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Spatiotemporal and Wavenumber Resolved Bicoherence at the Low to High Confinement Transition in the TJ-II Stellarator

Plasma turbulence is studied using Doppler reflectometry at the TJ-II stellarator. By scanning the tilt angle of the probing beam, different values of the perpendicular wave numbers are probed at the reflection layer. In this way, the interaction between zonal flows and turbulence is reported with (a) spatial, (b) temporal, and (c) wavenumber resolution for the first time in any magnetic confinement fusion device. We report measurements of the bicoherence across the Low to High (L--H) confinement transition at TJ-II. We examine both fast transitions and slow transitions characterized by an intermediate (I) phase. The bicoherence, understood to reflect the non-linear coupling between the perpendicular velocity (zonal flow) and turbulence amplitude, is significantly enhanced in a time window of several tens of ms around the time of the L--H transition. It is found to peak at a specific radial position (slightly inward from the radial electric field shear layer in H mode), and is associated with a specific perpendicular wave number ($k_\perp \simeq 6-12$ cm$^{-1}$, $k_\perp ρ_s \simeq 0.8-2$). In all cases, the bicoherence is due to the interaction between high frequencies ($\simeq 1$ MHz) and a rather low frequency ($\lesssim 50$ kHz), as expected for a zonal flow.