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Bicoherence during confinement transitions in the TJ-II stellarator

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 (Eθ). During biasing, the auto-bicoherence of Eθ was significant only in a narrow radial range, in contrast to the fluctuation level of Eθ and the linear coherence between Eθ and Er (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 Eθ 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.