arXiv subjects

Devshree Mandal

Publications and source records attributed to Devshree Mandal.

11 recordsLinked to original sources

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↗

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

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

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

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↗

arXiv2020

Excitation of lower hybrid and magneto-sonic perturbations in laser plasma interaction

Lower hybrid (LH) and magneto-sonic (MS) waves are well known modes of magnetized plasma. These modes play important roles in many phenomena. The lower hybrid wave is often employed in magnetic confinement fusion experiments for current drive and heating purposes. Both LH and MS waves are observed in various astrophysical and space plasma observations. These waves involve ion motion and have not therefore been considered in high power pulsed laser experiments. This paper shows, with the help of Particle - In - Cell (PIC) simulations, a simple mechanism for excitation of lower hybrid and magnetosonic excitations in the context of laser plasma interaction. A detailed study characterising the formation and propagation of these modes have been provided. The scheme for generating these perturbations relies on the application of a strong magnetic field in the plasma to constrain the motion of lighter electron species in the laser electric field. The magnetic field strength is chosen so as to leave the heavier ions un-magnetized at the laser frequency. This helps in the excitation of the LH waves. On the other hand at the slower time scale associated with the laser pulse duration, even the ions show a magnetized response and magnetosonic excitations are observed to get excited.

physics.plasm-ph↗

arXiv2020

Spontaneous formation of magnetic dipoles by interaction of intense laser with overdense plasma

When a laser field is incident on an overdense plasma it is unable to penetrate inside it. Nevertheless, a part of its energy gets transferred to the electrons through a variety of mechanisms (e.g. vacuum and $\vec{J}\times \vec{B}) heating [1, 2]). The dynamics of these energetic electrons inside the plasma is a field of great interest. It is demonstrated here using 2-D PIC (Particle-In-Cell) simulation that when a high intensity laser pulse is incident on an overdense target, energetic electrons get generated which spontaneously organize themselves to form coherent structures. These coherent structures are observed to have similar dynamical traits as displayed by the solutions of the EMHD (Electron Magnetohydrodynamic) model [3]. This is noteworthy that EMHD is an approximate model and is applicable for the dynamics of non-relativistic electrons. However, in these simulations, the electron energy is in the relativistic regime as the laser intensity is significantly high. Thus, it shows that the relativistic dynamics also permits the existence of robust coherent structures. Interesting kinetic behaviour at particle level is observed in the simulation which shows that the coherent structures take background electrons in its fold and subsequently emit them at a higher energy.

physics.plasm-ph↗

arXiv2019

Effect of Transverse Beam Size on the Wakefields and Driver Beam Dynamics in Electron Beam Driven Plasma Wakefield Acceleration

In this paper, wakefields driven by a relativistic electron beam in a cold homogeneous plasma is studied using 2-D fluid simulation techniques. It has been shown that in the limit when the transverse size of a rigid beam is greater than the longitudinal extension, the wake wave acquires purely an electrostatic form and the simulation results show a good agreement with the 1-D results given by Ratan et al. [Phys. Plasmas, 22, 073109 (2015)]. In the other limit, when the transverse dimensions are equal or smaller than the longitudinal extension, the wake waves are electromagnetic in nature. Furthermore, a linear theoretical analysis of 2-D wakefields for a rigid bi-parabolic beam has also been done and compared with the simulations. It has also been shown that the transformer ratio which a key parameter that measures the efficiency in the process of acceleration, becomes higher for a 2-D system (i.e. for a beam having a smaller transverse extension compared to longitudinal length) than the 1-D system (beam having larger transverse extension compared to longitudinal length). Furthermore, including the self-consistent evolution of the driver beam in the simulation, we have seen that the beam propagating inside the plasma undergoes the transverse pinching which occurs much earlier than the longitudinal modification. Due to the presence of transverse dimensions in the system the 1-D rigidity limit given by Tsiklauri et al. [Phys. Plasmas, 25, 032114 (2018)] gets modified. We have also demonstrated the modified rigidity limit for the driver beam in a 2-D beam-plasma system.

physics.plasm-ph↗

arXiv2019

A new mechanism of direct coupling of laser energy to ions

The well-known schemes (e.g. Brunel, resonance absorption, JxB heating etc.) couple laser energy to the lighter electron species of the plasma. In this work, a fundamentally new mechanism of laser energy absorption directly to the heavier ion species has been proposed. The mechanism relies on the difference between the ExB drifts of electron and ions in the oscillating electric field of the laser and an external magnetic field to create charge density perturbations. The proposed mechanism is verified with the help of Particle - In - Cell (PIC) simulations using OSIRIS4.0.

physics.plasm-ph↗