Observation of phase pattern accelerating zonal flow

2019 
We report the first experimental observation of zonal flow(ZF) formation through phase patterning. Here the `phase' refers to the eikonal phase carried by streamer-like mode. It is observed that the phase-gradient profile tends to form `shock' layer structures in regions where there are strong streamer-ZF interactions. The emergence of phase-gradient shock layers invalidates the constant-phase-gradient hypothesis, which is frequently employed in the modulational instability models of ZF generation, and is consistent with a recent theoretical work[Z.B. Guo, et al, Phys. Rev. Lett. 117, 125002 (2016)], which predicts that the phase-curvature(gradient of the phase-gradient) can produce a new Reynolds force and accelerate the ZF. By decomposing the Reynolds' force of the tilted streamers into a phase curvature driven piece and an amplitude inhomogeneity driven one, it is found that inside the shock layers the phase curvature plays a prominent role in accelerating the ZF. We also explore the formation mechanism of the phase pattern and its consistent dynamics with phase-curvature-driven ZF. These findings potentially open a new way to understand the various illusive self-organization phenomena in plasma turbulence.
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