The turbulent structure formation, where strongly-inhomogeneous turbulence and global electromagnetic fields are self-organized, is a fundamental mechanism that governs the evolution of high-temperature plasmas in the universe and laboratory (e.g., the generation of edge transport barrier (ETB) of the H-mode in the toroidal plasmas). The roles of inhomogeneities of radial electric field (E r) are known inevitable. In this mechanism, whether the first derivative of E r (shear) or the second derivative of E r (curvature) works most is decisive in determining the class of nontrivial solutions (which describe the barrier structure). Here we report the experimental identification of the essential role of the E r -curvature on the ETB formation, for the first time, based on the high-spatiotemporal resolution spectroscopic measurement. We found the decisive importance of E r -curvature on ETB formation during ELM-free phase, but there is only a low correlation with the E r -shear value at the peak of normalized ion temperature gradient. Furthermore, in the ELMing phase, the effect of curvature is also quantified in terms of the relationship between pedestal width and thickness of the layer of inhomogeneous E r. This is the fundamental basis to understand the structure of transport barriers in fusion plasmas.
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