Study of Improved Confinement in Helical Plasmas

在本研究项目中，研究了螺旋等离子体中改进约束的可能性，重点研究了“电场分岔和湍流抑制”的机制。此外，还探讨了利用CHS装置进行实验测试的方案。在分析中，我们首先研究了在螺旋等离子体中实现电场界面的条件。具有不同电场极性的畴相互接触的电场界面与强径向电场梯度有关。这种电场梯度可以达到抑制微观等离子体湍流的水平。这种结构已经被预测存在，问题是研究它是否在静止状态下自我维持，以及这种静止状态是否容易达到。本项目通过探索一个理论模型和包含接口存在的传输仿真代码来研究这些问题。将微观湍流的理论模型与界面的研究相结合，导出了一个阈值条件。验证了自维持机构在日光加速器/扭转加速器型配置下的工作，并在控制参数空间中分析了达到这种状态的可达性。得到了相图。在理论框架的基础上，进行了实验研究。结果表明，电场界面可以抑制等离子体的微观湍流。在W7-AS和LHD器件中也发现了电子能量的内部输运势垒，证实了理论模型的有效性。通过这个项目，等离子体湍流、跃迁和结构形成的理论有了实质性的进展。

In this research project, a possibility of improved confinement in helical plasmas is investigated, putting an emphasis on the mechanism of "electric field bifurcation and turbulence suppression". In addition, a proposal of experimental test by use of CHS device is examined.In the analysis, we first study the condition that the electric field interface is realized in helical plasmas. The electric field interface, across which the domains with different electric field polarity contact each other, is associated with the strong gradient of radial electric field. This gradient of electric field could reach the level where the microscopic plasma turbulence could be suppressed. This structure has been predicted to exist, and the issue is to investigate whether it is self-sustained in a stationary state and whether such a stationary state is easily accessible or not.These problems were studied in this project, by exploring a theoretical model together with the transport simulation code which includes the presence of interface. Theoretical model of microscopic turbulence is combined with the study of interface, and a threshold condition is derived. The self-sustaining mechanism is verified to work in heliotron/torsatron type configurations, and the accessibility to such a state is analyzed in the space of controlling parameters. A phase diagram was obtained. Based on the theoretical framework, experimental studies were performed. It was shown that the electric field interface is possible to occur in CHS so as to suppress the microscopic plasma turbulence. The internal transport barrier for electron energy was also found in W7-AS and LHD devices, confirming the validity of the theoretical model.Through this project, theory for plasma turbulence, transition and structure formation has been progressed substantially.