[论文解读] A cylindrical implosion platform for the study of highly magnetized plasmas at LMJ

G. Pérez-Callejo, C. Vlachos+28 more|arXiv (Cornell University)|Mar 22, 2022

Laser-Plasma Interactions and Diagnostics被引用 1

一句话总结

本文提出在激光巨量设施（LMJ）中构建一种激光驱动的圆柱形内爆平台，利用激光驱动线圈靶产生轴向种子磁场，以研究高度磁化的等离子体。模拟结果显示，初始5 T磁场被压缩至25 kT，使电子磁化，并将热点温度从1 keV提升至5 keV，同时密度从40 g/cm³降低至7 g/cm³。诊断方法提出采用双掺杂剂（Ar和Kr）实现空间分辨光谱测量。

ABSTRACT

Investigating the potential benefits of the use of magnetic fields in Inertial Confinement Fusion (ICF) experiments has given rise to new experimental platforms like the Magnetized Liner Inertial Fusion (MagLIF) approach at the Z-machine (Sandia National Laboratories), or its laser-driven equivalent at OMEGA (Laboratory for Laser Energetics). Implementing these platforms at MJ-scale laser facilities, such as the Laser MegaJoule (LMJ) or the National Ignition Facility (NIF), is crucial to reaching self-sustained nuclear fusion and enlarges the level of magnetization that can be achieved through a higher compression. In this paper, we present a complete design of an experimental platform for magnetized implosions using cylindrical targets at LMJ. A seed magnetic field is generated along the axis of the cylinder using laser-driven coil targets, minimizing debris and increasing diagnostic access compared with pulsed power field generators. We present a comprehensive simulation study of the initial B-field generated with these coil targets, as well as 2-dimensional extended magneto-hydrodynamics (MHD) simulations showing that a 5T initial B-field is compressed up to 25kT during the implosion. Under these circumstances, the electrons become magnetized, which severely modifies the plasma conditions at stagnation. In particular, in the hot spot the electron temperature is increased (from 1keV to 5keV) while the density is reduced (from 40gcc to 7gcc). We discuss how these changes can be diagnosed using X-ray imaging and spectroscopy, and particle diagnostics. We propose the simultaneous use of two dopants in the fuel (Ar and Kr) to act as spectroscopic tracers. We show that this introduces an effective spatial resolution in the plasma which permits an unambiguous observation of the B-field effects. Additionally, we present a plan for future experiments of this kind at LMJ.

研究动机与目标

• 开发一种可扩展、便于诊断的平台，用于在LMJ等MJ量级激光装置中研究磁化惯性约束聚变。
• 通过使用激光驱动线圈（LDC）靶，克服脉冲功率系统磁场产生方式的局限性，如碎片污染和轴向诊断路径受阻问题。
• 通过等离子体压缩实现更高磁化水平，推动在更高能量尺度下探索磁化ICF物理。
• 设计一种诊断策略，利用双掺杂剂（Ar和Kr）以明确观测等离子体中磁场效应。

提出的方法

• 采用激光驱动线圈（LDC）靶产生轴向种子磁场，最大限度减少碎片并保持轴向诊断通路畅通。
• 利用二维扩展磁流体动力学（MHD）模拟研究内爆动力学与磁场放大过程。
• 模拟显示初始5 T种子磁场在内爆过程中被压缩至25 kT，实现电子磁化。
• 在燃料中引入双掺杂剂（氩和氪），作为光谱示踪剂，实现空间分辨的等离子体诊断。
• 提出X射线成像与光谱测量作为主要诊断手段，用于测量温度、密度及磁场效应。
• 利用质子偏转与门控X射线成像验证磁场强度及空间梯度。

实验结果

研究问题

• RQ1 激光驱动线圈靶是否能够生成稳定、足够的轴向种子磁场，以支持在LMJ上开展磁化内爆研究？
• RQ2 磁场压缩在多大程度上可将初始磁场放大至足以磁化电子的水平？
• RQ3 在停滞阶段，磁化电子如何改变热点条件，特别是温度与密度？
• RQ4 双掺杂剂（Ar和Kr）是否能提供有效的光谱诊断空间分辨率，以明确分离磁场效应？
• RQ5 在测量磁场诱导的等离子体变化方面，哪种诊断配置（X射线成像、光谱测量、粒子束）最为有效？

主要发现

• 通过激光驱动线圈靶产生的5 T轴向种子磁场在圆柱形内爆过程中被压缩至25 kT，实现电子磁化。
• 电子磁化使热点温度从1 keV提升至5 keV，同时密度从40 g/cm³降低至7 g/cm³。
• 采用双掺杂剂（氩和氪）可实现光谱测量中的有效空间分辨率，从而明确检测磁场效应。
• X射线成像与光谱测量被证明是观测磁场诱导等离子体状态变化的可行诊断工具。
• 提出并验证了质子偏转诊断方法，用于测量内爆区域的磁场强度与梯度。
• 该平台设计最大限度减少碎片并保持轴向视线通路，相比脉冲功率系统，显著提升了诊断可及性。