Tanmay Karmakar

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.

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.

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.

An innovative instrumentation technique is developed to enable precise and efficient positional scans with motorized probe diagnostics, distributed axially and radially in Inverse Mirror Plasma Experimental Device (IMPED). The developed system automates diagnostic operations by positioning the probes, triggering the conditioning circuits, and acquiring and archiving structured data. A client–server based architecture is implemented and structured into discrete hierarchical layers enabling efficient remote operation and control over the Ethernet network. At the top layer, a supervisory node integrates multiple diagnostic probe drive units distributed over the local area network and implements positional scans using a programmatic input table in integration with the data acquisition system. The intermediate layer consists of embedded Transmission Control Protocol server nodes developed for each probe drive overseeing drive status, trajectory execution, and fault-tolerant operation. This layer communicates using the Standard Commands for Programmable Instruments, which are integrated through distributed embedded nodes. The bottom layer involves real-time servo controllers executing precise probe drive motion in closed loop control. The developed control technique of positional scan allows experiments with a user-defined extensive range of spatial locations with sub-millimeter precision (±0.2 mm). This system presently integrates 12 distributed motorized probe drives, allowing for simultaneous spatial positioning. For IMPED, a typical radial scan of 80 mm with 40 uniform steps is executed in ∼150 s. This automation enhances the reproducibility and efficiency of machine operation by significantly reducing the diagnostic time compared to manual operation. Scalability and flexibility of the architecture allow for future expansion, making it adaptable to similar scientific experiments.

Experimental observations of the intrinsic excitation and non-linear interactions of drift wave (DW) and Kelvin–Helmholtz (KH) instabilities in a linear magnetized plasma column are presented. The experiments are carried out in the inverse mirror plasma experimental device (IMPED)—a cylindrical, magnetized, linear plasma machine designed to study low-frequency waves and instabilities in plasma. A novel feature of IMPED is the ability to control plasma profiles, such as the density n(r), electron temperature Te(r), and plasma potential Vp(r) by varying the ratio Rm of the magnetic field in the main chamber to that in the source chamber. At high values of Rm, higher-density gradient scale length promotes the drift wave (DW) instability while lower Rm value results in a higher radial electric field, inducing a sheared poloidal flow that enhances the dominance of the Kelvin–Helmholtz (KH) mode. The background and fluctuating plasma parameters are characterized using various configurations of multiple in situ electric probes at different spatial locations to quantify the local gradients that excite the low-frequency primary instabilities. Statistical, spectral, and bispectral analysis of the density and potential signals help identify these modes in terms of wave number, frequency, phase, and amplitude and also delineate the nature of their non-linear interactions.