Study of Improved Confinement in Helical Plasmas

本研究计划主要探讨螺旋等离子体中改善约束的可能性，并着重探讨“电场分叉与湍流抑制”的机制。在分析中，我们首先研究了螺旋等离子体中电场界面实现的条件。不同电场极性的畴相互接触的电场界面与强的径向电场梯度有关。这种电场梯度可以达到可以抑制微观等离子体湍流的水平。这种结构已经被预测存在，问题是调查它是否是自我维持在一个稳定的状态，以及是否这样的稳定状态是容易访问或不。这些问题在本项目中进行了研究，通过探索一个理论模型，连同运输模拟代码，其中包括接口的存在。将微观湍流的理论模型与界面的研究相结合，导出了一个临界条件。验证了自持机制在Heliotron/Torsatron型构型中工作，并在控制参数空间中分析了这种状态的可达性。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，在CHS中可能存在电场界面，从而抑制微观等离子体湍流。在W7-AS和LHD装置中也发现了电子能量的内输运势垒，证实了理论模型的正确性。

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.