arXiv subjects

Amita Das

Publications and source records attributed to Amita Das.

At least 73 records · Page 4Linked to original sources

arXiv2017

Merger and reconnection of Weibel separated relativistic electron beam

The relativistic electron beam (REB) propagation in a plasma is fraught with beam plasma instabilities. The prominent amongst them being the collisionless Weibel destabilization which spatially separates the forward propagating REB and the return shielding currents. This results in the formation of REB current filaments which are typically of the size of electron skin depth during the linear stage of the instability. It has been observed that in the nonlinear stage the filaments size increases as they merge with each other. With the help of 2D PIC simulations in the plane perpendicular to the REB propagation, it is shown that these mergers occur in two distinct nonlinear phases. In the first phase, the total magnetic energy increases. Subsequently, however, during the second phase, one observes a reduction in magnetic energy. It is shown that the transition from one nonlinear regime to another occurs when the typical current associated with individual filaments hits the Alfven threshold. In the second nonlinear regime, therefore, the filaments can no longer permit any increase in current. Magnetic reconnection events then dissipate the excess current (and its associated magnetic energy) that would result from a merger process leading to the generation of energetic electrons jets in the perpendicular plane. At later times when there are only few filaments left the individual reconnection events can be clearly identified. It is observed that in between such events the magnetic energy remains constant and shows a sudden drop as and when two filaments merge. The electron jets released in these recon- nection events are thus responsible for the transverse heating which has been mentioned in some previous studies [Honda et al. Phys. Plasmas 7, 1302 (2000)].

physics.plasm-ph↗

arXiv2017

Observation of 1-D time dependent non-propagating laser plasma structures using Fluid and PIC codes

The manuscript reports the observation of time dependent localized and non-propagating structures in the coupled laser plasma system through 1-D fluid and PIC simulations. It is reported that such structures form spontaneously as a result of collision amongst certain exact solitonic solutions. They are seen to survive as coherent entities for a long time up to several hundreds of plasma periods. Furthermore, it is shown that such time dependence can also be artificially recreated by significantly disturbing the delicate balance between the radiation and the density fields required for the exact non-propagating solution obtained by Esirkepov et al. [1]. The ensuing time evolution is an interesting interplay between kinetic and field energies of the system. The electrostatic plasma oscillations are coupled with oscillations in the electromagnetic field. The inhomogeneity of the background and the relativistic nature, however, invariably produce large amplitude density perturbations leading to its wave breaking. In the fluid simulations, the signature of wave breaking can be discerned by a drop in the total energy which evidently gets lost to the grid. The PIC simulations are observed to closely follow the fluid simulations till the point of wave breaking. However, the total energy in the case of PIC simulations is seen to remain conserved all throughout the simulations. At the wave breaking the particles are observed to acquire thermal kinetic energy in the case of PIC. Interestingly, even after wave breaking, compact coherent structures with trapped radiation inside high-density peaks, continue to exist both in PIC and fluid simulations. Though the time evolution does not exactly match in the two simulations as it does prior to the process of wave breaking, the time-dependent features exhibited by the remnant structures are characteristically similar.

physics.plasm-ph↗

arXiv2017

Energy principle for 2D electromagnetic, relativistic, interpenetrating, counter streaming plasma flows

A relativistic electron beam propagating through plasma induces a return current in the system. Such a system of counterstreaming forward and return current is susceptible to host of instabilities out of which Weibel remains a dominant mode for destabilizing the system. Weibel instability has been widely investigated in simulations, experiments, as well as, analytically using fluid and kinetic treatments. A purely growing mode like Weibel instability can also be understood by using conservation theorems and energy principle analysis. An electrostatic analog to the Weibel instability, two stream instability in a beam plasma system has already been investigated using energy principle analysis [Lashmore-Davies 2007]. A detailed analytical description of conservation theorem for 2D Weibel instability in a beam plasma system has been carried out in this manuscript.

physics.plasm-ph↗

arXiv2017

Observation of enhanced absorption of laser radiation by nano structured targets in PIC simulations

It is well known that Brunel's vacuum heating mechanism is operative for laser energy absorption when the target plasma density rises sharply. For non-relativistic laser intensities and planar targets it is also necessary that the laser should strike the target at oblique incidence. The laser electric field at oblique incidence has a component normal to the surface to extract electrons from the target in the vacuum region for Brunel's mechanism to be operative. At relativistic laser intensities, oblique incidence is not necessarily required as the J x B force is significant and can extract electrons from the target even when the laser is at normal incidence. In this manuscript, the interaction of short and intense laser pulse with structured overdense plasma targets have been studied using 2D particle-in-cell simulations. It is shown that for structured targets the absorption increases many fold. A detailed study and understanding of the absorption process for the structured targets in terms of structure scale length and amplitude have been provided.

physics.plasm-ph↗

arXiv2017

Magnetic field generation in finite beam plasma system

For finite systems boundaries can introduce remarkable novel features. A well known example is the Casimir effect [1, 2] that is observed in quantum electrodynamic systems. In classical systems too novel effects associated with finite boundaries have been observed, for example the surface plasmon mode [3] that appears when the plasma has a finite extension. In this work a novel instability associated with the finite transverse size of a beam owing through a plasma system has been shown to exist. This instability leads to distinct characteristic features of the associated magnetic field that gets generated. For example, in contrast to the well known unstable Weibel mode of a beam plasma system which generates magnetic field at the skin depth scale, this instability generates magnetic field at the scales length of the transverse beam dimension [4]. The existence of this new instability is demonstrated by analytical arguments and by simulations conducted with the help of a variety of Particle - In - Cell (PIC) codes (e.g. OSIRIS, EPOCH, PICPSI). Two fluid simulations have also been conducted which confirm the observations. Furthermore, laboratory experiments on laser plasma system also provides evidence of such an instability mechanism at work.

physics.plasm-ph↗

arXiv2017

Observation of the Korteweg-de Vries soliton in molecular dynamics simulations of a dusty plasma medium

The excitation and evolution of Korteweg-de Vries (KdV) solitons in a dusty plasma medium are studied using Molecular Dynamics (MD) simulations. The dusty plasma medium is modelled as a collection of dust particles interacting through Yukawa potential, which takes into account dust charge screening due to the lighter electron and ion species. The collective response of such screened dust particles to an applied electric field impulse is studied here. An excitation of a perturbed positive density pulse propagating in one direction along with a train of negative perturbed rarefactive density oscillations in the opposite direction is observed. These observations are in accordance with evolution governed by the KdV equation. Detailed studies of (a) amplitude vs. width variation of the observed pulse, (b) the emergence of intact separate pulses with an associated phase shift after collisional interaction amidst them, etc., conclusively qualify the positive pulses observed in the simulations as KdV solitons. It is also observed that by increasing the strength of the electric field impulse, multiple solitonic structures get excited. The excitations of the multiple solitons are similar to the experimental observations reported recently by Boruah et al. [Phys. Plasmas 23, 093704 (2016)] for dusty plasmas. The role of coupling parameter has also been investigated here, which shows that with increasing coupling parameter, the amplitude of the solitonic pulse increases whereas its width decreases.

physics.plasm-ph↗

arXiv2016

Particle-in-Cell simulation of two-dimensional electron velocity shear driven instability in relativistic domain

We carry out Particle-in-Cell (PIC) simulations to study the instabilities associated with a 2-D sheared electron flow configuration against a neutralizing background of ions. Both weak and strong relativistic flow velocities are considered. In the weakly relativistic case, we observe the development of electromagnetic Kelvin Helmholtz instability with similar characteristics as that predicted by the electron Magnetohydrodynamic (EMHD) model. On other hand, in strong relativistic case the compressibility effects of electron fluid dominate and introduce upper hybrid electrostatic oscillations transverse to the flow which are very distinct from EMHD fluid behaviour. In the nonlinear regime, both weak and strong relativistic cases lead to turbulence with broad power law spectrum.

physics.plasm-ph↗

arXiv2016

Shear Viscosity of Turbulent Chiral Plasma

It is well known that the difference between the chemical potentials of left-handed and right-handed particles in a parity violating (chiral) plasma can lead to an instability. We show that the chiral instability may drive turbulent transport. Further we estimate the anomalous viscosity of chiral plasma arising from the enhanced collisionality due to turbulence.

hep-ph↗

arXiv2016

Relativistic electron beam driven longitudinal wake-wave breaking in a cold plasma

Space-time evolution of relativistic electron beam driven wake-field in a cold, homogeneous plasma, is studied using 1D-fluid simulation techniques. It is observed that the wake wave gradu- ally evolves and eventually breaks, exhibiting sharp spikes in the density profile and sawtooth like features in the electric field profile [1]. It is shown here that the excited wakefield is a longitudi- nal Akhiezer-Polovin mode [2] and its steepening (breaking) can be understood in terms of phase mixing of this mode, which arises because of relativistic mass variation effects. Further the phase mixing time (breaking time) is studied as a function of beam density and beam velocity and is found to follow the well known scaling presented in ref.[3].

physics.plasm-ph↗

arXiv2015

Sub and super-luminar propagation of structures satisfying Poynting like theorem for incompressible GHD fluid model depicting strongly coupled dusty plasma medium

The strongly coupled dusty plasma has often been modelled by the Generalized Hydrodynamic (GHD) model used for representing visco-elastic fluid systems. The incompressible limit of the model which supports transverse shear wave mode is studied in detail. In particular dipole structures are observed to emit transverse shear waves in both the limits of sub and super - luminar propagation, where the structures move slower and faster than the phase velocity of the shear waves, respectively. In the sub - luminar limit the dipole gets engulfed within the shear waves emitted by itself, which then backreacts on it and ultimately the identity of the structure is lost. However, in the super - luminar limit the emission appears like a wake from the tail region of the dipole. The dipole, however, keeps propagating forward with little damping but minimal distortion in its form. A Poynting like conservation law with radiative, convective and dissipative terms being responsible for the evolution of W , which is similar to `enstrophy' like quantity in normal hydrodynamic fluid systems, has also been constructed for the incompressible GHD equations. The conservation law is shown to be satisfied in all the cases of evolution and collision amidst the nonlinear structures to a great accuracy. It is shown that monopole structures which do not move at all but merely radiate shear waves, the radiative term and dissipative losses solely contribute to the evolution of W. The, dipolar structures, on the other hand, propagate in the medium and hence convection also plays an important role in the evolution of W.

physics.plasm-ph↗

arXiv2015

Effect of finite beam width on current separation in beam plasma system: Particle-in-Cell simulations

The electron beam propagation in a plasma medium is susceptible to several instabilities. In the relativistic regime typically the weibel instability leading to the current separation dominates. The linear instability analysis is carried out for a system wherein the transverse extent of the beam is infinite. Even in simulations, infinite transverse extent of the beam has been chosen. In real situations, however, beam width will always be finite. keeping this in view the role of finite beam width on the evolution of the beam plasma system has been studied here using Particle - in - Cell simulations. It is observed that the current separation between the forward and return shielding current for a beam with finite beam occurs at the scale length of the beam width itself. Consequently the magnetic field structures that form have maximum power at the scale length of the beam width. This behaviour is distinct from what happens with a beam with having an infinite extent represented by simulations in a periodic box, where the initial scale length at which the magnetic field appears is that of the electron skin depth.

physics.plasm-ph↗

arXiv2015

2-D studies of Relativistic electron beam plasma instabilities in an inhomogeneous plasma

Relativistic electron beam propagation in plasma is fraught with several micro instabilities like two stream, filamentation etc., in plasma. This results in severe limitation of the electron transport through a plasma medium. Recently, however, there has been an experimental demonstration of improved transport of Mega Ampere of electron currents (generated by the interaction of intense laser with solid target) in a carbon nanotube structured solid target [Phys. Rev Letts. 108, 235005 (2012)]. This then suggests that the inhomogeneous plasma (created by the ionization of carbon nano tube structured target) helps in containing the growth of the beam plasma instabilities. This manuscript addresses this issue with the help of a detailed analytical study and simulations with the help of 2-D Particle - In - Cell code. The study conclusively demonstrates that the growth rate of the dominant instability in the 2-D geometry decreases when the plasma density is chosen to be inhomogeneous, provided the scale length 1/ks of the inhomogeneous plasma is less than the typical plasma skin depth (c/ωp0 ) scale. At such small scale lengths channelization of currents are also observed in simulation.

physics.plasm-ph↗

arXiv2015

1D3V PIC simulation of propagation of relativistic electron beam in an inhomogeneous plasma

A recent experimental observation has shown efficient transport of Mega Ampere of electron currents through aligned carbon nanotube arrays [Phys. Rev Letts. 108, 235005 (2012)]. The result was subsequently interpreted on the basis of suppression of the filamentation instability in an inhomogeneous plasma [Phys. Plasmas 21, 012108 (2014)]. This inhomogeneity forms as a result of the ionization of the carbon nanotubes. In the present work a full 1D3V Particle-in-Cell (PIC) simulations have been carried out for the propagation of relativistic electron beams (REB) through an inhomogeneous background plasma. The suppression of the filamentation instability, responsible for beam divergence, is shown. The simulation also confirms that in the nonlinear regime too the REB propagation is better when it propagates through a plasma whose density is inhomogeneous transverse to the beam. The role of inhomogeneity scale length, its amplitude and the transverse beam temperature etc., in the suppression of the instability is studied in detail.

physics.plasm-ph↗

arXiv2014

Visco-elastic fluid simulations of coherent structures in strongly coupled dusty plasma medium

A generalized hydrodynamic (GHD) model depicting the behaviour of visco-elastic fluids has often been invoked to explore the behaviour of a strongly coupled dusty plasma medium below their crystallization limit. The model has been successful in describing the collective normal modes of the strongly coupled dusty plasma medium observed experimentally. The paper focuses on the study of nonlinear dynamical characteristic features of this model. Specifically, the evolution of coherent vorticity patches are being investigated here within the framework of this model. A comparison with Newtonian fluids and Molecular Dynamics (MD) simulations treating the dust species interacting through the Yukawa potential has also been presented.

physics.plasm-ph↗

arXiv2014

Turbulence in two dimensional visco - elastic medium

The properties of decaying turbulence is studied with the help of a Generalized Hydrodynamic (GHD) fluid model in the context of two dimensional visco - elastic medium such as a strongly coupled dusty plasma system. For the incompressible case considered here however, the observations are valid for a wider class of visco - elastic systems not necessarily associated with plasmas only. Our observations show that an initial spectrum that is confined in a limited domain of wave numbers becomes broad, even when the Reynold's number is much less than the critical value required for the onset of turbulence in Newtonian fluids. This is a signature of elastic turbulence where Weissenberg's number also plays a role in the onset of turbulence. This has been reported in several experiments. It is also shown that the existence of memory relaxation time parameter and the transverse shear wave inhibit the normal process (for 2-D systems) of inverse spectral cascade in this case. A detailed simulation study has been carried out for the understanding of this inhibition.

physics.plasm-ph↗

arXiv2011

Exact propagating nonlinear singular disturbances in strongly coupled dusty plasmas

The nonlinear longitudinal response of a strongly coupled dusty plasma system is analytically investigated using the Generalized Hydrodynamic (GHD) model. It is shown that the Galilean invariant form of the model does not have soliton solutions, but can support a variety of nonlinear singular (yet conservative) solutions like shock structures of zero strength and propagating solutions with cusp like singularities. These novel entities should be detectable in numerical simulations and experiments studying large amplitude longitudinal excitations in such systems.

physics.plasm-ph↗

arXiv2008

Stability of nonlinear one-dimensional laser pulse solitons in a plasma

In a recent one-dimensional numerical fluid simulation study [Saxena et al., Phys. Plasmas 13,032309 (2006)], it was found that an instability is associated with a special class of one-dimensional nonlinear solutions for modulated light pulses coupled to electron plasma waves in a relativistic cold plasma model. It is shown here that the instability can be understood on the basis of the stimulated Raman scattering phenomenon and the occurrence of density bursts in the trailing edge of the modulated structures are a manifestation of an explosive instability arising from a nonlinear phase mixing mechanism.

physics.plasm-ph↗

arXiv2008

Propagation of Electron Magnetohydrodynamic structures in a 2-D inhomogeneous plasma

The fully three dimensional governing equations in the electron magnetohydrodynamic (EMHD) regime for a plasma with inhomogeneous density is obtained. These equations in the two dimensional (2-D) limit can be cast in terms of the evolution of two coupled scalar fields. The nonlinear simulations for the two dimensional case are carried out to understand the propagation of EMHD magnetic structures in the presence of inhomogeneity. A novel effect related to trapping of dipolar magnetic structures in the high density plasma region in the EMHD regime is observed. The interpretation of this phenomena as well as its relevance to the problem of hot spot generation in the context of fast ignition is presented.

physics.plasm-ph↗