托卡马克等离子体中低杂波参量不稳定性的模拟和理论研究

As an important tool for steady state operation of tokamak, lower hybrid(LH) waves are widely used in the fusion experiments for the sustainment of fully non-inductive discharges. However, as the injected wave power and plasma density increase, the LH wave power coupling efficiency and current drive efficiency decrease anomalously, and parametric instability (PI) is considered as the primary cause of this issue, which has been observed and confirmed in many experiments. Thus, in order to develop a steady state current drive method for present tokamaks in high parameter regime and for future reactor-scale experiments, it is important to understand and mitigate LH wave PIs..Firstly, in this work, GTC-RF code based on nonlinear particle-in-cell simulation method will be applied to study LH wave PIs. The decay channels and mechanisms of PI in different parameter regimes can be identified by measuring the frequency and growth rate of low frequency mode from GTC-RF simulation and comparing with theory. The scaling of LH wave PI strength on different macroscopic parameters will be summarized. Secondly, we will study LH wave PIs by using experimental data of domestic tokamak device, which aim to provide analysis for understanding the experimental observation and optimizing the future experiments, as well as providing ideas for improving LH wave current drive efficiency in the future large tokamak such as ITER and CFETR.

低杂波被广泛应用于驱动托卡马克等离子体中的非感应电流，是维持托卡马克稳态运行的重要工具。然而，随着低杂波功率和托卡马克等离子体密度不断提高，低杂波耦合效率和电流驱动效率明显下降，大量实验证据表明参量不稳定性是造成该问题的最重要原因。因此，为了在目前高参数托卡马克以及未来反应堆级别的托卡马克实验中找到有效的电流驱动方法，理解和缓解低杂波参量不稳定性极其重要。 .第一，本工作拟采用非线性粒子模拟方法，利用已开发的GTC-RF程序对低杂波参量不稳定性展开研究，通过测量模拟得到的低频模式频率和增长率并结合理论验证，分析不同宏观参数下的参量衰变渠道和机制，并给出参量不稳定性强度随各宏观参数的定标关系。第二，本工作拟针对国内装置的实验数据进行模拟分析，解释实验结果并提出优化方案，也为未来大型托卡马克如ITER和CFETR上低杂波驱动电流提供模拟依据。

高功率低杂波注入等离子体时会通过非线性波-波耦合激发参量不稳定性，造成波功率损失以及耦合效率下降。理解和控制低杂波参量不稳定性，对提升电流驱动效率具有重要意义。本项目围绕低杂波参量不稳定性开展理论与模拟研究。在理论方面，基于约化电磁模型推导出参量不稳定性非线性电磁色散关系，修正了广泛使用的非线性静电色散关系，对于刮削层参数条件下研究参量不稳定性更加准确；通过计算不同散射夹角下的离子回旋波准模和离子声波准模衰变，发现电磁效应主要通过平行方向非线性项降低参量不稳定性增长率，并发现提高泵波频率和平行方向折射率、提高电子温度、提高平衡磁场强度有助于抑制低杂波参量不稳定性。在模拟方面，在真实托卡马克几何位形下并考虑等离子体和泵波波场非均匀性，采用GTC-RF程序模拟了低杂波参量不稳定性的完整动力学过程，得到了相应的功率谱和波场结构，基于第一性原理证实了边频波与泵波沿平行磁场方向反向传播；另外，本项目开发了低杂波射线追踪程序LHRAY，适用于实验上建模优化低杂波参数；开发了等离子体不稳定性本征值程序MAS，适用于实验上分析波加热和电流驱动过程中产生的高能量粒子激发的阿尔芬本征模。本项目的研究成果为实验上缓解低杂波参量不稳定性、提升高功率低杂波与等离子体耦合效率提供了理论与模拟参考。