In this work, we investigate the nonlinear dynamics of isolated current-carrying edge-localized mode (ELM) filaments using a reduced electromagnetic fluid model in slab geometry. Numerical simulations show that unidirectional parallel current significantly suppresses radial filament velocity and reduces the outward propagation velocity by weakening the curvature-driven interchange force. The reduction in radial velocity is found to follow a modified scaling relation, demonstrating that increasing current progressively weakens outward filament propagation. Analysis of the vorticity equation shows that the electromagnetic current source changes from a dipolar structure to a remarkable spiral pattern, and overcomes the conventional curvature drive in the nonlinear phase. This current-driven source directly imprints its topology on the vorticity field, resulting in spiral vorticity, enhanced angular momentum, increased rotational energy, and localized shear layers. The filament therefore undergoes a transition from a conventional propagating state to a rotationally self-organized electromagnetic structure. These findings demonstrate that parallel current acts as an effective electromagnetic vorticity source and provides new insight into the nonlinear dynamics of ELM filaments in tokamak edge plasmas.

Magnetized plasmas with equilibrium density gradients support drift-wave turbulence, which is often regulated by self-generated zonal flows. 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 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 investigate precursor soliton dynamics driven by finite-sized charged sources in a two-dimensional plasma using two-dimensional three-velocity particle-in-cell simulations with OSIRIS. The study examines how source geometry, charge density, vertical alignment, and external magnetic fields influence soliton formation, structure, and coherence. A single finite line source generates forward-propagating solitons with trailing wakes, while the precursors generated by two vertically aligned sources interact nonlinearly in a non-elastic manner, forming bow- or crescent-shaped fronts. An external magnetic field aligned with the dual-source motion suppresses transverse expansion, eliminating bow- or crescent-shaped fronts and producing narrow, axially elongated, field-aligned solitons with enhanced longitudinal coherence. Unequal source-to-plasma charge ratios induce asymmetric precursor solitons with tilted fronts and uneven downstream structures. Increasing source charge enhances soliton amplitude and propagation distance, whereas low electron-to-ion temperature ratios strengthen Landau damping and reduce coherence. Systematic variations in source shape-including triangular and V-shaped profiles-reveal that the number and orientation of leading slopes govern whether solitons appear one-sided, dual-sided, symmetric, or asymmetric. Scalability tests confirm predictable dependence on the ion-acoustic speed, and simulations under low-Earth-orbit conditions reveal stationary ("pinned") solitons. These results provide critical insights into how source configuration, plasma parameters, magnetic effects, and non-elastic interactions collectively shape nonlinear wakefields and debris-plasma interactions in laboratory and space plasmas.

Edge-localized-mode (ELM) filaments are crucial for cross-field transport at the tokamak edge; yet, their dynamics are often analyzed using the cold-ion approximation, despite experimental data indicating that Ti Te . This study employs a normalized three-dimensional fluid model to investigate the influence of finite ion temperature on the dynamics of unidirectional current-carrying ELM-like filaments. We demonstrate that increasing ion temperature substantially alters filament propagation and interaction, resulting in a delay of filament merging despite an increase in total kinetic energy due to a stronger pressure-gradient drive. The examination of single-filament dynamics indicates that finite ion temperature generates asymmetric potential structures, strong poloidal flows, and persistent rotational motion, which channel kinetic energy from radial propagation into vortical dynamics. A comprehensive examination of the ion-to-electron temperature ratio reveals a distinct transition from radially dominated to rotation-dominated behavior as ion temperature increases. These results provide a unified physical explanation for reduced radial transport and delayed merging in the warm-ion domain, emphasizing the necessity of incorporating ion temperature effects in the modeling of ELM filament dynamics and edge plasma transport.

We investigate the nonlinear coalescence of two current-carrying ELM filaments using a three-dimensional electromagnetic fluid model. In the flat-density limit, the coalescence exhibits magnetic island-like reconnection, characterized by X-point formation, current-sheet development, and Sweet-Parker-like resistive scaling. Introducing a blob-like density perturbation modifies the reconnection dynamics: while the peak reconnection rate remains nearly unchanged for weak perturbations, it decreases and is increasingly delayed for larger density amplitudes. Analysis of the induction equation reveals a transition from resistive to increasingly density-dependent advective dynamics. Finite density perturbations also enhance the post-compression rebound, or sloshing, of the filaments. The sloshing amplitude increases with the density-gradient pressure force, establishing density perturbation as an additional control parameter for both reconnection and filament sloshing. These results highlight the coupled electromagnetic and pressure-driven dynamics governing the nonlinear evolution of ELM filaments in the tokamak edge.

Magnetic islands play a crucial role in regulating plasma confinement in tokamaks by interacting with micro-instabilities, such as the ion temperature gradient (ITG) mode. This work presents a detailed investigation of the effects of static magnetic islands on ITG instability, relevant to the ADITYA-U tokamak, using the Global Gyrokinetic Code in Cylindrical Coordinates (G2C3), a particle-in-cell (PIC) framework that employs a neural-network-assisted projection scheme. A two-phase simulation strategy is adopted. In the first phase, static magnetic islands with mode numbers (m, n) = (2, 1) and (3, 1) are introduced by perturbing the equilibrium magnetic flux functions. Particle dynamics within these modified topologies result in the flattening of plasma density profiles in the island regions, confirming island formation and its impact on the equilibrium profiles. In the second phase, the flattened profiles serve as new equilibria for linear electrostatic gyrokinetic simulations with adiabatic electrons, enabling the study of the modified ITG behavior. Magnetic islands significantly restructure the ITG mode, producing a spatial redistribution of potential fluctuations within and around the island region. Moreover, as the island width increases, the growth rates of different toroidal ITG modes converge, suggesting a universal stabilization trend. A comparison between the (2,1) and (3,1) islands indicates that higher-q islands lead to a more spatially extended ITG mode structure, reflecting the longer magnetic connection lengths and weaker curvature drive at outer flux surfaces. These results demonstrate the pivotal role of island-induced equilibrium modifications in determining ITG stability and mode structure in tokamak plasmas.

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 (similar to 700 Hz) ZF, with nearly zero k(theta), finite k(r) along it is 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 is 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 (phi) over tilde (f) 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 ZF peak, indicating turbulence regulation by ZFs, 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.

We study the effect of ambient magnetic fields on ion-acoustic precursor solitons generated by a finite-sized charged object moving supersonically through a plasma using 2D3V particle-in-cell simulations. A magnetic field parallel to the source velocity ( B-x) couples with the out-of-plane velocity ( v(z)), producing inward transverse forces that suppress lateral expansion and accelerate forward propagation, forming compact, focused soliton structures. In-plane perpendicular ( B-y) and out-of-plane ( B-z) fields induce longitudinal and transverse suppression, with By opposing the disturbance and B-z generating a clockwise shear that inhibits propagation. Oblique fields tilt soliton structures along the field direction, resulting in asymmetric evolution. Comparison across plasma densities shows that soliton formation is primarily governed by the magnetization parameter, Omega(c)/omega(p), where Omega(c) is the cyclotron frequency and omega p is the plasma frequency for ions and electrons. These effects associated with the magnetic field orientation and strength provide new insights into the dynamics of precursor solitons that can be tested in laboratory experiments and can prove useful in space plasma applications.

The gradient-driven microturbulence in ADITYA-U tokamak plasmas has been suppressed by injecting short gas puffs. The suppression of microturbulence increases the core temperature and subsequently the energy confinement time following the gas puff. The gas injection modifies the radial density profile, making it relatively flatter near the mid-radius. Global electrostatic gyrokinetic simulations show that this modification to the radial density profile due to gas injection suppresses the existing trapped electron mode (TEM). Simulation results show that the TEM-dominated turbulence suppression reduces the turbulence-driven heat transport, leading to an increase in core temperature. Applying multiple periodic gas-puffs leads to multiple periodic events of TEM suppression, improving the overall energy confinement time, and is used as an active control mechanism to influence microturbulence in ADITYA-U tokamak.

We report the experimental observation of highly nonlinear coherent structures in a linear magnettized plasma characterized by a strong background density gradient and significant ExB velocity shear under high ion-neutral collisionality. These structures, identified as drift acoustic waves, exhibit large normalized density fluctuations reaching amplitudes of up to 10

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 x 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 x 10^-4 mbar), the ZF growth is reduced and streamers emerge through nonlinear coupling of neighboring drift modes mediated by a mediator mode. At high collisionality (2 x 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 establish the ion-neutral collision frequency as an effective control knob for regulating turbulent structures and edge transport in magnetized plasmas.

This study reexamines the excitation of ion-acoustic precursor solitons by a supersonically moving charged debris object, incorporating two previously overlooked physical factors: the dynamic charging of the debris and the impermeable nature of its surface. The influence of charging dynamics is explored using an enhanced one-dimensional fluid-Poisson model, where the source charge is treated as a dynamical variable and solved self-consistently alongside the core plasma equations. By comparing these results with prior fixed-charge models, we evaluate the effects on soliton onset and propagation, finding that charging dynamics does not hinder soliton generation or evolution. To assess the impact of the impermeability of debris surface, a two-dimensional fluid model simulates the interaction between an electrostatically biased, impenetrable object and a flowing plasma. Modeling the object as an infinite wall disconnects the upstream and downstream plasma regions, forming a sheath without solitons-consistent with earlier fluid and particle-in-cell simulations. However, replacing the wall with a finite object enables plasma flow around it, restoring upstream-downstream connectivity and naturally generating precursor solitons.

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 B-m. The magnetic field ratio R-m=B-m/Bs is a key control parameter that governs the radial profiles of ion density ( n(i)), electron temperature ( T-e), and plasma pressure (p). A low-frequency (5-10 kHz) drift wave (DW) mode is observed in the n 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 k(theta)=0, k(r)not equal 0 and the radial polarity reversal of kr in the phi(f) 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 R(m)is decreased, the peak of the radial gradients of P, n(i), T(e )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.

We report on a model study of the merging of two electromagnetically interacting blobs, which have unidirectional currents and are located in the tokamak edge region in a high beta plasma. Unlike the conventional plasma blobs with dipolar currents that originate from resistive drift/interchange plasma turbulence, these unidirectional filamentary blobs arise from edge-localized mode ejection events. Two such blobs can interact strongly in the edge region to merge with each other in the poloidal direction. Our detailed simulations reveal that the blobs rotate about each other during the merging process and the merging occurs with a rate of acceleration in the poloidal direction that is directly proportional to the square of the current density of the blobs and inversely proportional to its density. We also derive an analytic condition for two poloidally separated plasma blobs to merge poloidally without a significant movement in the radial direction. Numerical simulations support this analytical condition. The separation distance between two high current density blobs is also seen to exhibit a sloshing behavior. For a given blob radius and density, the radial velocity during merging decreases with the strength of the unidirectional current density.

We report on experimental observations of the bending of a dust acoustic shock wave around a dust void region. This phenomenon occurs as a planar shock wavefront encounters a compressible obstacle in the form of a void whose size is larger than the wavelength of the wave. As they collide, the central portion of the wavefront, that is the first to touch the void, is blocked while the rest of the front continues to propagate, resulting in an inward bending of the shock wave. The bent shock wave eventually collapses, leading to the transient trapping of dust particles in the void. Subsequently, a Coulomb explosion of the trapped particles generates a bow shock. The experiments have been carried out in a Direct Current (DC) glow discharge plasma, where the shock wave and the void are simultaneously created as self-excited modes of a three-dimensional dust cloud. The salient features of this phenomenon are reproduced in molecular dynamics simulations, which provide valuable insights into the underlying dynamics of this interaction. (c) 2025 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/4.0/).

A Fast Visible Imaging Diagnostic (FVID) has been installed in Aditya-U tokamak to capture the visible image of poloidal cross-section of the vacuum vessel, where first interaction of pellet (micro-particle) with plasma takes place. The pellet particles are injected into the plasma using Inductively Driven Pellet Injector to investigate the assimilation process during plasma-pellet impurity interaction which induces the plasma disruptions. A complementary metal-oxide-semiconductor (CMOS) based high speed camera has been used that can capture upto150 kilo frames per second at resolution of 16x8 pixels. The system has been operated at high temporal and spatial resolution (26 kfps at 256x256 pixels) of 38 mu s/frame and 2.9 mm/pixel at tangency plane respectively. In Aditya-U tokamak, plasma experiments with inductively driven pellet injector were aimed to study the suitable disruption control mechanism for ITER like fusion machines where a projectile containing an impurity payload, weighing similar to 50-200 mg, is accelerated towards the core of the plasma with velocities of similar to 200-220 m/s using a linear coil gun accelerator, which can deposit micron-sized particles deep into the plasma core and radiate away the plasma stored energy within few milliseconds. The spatial and temporal evolution of visible radiation during plasma-pellet impurity interaction have been investigated for different payload weights of lithium-titanate (Li2TiO3) and Lithium carbonate (Li2CO3) with particle size of 50-80 micron. Dynamics of plasma column in aspect of size and its movement was studied using various image processing techniques.

We investigate the two-stream instability in a strongly coupled plasma using classical molecular dynamics simulations with long-range Coulomb interactions between particles. The nonlinear evolution of the instability is identified by the emergence of a Bernstein-Greene-Kruskal (BGK) mode. Our simulations capture key microscopic effects, such as inter-particle correlations, collisional dynamics, and coherent wave-particle interactions-features often absent in traditional fluid and kinetic models, including particle-in-cell and Vlasov approaches. In the linear regime, the instability grows rapidly and saturates within a few tens of plasma periods. As the system transitions into the nonlinear saturation phase, a single BGK mode emerges. This mode (or phase-space hole) becomes dynamically unstable in the nonlinear regime, characterized by a continuous decay of electrostatic energy over time. An energy budget analysis reveals a bump in an otherwise thermal spectrum, indicating the excitation of a coherent mode, further confirmed through a numerical rendering of the dispersion relation. The pairwise interaction plays a crucial role: pronounced instability and BGK mode formation occur with long-range Coulomb forces, while such structures are suppressed under shielded Coulomb interactions. We observe the emergence of a single BGK mode across all coupling strengths in the fluid regime, provided the streaming velocity exceeds a critical threshold.

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.

The development and commissioning of a new capacitively coupled dusty plasma experimental (CCDPx) device is reported. The plasma discharge of argon gas is produced using a dual-channel radio frequency source. The dusty plasma is produced by the introduction of monodispersive microspheres of melamine formaldehyde in the discharge. The characterization of the plasma is performed using an RF-compensated Langmuir probe. The device features an innovative lower electrode that is capable of creating a variety of potential wells to trap the dust particles in 1-D, 2-D, or 3-D equilibrium configurations. Importantly, the transverse confinement of the particles can be controlled in real time in a continuous and dynamic manner to observe transitions from a 1-D chain to 2-D and 3-D structures. This design feature provides a unique and powerful ability to explore new areas of dusty plasma research related to phase transitions and structural transitions. A detailed description of the design features, diagnostic facilities, and operational characteristics are provided. Preliminary experimental findings of dusty plasmas of differing dimensionalities are presented, and the potential for exploiting the device’s unique facilities for future research is discussed.

We compare model solutions of a forced Kadomtsev-Petviashvili (fKP) equation with experimental observations of dust acoustic precursor solitons excited by a supersonically moving charged cylindrical object in a dusty plasma medium. The fKP equation is derived from a three-fluid-Poisson model of the dusty plasma using the reductive perturbation technique and numerically solved for parameters close to the experimental investigations of cylindrical precursor solitons. The fKP model solutions show excellent agreement with the experimental results in reproducing the prominent geometric features of the two-dimensional solitons and closely matching the quantitative values of their velocities, amplitudes, and temporal evolutions. Our findings suggest that the fKP equation can serve as a very realistic model to investigate the dynamics of precursor solitons and can be usefully employed in practical applications such as space debris detection and tracking techniques that are based on observing/predicting nonlinear plasma excitations induced by the debris in the ionosphere.