Experimental validation of non-uniformity effect of the radial electric field on the edge transport barrier formation in JT-60U H-mode plasmas

Abstract

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 (Er) are known inevitable. In this mechanism, whether the first derivative of Er (shear) or the second derivative of Er (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 Er-curvature on the ETB formation, for the first time, based on the high-spatiotemporal resolution spectroscopic measurement. We found the decisive importance of Er-curvature on ETB formation during ELM-free phase, but there is only a low correlation with the Er-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 Er. This is the fundamental basis to understand the structure of transport barriers in fusion plasmas.

Frequency and plasma condition dependent spatial structure of low frequency global potential oscillations in the TJ-II stellarator

“…The transition from L-to H-mode plasmas is characterized by the development of an edge transport barrier, the suppression of turbulence, the formation of a negative radial electric field well in the edge and the reduction of emission [166,167]. During the L-H transition, the turbulence is decorrelated by × shear flows and by inhomogeneities in radial electric field [168]. The understanding of H-mode regimes has led to a more complete understanding of turbulent transport in high temperature plasmas for nuclear fusion.…”

Turbulent transport reduction induced by transition on radial electric field shear and curvature through amplitude and cross-phase in torus plasma

“…In the present case, is considered to hold 7 so that the toroidal velocity correction term for Z 2 is neglected. Recent investigation in JT-60U 8 showed an important role of not only Z 1 but also of Z 2 by referring to temperature gradient profile as an indicator of confinement intensity.…”

“…Numerous studies have shown essential roles of an edge-localized poloidal E × B flow structure on confinement improvement 4 – 7 . However, definitive conclusion regarding what aspect of the E × B flow suppresses the turbulent transport is still under debate 8 . In order to obtain an understanding based on first principles, interplay between the flow structure and the turbulence must be diagnosed with high spatiotemporal resolutions, which remains challenging.…”

“…In that model, responsible physics of shear and curvature were considered as the E × B flow shear decorrelation of turbulence eddy 13 – 15 and the modulational coupling causing energy transfer from turbulence to macroscopic E × B flow structure 2 , 12 , 16 , respectively. Experimental examinations of those models have been performed 8 , 17 – 25 . Focusing on detailed physics of transport suppression, direct fluctuation measurement by use of electrostatic probes has been promoted.…”

Bifurcation phenomena in magnetically confined toroidal plasmas

“…The product of radial electric field and the 2nd derivative of the radial electric field -E r E 0 r $ as well as E 0 r was predicted to suppress the turbulence and reduce the transport. The impact of -E r E 0 r and E 0 r on the transport reduction was experimentally tested in JFT-2 M H-mode plasma [56].…”