Rosh Roy

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.

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.