Turbulence Nonlinearities Shed Light on Geometric Asymmetry in Tokamak Confinement Transitions

A comprehensive study of fully frequency-resolved nonlinear kinetic energy transfer has been performed for the first time in a diverted tokamak, providing new insight into the parametric dependences of edge turbulence transitions. Measurements using gas puff imaging in the turbulent L-mode state illuminate the source of the long known but as yet unexplained “favorable-unfavorable” geometric asymmetry of the power threshold for transition to the turbulence-suppressed H mode. Results from the recently discovered I mode point to a competition between zonal flow (ZF) and geodesic-acoustic modes (GAM) for turbulent energy, while showing new evidence that the I-to-H transition is still dominated by ZFs. The availability of nonlinear drive for the GAM against net heat flux through the edge corresponds very well to empirical scalings found experimentally for accessing the I mode.

本研究首次在偏滤器托卡马克（diverted tokamak）中开展了全频率分辨的非线性动能输运综合研究，为阐明边缘湍流转变的参数依赖关系提供了全新视角。借助湍流L模（L-mode）状态下的喷气成像（gas puff imaging）测量，研究明确了长期被观测到但尚未得到合理解释的、向湍流抑制H模（H-mode）转变的功率阈值几何不对称性的“有利-不利”效应来源。针对近期发现的I模（I mode）的研究结果显示，带状流（zonal flow, ZF）与测地声模（geodesic-acoustic modes, GAM）在湍流能量的争夺中存在竞争关系，同时为“I模向H模的转变仍以带状流为主导”这一结论提供了新的实验证据。边缘区域净热通量条件下测地声模能够获得非线性驱动的情况，与实验中获取I模的经验定标律吻合极佳。