Bicoherence during confinement transitions in the TJ-II stellarator

Abstract

Bicoherence was computed for a number of discharges having confinement transitions, either induced by biasing or spontaneous, at the TJ-II stellarator. The bicoherence was computed for a variety of signals obtained using a reciprocating Langmuir probe system, but the main emphasis here is on the analysis of the poloidal electric field (E θ). During biasing, the auto-bicoherence of E θ was significant only in a narrow radial range, in contrast to the fluctuation level of E θ and the linear coherence between E θ and E r (the radial electric field), which were affected over a very broad radial extension. With spontaneous transitions, significant bicoherence was again detected only in a narrow radial range, associated with the position of the flow shear layer. The observations are consistent with the standard sheared flow model for confinement transitions. The temporal asymmetry A of E θ was also computed. During biasing, the region of strongly modified asymmetry was located inwards from the bicoherence maximum. This suggests that the detected bicoherence is not merely due to a change in the temporal shape of the signal.

Integrated data analysis at TJ-II: The density profile

“…The edge density gradient responds rapidly to the applied bias, more so than the mean particle density (especially at biasing switch-off), thus confirming the idea that electrode biasing generates an edge particle (electron) barrier. In relation to this, it is interesting that during the biasing time interval, the bicoherence, also assumed to be related to the formation of the edge transport barrier, was found to be concentrated in the radial range 0.9 < ρ < 1 30 . …”

“…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.…”

Non-local transport nature revealed by the research in transient phenomena of toroidal plasma

“…Figure 14a with a fishbone frequency at ∼ 20 kHz. Figure 14e, f shows auto bi-coherence during biasing (1125 < t < 1150 ms) and total bi-coherence during and after biasing (1175 < t < 1200 ms) measured with a Langmuir probe in the TJ-II stellarator (van Milligen et al 2008). Bi-coherence with a peak of low frequency around 10 kHz was observed.…”

Analysis of the influence of external biasing on Texas Helimak turbulence

“…16 One of most fruitful procedures to identify the mode coupling and estimate its importance is the bispectral analysis. 9,36 We apply this analysis to the turbulence in Helimak to identify the bispectrum differences in discharges with positive and negative bias.…”