Evidence of Long-Distance Correlation of Fluctuations during Edge Transitions to Improved-Confinement Regimes in the TJ-II Stellarator

Abstract

Long-distance coupling between edge parameters' fluctuations has been investigated in the TJ-II stellarator. Results show long-range correlations in potential fluctuations, which are amplified by the development of radial electric fields during transitions to improved-confinement regimes, whereas there is no correlation between ion saturation current signals. These experimental findings suggest the importance of long-range correlations as a new fingerprint of the plasma behavior during the development of edge shear flows and the key role of electric fields to amplify them.

Integrated data analysis at TJ-II: The density profile

“…11. These discharges have been analyzed in previous work 29,30 . Significant long-range correlations (using toroidally separated Langmuir probes in the edge region) and bicoherence were detected, indicating the formation of an edge transport barrier associated with bias-induced shear flow.…”

Long-distance correlation and zonal flow structures induced by mean E×B shear flows in the biasing H-mode at TEXTOR

“…12 and 13. Very recently, in the TJ-II stellarator 14 an amplification of the LDC by biasing electric fields has been found in potential fluctuations during the transition to improved confinement. In the ISTTOK tokamak, 15 the LDC of potential fluctuations displays bursty features and couples with reduced turbulent transport.…”

Dynamic transport regulation by zonal flow-like structures in the TJ-II stellarator

“…Experimentally, global, fluctuating flow patterns manifest themselves as electric potential structures exhibiting long-range correlation (LRC), and have been observed in different devices using Langmuir probes and Heavy Ion Beam probes [5,6,7,8]. These LRC potential structures oscillate but are constant on a flux surface (which amounts to having k ≈ k ⊥ ≈ 0), but to qualify as a zonal flow-like structure they must also be 'zonal' (i.e.…”