Bicoherence During Confinement Transitions In The Tj-ii Stellarator

摘要

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).rnDuring biasing, the auto-bicoherence of E_θ was significant only in a narrow radial range, in contrast to the fluctuation level of Eg and the linear coherence between E_θ and E_r (the radial electric field), which were affected over a very broad radial extension.rnWith 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.rnThe temporal asymmetry A of Eg 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.