Zonal flow dynamics, modulation of mean-flow-driven fluctuations, and particle flux control in a linear magnetized plasma.

Abstract Magnetized plasmas with radial inhomogeneities in equilibrium density and temperature naturally support a variety of electrostatic fluctuations, with drift-wave turbulence being a dominant source of outward particle transport. Controlling such turbulence is essential for improving confinement, and both mean flows and zonal flows (ZFs) are known to play key roles in its suppression. We report experimental evidence of the coexistence and nonlinear interaction of mean flow shear driven fluctuations and ZFs in a magnetized plasma column, where the two flows are localized in different radial zones. The mean flow, generated by a radial DC electric field, excites a coherent Kelvin Helmholtz instability (KHI) at 5.6 kHz, while a low-frequency (∼700 Hz) ZF, with nearly zero k_θ , finite kr along it’s reversal emerges at the location of the steep Reynolds stress gradient. A temporal evolution study reveals that the KHI is being modulated by the ZF and it’s amplitude increases as the ZF amplitude decreases, with the ZF phase consistently leading that of KHI, indicating energy transfer from ZF to KHI. An auto-bicoherence analysis of Φ_f_tilde spectra confirms the nonlinear coupling between the ZF, KHI, and its sidebands. Particle flux measurements show negligible net transport in the peak ZF region, while the maximum mean-flow shear region exhibits negative particle flux, implying inward transport. A probability distribution function (PDF) analysis of density fluctuations reveals three distinct radial regimes: a nearly Gaussian distribution at the zonal flow peak, indicating turbulence regulation by zonal flows, a negatively skewed distribution in the mean flow shear region associated with hole-like structures and a positively skewed distribution at the edge linked to blob like transport. These results suggest that the mean flow has a more dominant role than the ZF in suppressing outward transport, and underscores the importance of investigating transport behavior when both flows coexist within the same spatial region.