designs suffered from inflight symmetry swings, high SRS backscatter on the inner cones, and significant hot electron generation posing a threat to DT fuel preheat. Subsequent experiments in larger, low (≤0.6 mg/cm3 4He) gas fill Hohlraums demonstrated round implosions by varying the inner cone fraction throughout the laser drive at Δλ = 0 A while keeping backscatter and hot electron generation very low. To enable driving larger capsules at a given Hohlraum size, additional tools for implosion symmetry control are required. With this goal in mind, this paper presents a detailed experimental study of using CBET in low gas fill Hohlraums near NIF's current peak power capability. We find a ∼2.5× higher sensitivity of the P2 Legendre mode with respect to Δλ changes compared to that of high gas fill designs. We attribute this observation to the fact that backscatter remains very low and that CBET remains in a linear regime, as suggested by simulations. As a result, a much smaller Δλ of order 1 A is sufficient for sustaining implosion symmetry while keeping laser-to-Hohlraum coupling high and hot electron generation very low. While this study used plastic ablator capsules, our findings can be generalized to other ablator materials and, hence, show great promise for using wavelength detuning as a strong lever for implosion symmetry control in future low gas fill designs that require smaller case to capsule ratios in order to increase the energy coupled to the capsule.