Description
In this work, we report an experimental study of electron-to-ion (e-i) root transitions in the TJ-II stellarator, a confinement transition associated with the formation of a sheared radial electric field in the plasma edge. The line-averaged electron density, was kept close to the value at which such transitions are typically observed, while the Electron Cyclotron Resonance Heating (ECRH) power was modulated in order to investigate how changes in heating conditions affect the transition.
We find that the density at which the e-i transition occurs depends on the applied ECRH power. This indicates that the line-averaged electron density alone does not determine the transition, and supports the view that the relevant control parameter is linked to the edge plasma potential profile, or equivalently to the radial electric field and its shear.
Long-range correlation analysis using spatially separated Langmuir probes reveals a clear change in the nature of edge turbulence across the transition. Below the threshold, correlations are consistent with radially propagating filamentary structures, while above the threshold they are localized in a narrow radial region, consistent with the presence of a zonal-flow-like structure of width about 1 cm.
In the high-confinement state, wavelet spectra show a strong suppression of low-frequency fluctuations (below 50 kHz) together with a significant reduction of the turbulence correlation time, indicating effective turbulence decorrelation by shear flows. In addition, intermittency is reduced, suggesting a decrease in multifractality and an increased role of low-order coherent activity. Measures of entropy and statistical complexity suggest that this transition can be considered a phase transition.
Altogether, these observations show that the plasma edge dynamics during the e-i root transition cannot be understood as a purely profile-driven process, but involve nonlinear interactions between profiles and turbulence.