热核等离子体时间演化和不对称性对聚变产物能谱的影响研究

The primary goal of nuclear fusion research in the lab nowadays is ignition and burn. An essential prerequisite for achieving ignition is to compress fusion fuel to a state with high enough areal density and temperature which meets the Lawson criterion by optimized implosion design. Diagnostics of plasma state near stagnation hence plays an important role in the study of implosion physics. One way to diagnose the imploded capsule is to measure the spectra of the fusion products, including secondary neutrons and protons from DD implosions, down-scattered neutrons and knock-on charged particles from DT implosions, and primary protons form D3He implosions. To realize this diagnostic method, the relationship between the state of the imploded capsule and the spectra of fusion products must be established first. The majority of existing theoretical analysis on spectra of fusion products is based on the assumption that the capsule is static and spherically symmetric. The impact of time evolution and asymmetry is, however, neglected. This may cause some misinterpretation of the fusion spectra and some misunderstanding of the plasma state. To solve this problem, our project aims to study: (1) The correlation between time evolution of plasma state and time evolution of fusion products; (2) The correlation between low-mode polar asymmetries of the imploded capsule and the angular distribution of the fusion products. Our research mainly relies on Monte Carlo particle transport simulations and radiation-hydrodynamics simulations.

点火和燃烧是当前实验室核聚变研究的首要目标。通过优化的内爆过程将聚变燃料压缩到满足Lawson判据要求的温度和密度状态是实现惯性约束聚变点火的先决条件之一，因而阻滞阶段热核等离子体状态诊断对于惯性约束聚变内爆物理研究而言具有重要意义。诊断阻滞阶段等离子体状态的一种重要方法是通过测量聚变产物（包括DD次级质子和中子、DT下散射中子和反冲带电粒子、D3He初级质子）的能谱来实现。其应用的基础是建立靶丸状态和聚变能谱之间的定量关系。现有的聚变产物能谱理论分析多基于静态、球对称假设，忽略了时间演化和不对称性的影响，可能引起聚变能谱的错误解读，导致诊断出的靶丸状态偏差较大。为了解决这个问题，本项目拟研究：1）热核等离子体时间演化与聚变产物能谱时间演化的关联性；2）热核等离子体低阶模不对称性与聚变产物能谱角分布的关联性。本项目的研究手段以Monte Carlo粒子输运模拟为主，辐射流体程序模拟为辅。

聚变产物能谱及其特征量是表征激光聚变内爆阻滞阶段等离子体状态的最主要物理量。通过数值手段研究聚变产物能谱及其特征量与等离子体状态之间的变化规律可以为相应核诊断方法的建立奠定基础。本项目采用蒙特卡罗方法建立了聚变产物粒子与具有复杂时间和空间分布的等离子体相互作用的粒子输运程序。程序使用ENDF核反应数据库，考虑了中子与反应核的弹性散射和各种非弹性散射。程序使用文献中的理论和经验模型，考虑了等离子体对带电聚变粒子的能量损失，以及常见核反应。通过采用强迫碰撞的减方差技巧，大大提升了程序的模拟效率。以激光聚变核诊断中最常用的DT中子和DHe3质子为例，讨论了等离子体面密度时间演化和低阶不对称性分别对DT中子下散份额和DHe3质子能量下移的影响。本项目研究成果为开展核诊断研究奠定了基础，也对建立相关的核诊断方法提供了参考。