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.

Lower hybrid (LH) and magneto-sonic (MS) waves are well known modes of magnetized plasma. These modes play important roles in many phenomena. The LH 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 simulations, a simple mechanism for excitation of LH 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. 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.

The excitation of electrostatic waves in plasma by laser electromagnetic (EM) pulse is important as it provides a scheme by which the power from the laser 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 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 EM 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 EM pulse) felt by the two plasma species, viz., the electrons and the ions in a magnetized plasma.

The formation and the dynamics of coherent magnetic field structures in the context of laser plasma interaction has attracted considerable attention. In the literature the formation of these structures has, however, mostly been reported in the wake of a laser pulse propagating in an underdense plasma medium (Bulanov et al., Phys. Rev. Lett., vol. 76, 1996, pp. 3562–3565; Nakamura & Mima Phys. Rev. Lett., vol. 100, 2008, 205006; Bulanov et al., Plasma Phys. Rep., vol. 31, no. 5, 2005, pp. 369–381; Naumova et al., Phys. Plasmas, vol. 8, no. 9, 2001, pp. 4149–4155; Nakamura et al., Phys. Rev. Lett., vol. 105, no. 13, 2010, 135002). The study here focuses on the formation of coherent structures by an intense laser pulse when it interacts with an overdense plasma medium. The laser in this case gets reflected and partially dumps its energy to the lighter electrons species. Particle-in-cell simulation studies have been carried out in two dimensions to show that the energetic electrons (generated at the critical layer and having relativistic energies), together with the background plasma electrons often self-organize to form distinct electron current vortices. These electron vortices have associated magnetic fields with monopolar or dipolar symmetries depending on the rotation profile of the electron current. The formation, stability and dynamics of these structures in the context of overdense plasma is of special importance as they provide a possibility of energy transport into those regions of plasma which are inaccessible to lasers. For such applications, higher energy content (involvement of relativistic electrons in their formation) of these structures is desirable. It is shown that their salient propagation characteristics even at relativistic energies follow the rules of electron magnetohydrodynamics (EMHD) (Isichenko & Marnachev, Sov. Phys. JETP, vol. 66, 1987, p. 702; Biskamp et al., Phys. Rev. Lett., vol. 76, 1996, p. 1264) (Generalized - EMHD Yadav et al., Phys. Plasmas, vol. 15, no. 6, 2008, 062308; Yadav et al., Phys. Plasmas, vol. 16, no. 4, 2009, 040701) for homogeneous (inhomogeneous) plasma medium, respectively.