Lavkesh Lachhvani

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.

We present a nondestructive approach for reconstructing the transverse center-of-charge trajectory of a non-neutral plasma cloud in a grounded trap using capacitive wall probe signals. The method combines electrostatic response maps with a multiprobe inversion to relate measured induced charge histories to cloud motion. Validation with synthetic data shows accurate recovery of the centroid trajectory and total charge for compact clouds, including cases with profile mismatch and finite signal noise. The approach provides a practical route for motion inference in devices where direct imaging is unavailable and a simple analytic conversion from probe signals to position is not possible.

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.

The results of a model numerical study highlighting some limitations of capacitive probes, when used as diagnostics in non-neutral plasmas (NNPs), are presented. Plasma models with specific charge distributions that mimic the dynamics of frequently observed m=1,2 diocotron modes in NNP experiments are used. The induced signals are calculated for a range of plasma column trajectory sizes and velocities. For small displacements, nearly sinusoidal signals are recorded at the fundamental frequency. As the displacement increases, higher harmonics emerge owing to the nonlinear response of the probe even for a linear plasma motion. The influence of the spatially varying angular velocity is also examined. Nonuniform velocity-induced nonlinearities are found to enhance the phase-coupled harmonic content of the probe signal, as confirmed by FFT and bispectral analyses. The results indicate that the shape of the measured signal is determined by the nonlinear mapping inherent in the probe's response, as well as by the plasma dynamics. Thus, one needs to exercise caution in interpreting the probe signals to determine the intrinsic plasma dynamics as probe response nonlinearities need to be accounted for. It is suggested that using multiple probes can help in accurately reconstructing the plasma trajectory, and thereby obtaining a clearer understanding of the plasma dynamics.

The SMall Aspect Ratio Toroidal EXperiment in a C-shaped torus recently reported an unprecedented electron plasma confinement, τconfine, of >100s. Understanding the plasma transport mechanism is essential to validate the observed long confinement. Measurement of the total charge Qtotal and its spatial and temporal evolution offers insight into the plasma transport mechanism. The existing charge collector for measuring Qtotal is affected by fast transients, resulting in a low signal-to-noise ratio (SNR) and insufficient bit resolution, which makes its measurement difficult, especially at long confinement times. This restricted the measurement of Qtotal only to about 20 ms. The upgraded charge collector has modified switching circuitry and has utilized components that are carefully considered to mitigate the effect of transients. The resulting increase in SNR and improvement in bit resolution have thereby allowed measurement of Qtotal for extended confinement periods of up to 30 s.

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.

In this paper, we report the excitation of coherent density and potential fluctuations induced by magnetohydrodynamic (MHD) activity in the edge plasma region of ADITYA-U tokamak. When the amplitude of the MHD mode, mainly the m/n = 2/1, increases beyond a threshold value, |B̃θ|/Bθ ∼ 0.3%–0.4%, coherent oscillations in the density and potential fluctuations are observed having the same frequency as that of the MHD mode. The mode numbers of these MHD induced density and potential fluctuations are obtained by Langmuir probes placed at different radial, poloidal, and toroidal locations in the edge plasma region. Detailed analyses of these Langmuir probe measurements reveal that the coherent mode in edge potential fluctuation has a mode structure of m/n = 2/1, whereas the edge density fluctuation has an m/n = 1/1 structure. It is further observed that beyond the threshold, the coupled power fraction scales almost linearly with the magnitude of B̃θ/Bθ fluctuations. Furthermore, the rise rates of the coupled power fraction for coherent modes in density and potential fluctuations are also found to be dependent on the growth rate of magnetic fluctuations. The disparate mode structures of the excited modes in density and plasma potential fluctuations suggest that the underlying mechanism for their existence is the coupling of even harmonics of potential to the odd harmonics of pressure due to 1/R dependence of the toroidal magnetic field.