Bhoomi Khodiyar

Equilibrium density gradients support drift-wave turbulence, which is often regulated by self-generated zonal flow in magnetized plasmas. In this work, we experimentally examine the effect of increasing the magnetic field on turbulence characteristics in a linear plasma device. As the magnetic field is increased from 600 to 1000 G, zonal flow is suppressed while the mean flow increases. Spectral analysis of density and floating potential fluctuations shows a redistribution of power from low-frequency (0.1–1 kHz) to high-frequency (1–300 kHz) components, along with an increase in the spectral slope and the ratio PHF/PLF. This change is linked to a reduction in Reynolds stress due to the loss of correlation between radial and poloidal velocity fluctuations, which possibly weakens the drive for zonal flow generation. Similar behavior is observed near the peak gradient region, also indicating its global nature. The present results suggest a transition from a zonal-flow-dominated regime to a state dominated by smaller-scale fluctuations, possibly influenced by mean flow shear. These findings highlight how the magnetic field redistributes spectral energy across frequency scales in drift-wave turbulent plasmas.

We report experimental observations of a controlled transition from a zonal-flow (ZF) dominated regime to a coexistence regime of ZFs and streamers, and finally to a streamer-dominated state in a linear magnetized plasma column. The controlling parameter is the ion-neutral collision frequency. At low collisionality (2×10−5 mbar), the plasma turbulence is dominated by coherent ZFs (600–700 Hz) that are nonlinearly driven by drift-wave fluctuations. With increasing collisionality (5×10−4 mbar), the ZF growth is reduced, and streamers emerge through a nonlinear coupling of neighboring drift modes mediated by a mediator mode. At high collisionality (2×10−3 mbar), ZFs are strongly damped, and the turbulence becomes streamer-dominated. For each of these turbulent states, the corresponding edge fluctuations transition from coherent, symmetric to intermittent, asymmetric fluctuations with enhanced low-frequency content and larger spatial scales that can result in convective transport. Our results demonstrate the possibility of selective excitation of ZFs and streamers by regulating their collisional damping and showcase the ion-neutral collision frequency as an effective control knob for regulating turbulent structures and edge transport in magnetized plasmas.

This work presents the control of zonal flow excitation and radial localization using plasma profiles and neutral gas pressure in a cylindrical device. In IMPED, filamentary plasma which is produced by argon gas from the source region is guided by Bs into the main chamber and confined by Bm. The magnetic field ratio Rm=Bm/Bs is a key control parameter that governs the radial profiles of ion density (ni), electron temperature (Te), and plasma pressure (p). A low-frequency (5–10 kHz) drift wave (DW) mode is observed in the ñ and a broadband (400–500 kHz) electron temperature gradient (ETG) mode is observed in the electron current saturation fluctuation signal. Low-frequency (0.2–1 kHz) zonal flows (ZFs) are identified based on kθ=0, kr≠0 and the radial polarity reversal of kr in the ϕ̃f signal in ZF frequency range. The DW mode and ETG mode are localized in the peak pressure gradient and peak Te region, and their peak power amplitudes and locations are followed by the peak ZF power. As Rm is decreased, the peak of the radial gradients of P, ni, Te shifts toward the edge with steepening of the corresponding profiles. This alters the radial localization of DW and ETG turbulence and the associated Reynolds stress radial asymmetry, shifting the peak zonal flow location. These results suggest that ZFs in IMPED are driven by both DW and ETG turbulence. Moreover, increased neutral pressure raises collisionality, enhancing ZF damping and reducing their ability to suppress turbulence.

A detailed experimental study is conducted in a cylindrical magnetized plasma to investigate the characteristics of Rayleigh–Taylor and Kelvin–Helmholtz instabilities, delineate their dominant parameter regimes, and examine their nonlinear interactions. This is accomplished through modifying the density gradient scale length (Ln) and velocity shear scale length (Ls) by changing Rm that defines the ratio of the magnetic field in the main chamber to that in the source chamber of a linear magnetized plasma device. It is found that at Rm=49, where LnLs, KH instability prevails. In the intermediate regime where Ln∼Ls (Rm=35), a coupled RT–KH instability emerges, characterized by nonlinear three-wave interactions that facilitate energy transfer across modes, as qualitatively confirmed in the bicoherence spectrum of density and potential fluctuations. Furthermore, at higher pressure in the mid Rm regime reveal stronger multiple three-wave interactions, indicating enhanced mode coupling and nonlinear energy redistribution. These findings provide new insights into the dynamics of RT and KH instabilities in magnetized plasmas, leading to an understanding of the development of turbulence via energy cascades and instability-driven mixing. The results are relevant to laboratory, astrophysical and fusion plasmas, where these instabilities govern the evolution of turbulence and structure formation.