arXiv subjects

Amita Das

Publications and source records attributed to Amita Das.

At least 37 records · Page 2Linked to original sources

arXiv2024

Fluid Simulation for a Finite Size Plasma

Studies on finite-size plasma have attracted a lot of attention lately. They can form by ionizing liquid droplets by lasers. The dynamical behavior of such plasma droplets is, therefore, a topic of significant interest. In particular, questions related to the linear and nonlinear characteristics (associated with the inhomogeneous density typically at the edge of the droplet), the behavior of plasma expansion, etc., are of interest. A one-dimensional fluid simulation study has been carried out to investigate this behavior. It is observed that a slight imbalance in the charge density leads to oscillations that are concentrated and keep acquiring higher amplitude and sharper profile at the inhomogeneous edge region. Such oscillations lead to the expansion of the droplet. Though the fluid description breaks when the sharpness of these structures becomes comparable to the grid size, it provides a reasonable estimate of wave-breaking time. The presence of dissipative effects like diffusion is shown to arrest the sharpness of these structures. The dynamics of these structures in the presence of an externally applied oscillating electric field corresponding to a long wavelength radiation has also been studied.

physics.plasm-ph↗

arXiv2023

Novel Instabilities in Counter-Streaming Nonabelian Fluids

The dynamics of strongly interacting particles are governed by Yang-Mills (Y-M) theory, which is a natural generalization of Maxwell Electrodynamics (ED). Its quantized version is known as quantum chromodynamics (QCD) and has been very well studied. Classical Y-M theory is proving to be equally interesting because of the central role it plays in describing the physics of quark-gluon plasma (QGP)-which was prevalent in the early universe and is also produced in relativistic heavy ion collision experiments. This calls for a systematic study of classical Y-M theories. A good insight into classical Y-M dynamics would be best obtained by comparing and contrasting the Y-M results with their ED counterparts. In this article, a beginning has been made by considering streaming instabilities in Y-M fluids. We find that in addition to analogues of ED instabilities, novel nonabelian modes arise, reflecting the inherent nonabelian nature of the interaction. The new modes exhibit propagation/ growth, with growth rates that can be larger than what we find in ED. Interestingly, we also find a mode that propagates without getting affected by the medium.

physics.plasm-ph↗

arXiv2023

Two dimensional effects of laser interacting with magnetized plasma

Recent advancements in low-frequency short-pulse $CO_2$ lasers and the production of strong magnetic fields have made experimental studies on laser interactions with magnetized plasma a near-future possibility. Therefore, theoretical and numerical simulation studies have been pursued lately in this direction [A. Das, Review of Modern Plasma Physics 4, 1 (2020)] illustrating a host of novel phenomena related to laser energy absorption [Vashistha et al., New Journal of Physics, 22(6):063023 (2020); Goswami et al., Plasma Physics and Controlled Fusion 63, 115003 (2021)], harmonic generation [Maity et al., Journal of Plasma Physics, 87(5) (2021)], etc. However, most of these studies have been carried out in one-dimensional geometry with the laser having infinite transverse extent, and the plasma target was considered cold. This manuscript explores the manifestation of the 2-D and thermal effects on the problem of a laser interacting with magnetized plasma. As expected, additional transverse ponderomotive force is shown to be operative. A finite temperature of the target, along with transverse density stratification generates, leads to diamagnetic drift for the two plasma species. The imbalance of this drift between the two species can be an additional effect leading to an enhancement of laser energy absorption. The Particle - In - Cell (PIC) simulations with the OSIRIS4.0 platform is used to explore these features.

physics.plasm-ph↗

arXiv2023

Controlling intense, ultrashort, laser-driven relativistic mega-ampere electron fluxes by a modest, static magnetic field

The guiding and control of ultrahigh flux, femtosecond relativistic electron pulses through solid density matter is of great importance for many areas of high energy density science. Efforts so far include the use of magnetic fields generated by the propagation of the electron pulse itself or the application of hundreds of Tesla magnitudes, pulsed external magnetic fields driven by either short pulse lasers or electrical pulses. Here we experimentally demonstrate the guiding of hundreds of keV mega-ampere electron pulses in a magnetized neodymium solid that has a very modest, easily available static field of 0.1 tesla. The electron pulses driven by an ultrahigh intensity, 30 femtosecond laser are shown to propagate beam-like, a distance as large as 5 mm in a high Z target (neodymium), their collimation improved and flux density enhanced nearly by a factor of 3. Particle-in-cell simulations in the appropriate parameter regime match the experimental observations. In addition, the simulations predict the occurrence of a novel, near-monochromatic feature towards the high energy end of the electron energy spectrum, which is tunable by the applied magnetic field strength. These results may prove valuable for fast electron beam-driven radiation sources, fast ignition of laser fusion, and laboratory astrophysics.

physics.plasm-ph↗

arXiv2023

Mapping the complete evolution of magnetic excitation in beam-plasma system driven by an ultra-intense, femtosecond laser

Plasmas are beset with instabilities of all types, hydrodynamic, magneto-hydrodynamic, and electromagnetic. These instabilities are complex, occur over a large range of temporal and spatial scales, are most often unmanageable, and have seriously challenged our efforts at applications, even as they have shed light on the understanding of the physics of plasmas in the laboratory and astrophysical environments. A major reason for our limited success in their containment is the lack of direct experimental information on their origins and evolution, both temporal and spatial. In plasmas produced by high-intensity, short, and ultrashort pulse lasers, our knowledge of the instability stems from the (secondary) signals they generate e.g. scattering of electromagnetic waves in the form of Raman or Brillouin scattering. Rarely, if ever, has a direct measurement been made of the instantaneous evolution of the instabilities in plasmas. In this paper, we present direct measurements of the femtosecond evolution of the electromagnetic beam-driven instability that arises from the interaction of forward and return currents in an ultrahigh-intensity laser-produced plasma on a solid target.

physics.plasm-ph↗

arXiv2023

Ion heating in Laser interacting with magnetized plasma

The ion heating mechanism in the context of laser interacting with plasma immersed in a strong magnetic field is studied. The magnetic field is chosen to be strong for laser electromagnetic field propagation inside the plasma to be governed by the magnetized dispersion relation. Both X and RL mode configurations have been studied in detail using Particle - In - Cell (PIC) simulations. It is shown that the energy absorption process is governed by a resonant mechanism wherein the laser frequency matches with an underlying mode in the plasma. For X and RL mode configurations, these correspond to lower hybrid and ion cyclotron resonance, respectively. The absorption, however, is found to be most efficient at frequencies close to but not exactly matching with the resonance frequency. An understanding of the same has been provided. The role of laser polarization has been studied in detail.

physics.plasm-ph↗

arXiv2023

Harmonic generation in magnetized plasma for Electromagnetic wave propagating parallel to external magnetic field

The harmonic generation has always been of fundamental interest in studying the nonlinear nature of any physical system. In the present study, Particle - In - Cell (PIC) simulations have been carried out to explore the harmonic generation of Electromagnetic waves in a magnetized plasma. The EM wave propagation is chosen to be parallel to the applied external magnetic field. The simulations show the excitation of odd higher harmonics of RCP (Right circularly polarized) and LCP (Left circularly polarized) when the incident wave is linearly polarised. The harmonic generation is maximum when the incident EM wave frequency matches the electron cyclotron frequency. When the incident EM wave has a circular polarization, no harmonics get excited. A theoretical understanding of these observations has also been provided. The studies thus show that by appropriately tailoring of plasma parameters EM waves of higher frequencies and desired nature of circular polarization can be generated.

physics.plasm-ph↗

arXiv2022

Structure formation by electrostatic interactions in strongly coupled medium

The formation of correlated structures is of importance in many diverse contexts such as strongly coupled plasmas, soft matter, and even biological mediums. In all these contexts the dynamics are mainly governed by electrostatic interactions and result in the formation of a variety of structures. In this study, the process of formation of structures is investigated with the help of Molecular (MD) simulations in 2 and 3 dimensions. The overall medium has been modelled with an equal number of positive and negatively charged particles interacting via long-range pair Coulomb potential. A repulsive short-range Lennard-Jones (LJ) potential is added to take care of the blowing up of attractive Coulomb interaction between unlike charges. In the strongly coupled regime, a variety of classical bound states form. However, complete crystallization of the system, as typically observed in the context of one component strongly coupled plasmas, does not occur. The influence of localized perturbation in the system has also been studied. The formation of a crystalline pattern of shielding clouds around this disturbance is observed. The spatial properties of the shielding structure have been analyzed using the radial distribution function and Voronoi diagram. The process of accumulation of oppositely charged particles around the disturbance triggers a lot of dynamical activity in the bulk of the medium, wherein close encounters between widely separated particles occur. This leads to the formation of a larger number of bigger clusters. There are, however, also instances when bound pairs break up to provide the appropriate signed charge to the shielding cloud. A detailed discussion of these features has been provided in the manuscript.

physics.plasm-ph↗

arXiv2022

Localized absorption of laser energy in X-mode configuration of magnetized plasma

The heating of ions via lower hybrid waves has been observed in several astrophysical as well as laboratory plasmas. We have conducted Particle-In-Cell simulations to demonstrate absorption of the incident laser pulse at a chosen localized point in the target by manipulating the plasma density profile. We show that a part of the incident laser propagates inside plasma target, when its frequency lies below the lower hybrid resonance frequency. Thereafter, as it experiences a negative density gradient, it approaches the resonance point where its group velocity approaches zero. This is where the electromagnetic energy prominently gets converted into electrostatic and eventually into kinetic energy of ions. Thus by tailoring the plasma density profile one can have the absorption of incident electromagnetic wave energy at a designated location inside the plasma. This may have importance in various applications where energy deposition/heating of plasma at a localized region is desirable.

physics.plasm-ph↗

arXiv2022

Particle-In-Cell observations of Brillouin scattering for laser interacting with magnetized overdense plasma

One dimensional Particle-in-cell simulations using OSIRIS-4.0 has been conducted to study the interaction of a laser electromagnetic pulse with an overdense magnetized plasma target. The external magnetic field has been chosen to be directed along the laser propagation direction. This geometry supports the propagation of right (R) and left (L) circularly polarised electromagnetic waves in the plasma. The laser pulse is allowed to propagate inside the plasma when its frequency falls in the pass band of the dispersion curves of L and/or R waves. The strength of the applied external magnetic field is chosen as a parameter to ensure that the laser frequency lies in the appropriate pass band. It is demonstrated that for all possible polarization of the incident laser, parametric process involving a scattered Electromagnetic wave and an electrostatic mode occur. The parametric process has been identified as that due to the Brillouin back scattering process.

physics.plasm-ph↗

arXiv2022

Mode conversion and laser energy absorption by plasma under an inhomogeneous external magnetic field

The interaction of a high-frequency laser with plasma in the presence of an inhomogeneous external magnetic field has been studied here with the help of Particle-In-Cell simulation. It has been shown that laser enters inside the plasma as an extraordinary wave (X-wave), where the electric field of the wave oscillates perpendicular to both external magnetic field and propagation direction, and as it travels through the plasma, its dispersion property changes due to the inhomogeneity of the externally applied magnetic field. Our study shows that the X-wave's electromagnetic energy is converted to an electrostatic mode as it encounters the upper-hybrid (UH) resonance layer. In the later stage of the evolution, this electrostatic wave breaks and converts its energy to electron kinetic energy. Our study reveals two additional processes involved in decaying electrostatic mode at the UH resonance layer. We have shown that the energy of the electrostatic mode also converts to a low-frequency lower-hybrid mode and high-frequency electromagnetic harmonic radiations at the resonance layer. The dependence of energy conversion processes on the gradient of external magnetic field has also been studied and analyzed.

physics.plasm-ph↗

arXiv2021

Chaotic dynamics of small sized charged Yukawa Dust Clusters

In a recent work, [1] the equilibrium of a cluster of charged dust particles mutually interacting with screened Coulomb force and radially confined by an externally applied electric field in a 2-D configuration was studied. It was shown that the particles arranged themselves on discrete radial rings forming a lattice structure. In some cases with the specific number of particles, no static equilibrium was observed; instead, angular rotation of particles positioned at various rings was observed. In a two-ringed structure, it was shown that the direction of rotation was opposite. The direction of rotation was also observed to change apparently at random time intervals. A detailed characterization of the dynamics of small-sized Yukawa clusters has been carried out in the present work. In particular, it has been shown that the dynamical time reversal of angular rotation exhibits chaotic behavior.

physics.plasm-ph↗

arXiv2021

Harmonic generation in the interaction of laser with a magnetized overdense plasma

The mechanism of harmonic generation in both O and X-mode configurations for a magnetized plasma has been explored here in detail with the help of Particle-In-Cell (PIC) simulations. A detailed characterization of both the reflected and transmitted electromagnetic radiation propagating in the bulk of the plasma has been carried out for this purpose. The efficiency of harmonic generation is shown to increase with the incident laser intensity. Dependency of harmonic efficiency has also been found on magnetic field strength. This work demonstrates that there is an optimum value of the magnetic field at which the efficiency of harmonic generation maximizes. The observations are in agreement with theoretical analysis. For O-mode configuration, this is compelling as the harmonic generation provides for a mechanism by which laser energy can propagate inside an overdense plasma region.

physics.plasm-ph↗

arXiv2021

A 3D Magnetohydrodynamic simulation for the propagation of plasma plume transverse to applied magnetic field

We have carried out a 3D ideal-MHD (Magnetohydrodynamic) simulation to study the evolution of laser generated plasma plume in a moderate external magnetic field (0.13 T) oriented perpendicular to the flow direction of the plasma plume. The simulation shows that the plasma plume pushes the external magnetic field lines outward in the direction of the expansion. This leads to compression and bending of the magnetic field lines.The force resulting from the change in shape and the density of magnetic field lines opposes the expansion of the plume. An elliptic layer of shocked plasma is formed at the plasma/external field interface leaving a cavity in the plume core due to the outward expansion and the inertia of the plume. As the plasma pressure drops due to expansion, the imbalance between the magnetic energy and the internal energy results in the collapse of the cavity. These observations have striking similarities with the observations of the experiments [Phys. Plasmas 24, 033511 (2017)] performed recently to study the plasma plume expansion in the presence of an external transverse magnetic field. This similarity indicates that the physical mechanisms dominantly governing the plasma plume expansion in the moderate magnetic field are aptly described in the ideal MHD regime. The studies thus show that the laser generated plasma plume can be utilized to carry out interesting experiments on MHD phenomena in a simple laboratory set up.

physics.plasm-ph↗

arXiv2021

Kelvin-Helmholtz instability in strongly coupled dusty plasma with rotational shear flows and tracer transport

Kelvin-Helmholtz (KH) instability plays a significant role in transport and mixing properties of any medium. In this paper, we numerically explore this instability for a two-dimensional strongly coupled dusty plasma with rotational shear flows. We study this medium using generalized hydrodynamic fluid model which treats it as viscoelastic fluid. We consider the specific cases of rotating vorticity with abrupt radial profiles of rotation. In particular: single-circulation, and multi-circulation vorticity shell profiles have been chosen. We observe the KH vortices at each circular interface between two relative rotating flows along with a pair of ingoing and outgoing wavefronts of transverse shear waves. Our studies show that due to the interplay between KH vortices and shear waves in the strongly coupled medium, the mixing and transport behaviour are much better than inviscid hydrodynamic fluids. In interests of substantiating the mixing and transport behaviour, the generalized hydrodynamic fluid model is extended to include the Lagrangian tracer particles. The numerical dispersion of these tracer particles in a flow provides an estimate of the diffusion in such a medium. We present the preliminary observations of tracers distribution (cluster formation) and their diffusion (mean square displacement) across the medium.

physics.plasm-ph↗

arXiv2021

Ponderomotive force driven mechanism for electrostatic wave excitation and energy absorption of Electromagnetic waves in overdense magnetized plasma

The excitation of electrostatic waves in plasma by laser electromagnetic pulse is important as it provides a scheme by which the power from the laser electromagnetic (EM) field can be transferred into the plasma medium. The paper presents a fundamentally new ponderomotive pressure-driven mechanism of excitation of electrostatic waves in an overdense magnetized plasma by a finite laser pulse. Particle-in-cell (PIC) simulations using the EPOCH-4.17.10 framework have been utilized for the study of a finite laser pulse interacting with a magnetized overdense plasma medium. The external magnetic field is chosen to be aligned parallel to the laser propagation direction. In this geometry, the electromagnetic wave propagation inside the plasma is identified as whistler or R and L waves. The group velocity of these waves being different, a clear spatial separation of the R and L pulses are visible. In addition, excitation of electrostatic perturbation associated with the EM pulses propagating inside the plasma is also observed. These electrostatic perturbations are important as they couple laser energy to the plasma medium. The excitation of electrostatic oscillations are understood here by a fundamentally new mechanism of charge separation created by the difference between the ponderomotive force (of the electromagnetic pulse) felt by the two plasma species, viz., the electrons and the ions in a magnetized plasma.

physics.plasm-ph↗

arXiv2021

Electromagnetic wave transparency of X mode in strongly magnetized plasma

An Electromagnetic (EM) pulse falling on a plasma medium from vacuum can either reflect or propagate inside the plasma depending on whether it is overdense or underdense. In a magnetised plasma, however, there are usually several pass and stop bands for the EM wave depending on the orientation of the magnetic field with respect to the propagation direction. The EM wave while propagating in a plasma can excite electrostatic disturbances in the plasma [1, 2]. In this work Particle - In - Cell simulations have been carried out to illustrate the complete transparency of the EM wave propagation inside a strongly magnetised plasma. The external magnetic field is chosen to be perpendicular to both the wave propagation direction and the electric field of the EM wave, which is the X mode configuration. Despite the presence of charged electron and ion species the plasma medium behaves like a vacuum. The observation is understood with the help of particle drifts. It is shown that though the two particle species move under the influence of EM fields their motion does not lead to any charge or current source to alter the dispersion relation of the EM wave propagating in the medium. Furthermore, it is also shown that the stop band for EM wave in this regime shrinks to a zero width as both the resonance and cut-off points approach each other. Thus transparency to the EM radiation in such a strongly magnetised case appears to be a norm. This may have important implications in astrophysical scenarios. For instance, the plasma surrounding objects like pulsars and magnetars is often threaded with strong magnetic fields.

physics.plasm-ph↗

arXiv2021

Effect of Ion Motion on Breaking of Longitudinal Relativistically Strong Plasma Waves: Khachatryan mode revisited

Effect of ion motion on the spatio-temporal evolution of a relativistically strong space charge wave, is studied using a 1-D fluid simulation code. In our simulation, these waves are excited in the wake of a rigid electron beam propagating through a cold homogeneous plasma with a speed close to the speed of light. It is observed that the excited wave is a mode as described by Khachatryan [Phys. Rev. E 58, 7799 (1998)] whose profile gradually sharpens and the wave eventually breaks after several plasma periods exhibiting explosive behaviour. It is found that breaking occurs at amplitudes, which is far below the breaking limit analytically derived by Khachatryan [Phys. Rev. E 58, 7799 (1998)]. This phenomenon of wave breaking, at amplitudes well below the breaking limit, is understood in terms of phase mixing of the excited wave. It is further found that the phase mixing time (wave breaking time) scales inversely with the energy density of the wave.

physics.plasm-ph↗