托卡马克边缘等离子体中锂杂质输运和其对背景等离子体影响的模拟研究

Magnetically controlled nuclear fusion is well recognized to be one of possible ultimate ways of solving energy crisis for human beings. However, as the performance of confined plasma improves greatly and discharge time prolongs by orders of magnitude, the issues induced by plasma-material interactions turn to be so outstanding that they become a dominant factor determining whether nuclear fusion energy can be generated economically viably. As the interface of plasma-material interactions, the edge plasma plays an important role in the exchange of momentum and energy between core plasma and plasma facing-components; in the meantime, one has to fully utilize the edge plasma to regulate the plasma. Taming the edge plasma is one of key issues of the ITER project and future fusion reactors. Real and potential applications of lithium in fusion plasma all get involved with a fundamental physical issue, transport of lithium in the edge plasma, which will be the focus of this present project. We will establish a fluid model of lithium transport in the edge plasma by reducing Braginskii’s equations based on features and properties of lithium applications and edge plasma, and solve the fluid model in the frame of BOUT++. This project is planned to study the transport of lithium in the edge plasma in the experiments of real-time dropping lithium from the upper divertor in the EAST, and in other potential scenarios to quantitatively know how and where lithium deposits on the wall, understand why lithium has little capability of penetrating into the core plasma but has good capability of preventing other impurities from contaminating the core plasma, and how lithium fluxes can alleviate the energy fluxes of plasma to the divertor. The project is also to study the radiative lithium divertor, and assess its viability in the future fusion-reactors. If this proposal is funded, the execution of the aforementioned projection can uncover many physical mechanisms about lithium transport in the edge plasma to facilitate more successful applications of lithium in fusion plasmas in the future.

托卡马克磁控核聚变被认为是解决人类能源危机的终极方案。但随着等离子体参数的提高及放电时间的急剧增加，托卡马克中的等离子体与面壁相互作用日益严重，解决等离子体与面壁相互作用引起的问题已成为实现经济可行核聚变的关键。边缘等离子体作为等离子体和面壁相互作用的载体，以及人工调制等离子体行为必须借助的介质，已成为近期研究重点。针对锂在聚变等离子体中的实际和潜在应用，本项目重点研究它们所涉及的基本物理问题—锂杂质在边缘等离子体中的输运。根据实际应用及边缘等离子体的特点, 简化Braginskii方程，建立锂杂质和背景等离子体相互耦合的流体输运模型，在BOUT++框架下，开发相应的模块，模拟锂杂质在边缘等离子体中的输运，研究EAST实时锂化面壁过程中锂杂质在面壁的沉积规律，理解芯部等离子体中锂杂质浓度低的物理机制，解释锂杂质能降低偏滤器处能流的物理原因，评估辐射偏滤器的应用前景，为实验工作提供理论指导。

本项目围绕锂应用涉及的两个基本科学问题：锂杂质如何输运及分布的； 等离子体如何反应的的。利用SOLPS和BOUT++平台下开发的杂质输运模块首先对EAST实时注入锂粉实验模拟研究，搞清了各价态的锂杂质在边界等离子体的分布特点：中性锂原子和一价二价锂离子基本存在于刮削层，进入到磁场分界线（sepx）里的锂杂质所占比例很小，主要为三价锂离子；锂杂质可以辐射一定比例的等离子体能量，并且主要集中在注入区域和偏滤器区域；锂杂质导致等离子体密度剖面在径向台基底部小局域升高，边界温度剖面的高度在径向大区域降低。不同的注入位置，注入速度以及等离子体放电参数会影响这种改变。同时，利用SOLPS程序分析了锂化壁第一壁会对放电等离子体产生怎样的影响,发现除改变壁面处粒子的再循环外，与锂粉注入相比，其对高约束等离子体放电的影响较小，主要是因为锂杂质密度低两个两级左右，进入sepx 里面的锂杂质比例很小可以忽略；引入锂杂质后，达到靶面的热流宽度稍有展宽，但对高约束高参数放电的热流宽度展宽不明显。耦合杂质效应拓展了BOUT++六场模型，模拟研究一定小比例锂杂质进入到sepx内后在大ELM爆发前的不稳定性线性增长阶段，对剥离气球模起一定的致稳作用。针对弹丸注入调制ELM实验，发展了含时的弹丸烧蚀模型，并将计算得到的中性杂质密度作为初始信息输入到SOLPS程序中去，模拟研究了不同尺寸弹丸和不同注入条件下的EAST弹丸触发ELM实验，给出了不同成分的演化分布，总结出了ELM触发的弹丸阈值大小，与实验结果符合很好，同时解释了实验观察到的概率性ELMs 触发效率的现象。锂辐射偏滤器可能是未来聚变装置为满足材料热负荷的一种选择，粒子模拟的初步结果显示靶板表面的中性锂原子层可以使达到偏滤器靶面的能流减小40%，但是模拟显示锂离子可沿sepx进入到上游等离子体。在本项目的资助下，我们不但完成了项目的既定目标，也为更深入研究ELM的触发过程和研究辐射偏流器打下了基础。