Anuraj Panwar

Compressional Alfvénic rogue and solitary waves in magnetohydrodynamic plasmas

Generation of compressional Alfvenic rogue and solitary waves in magnetohydrodynamic plasmas is investigated. Dispersive effect caused by non-ideal electron inertia currents perpendicular to the ambient magnetic field can balance the nonlinear steepening of waves leading to the formation of a soliton. The reductive perturbation method is used to obtain a Korteweg–de Vries (KdV) equation describing the evolution of the solitary wave. The height of a soliton is proportional to the soliton speed “U” and inversely proportional to plasma “β” (ratio of plasma thermal pressure to pressure of the confining magnetic field) and the width of soliton is proportional to the electron inertial length. KdV equation is used to study the nonlinear evolution of modulationally unstable compressional Alfvenic wavepackets via the nonlinear Schrodinger equation. The characteristics of rogue wave influenced by plasma “β” and the electron inertial length are described.

Oblique ion-acoustic cnoidal waves in two temperature superthermal electrons magnetized plasma

A study is presented for the oblique propagation of ion acoustic cnoidal waves in a magnetized plasma consisting of cold ions and two temperature superthermal electrons modelled by kappa-type distributions. Using the reductive perturbation method, the nonlinear Korteweg de-Vries equation is derived, which further gives the solutions with a special type of cnoidal elliptical functions. Both compressive and rarefactive structures are found for these cnoidal waves. Nonlinear periodic cnoidal waves are explained in terms of plasma parameters depicting the Sagdeev potential and the phase curves. It is found that the density ratio of hot electrons to ions μ significantly modifies compressive/refractive wave structures. Furthermore, the combined effects of superthermality of cold and hot electrons κc,κh, cold to hot electron temperature ratio σ, angle of propagation and ion cyclotron frequency ωci have been studied in detail to analyze the height and width of compressive/refractive cnoidal waves. The findings in...

Stimulated Brillouin scattering of the beat wave of two lasers in a plasma

Stimulated Brillouin scattering of two collinear lasers in a plasma is investigated. Lasers exert a longitudinal ponderomotive force on electrons, imparting them oscillatory axial velocity at the beat frequency. This velocity acts as a driver for parametric excitation of an ion acoustic wave (ω,k) and a noncollinear sideband electromagnetic wave (ω′,k′). The driver velocity v0− couples to the sideband wave to exert a ponderomotive force at (ω,k) on the electrons, driving the ion acoustic wave. The density perturbation of ion acoustic wave beats with v0− to produce a nonlinear current at (ω′,k′), driving the sideband. In the case of finite spot size Gaussian laser beams, the beat wave has a Gaussian profile and excites an ion acoustic wave (ω,k) and a backscattered TM mode (ω′,kz′). The growth rate scales as the product of amplitudes of the lasers and maximizes at optimum values of scattering angles. The parametric instability of difference frequency driver is stronger than the sum frequency driver.

Relativistic forward stimulated Raman scattering of a laser in a plasma channel

A circularly polarized Gaussian laser beam propagating through a low density plasma creates a partially electron depleted channel. The laser undergoes stimulated forward Raman scattering, producing a plasma wave and two radially localized electromagnetic sideband waves. The laser and the sideband waves exert an axial ponderomotive force on electrons driving the plasma wave. The latter couples with the pump to drive the sidebands. The radial width of the electromagnetic sideband is of the order of the spot size of the pump, r0, whereas the radial width of the plasma wave is determined by the growth rate of the Raman process. The localization effect reduces the region of interaction and the growth rate. The algebraic equation of growth rate for forward Raman scattering is solved numerically for a typical laser wavelength and a plasma density The growth rate of the forward Raman process increases on increasing the normalized pump amplitude at lower values of pump amplitude, while the growth rate decreases on increasing at higher values of the pump amplitude. On increasing the laser spot size, maximum growth rate is obtained at higher values of a00. This analysis will be applicable in the study of x-ray lasers, inertial confinement fusion, and laser plasma accelerators.

Self-phase modulation of a laser in self created plasma channel

An analytical formalism of self focusing and self-phase modulation of an intense short pulse laser in a plasma due to relativistic and ponderomotive nonlinearities is developed. In the paraxial ray approximation, the pulse retains its Gaussian radial profile, however, its spot size varies with the distance of propagation in a periodic manner. It is influenced by self focusing. The frequency of the laser undergoes red shift. For a tan-hyperbolic temporal profile of pulse the red shift is maximum at the foot of the pulse and decreases slowly as one goes to portions of higher and higher intensity. The effect of ponderomotive nonlinearity is very significant in this respect. The maximum downshift occurs at a distance at which the laser acquires a minimum spot size. With retarded time normalized axial intensity increases more at z ∼ Rd and the radial intensity is also more narrowly peaked at d z ∼ Rd, where Rd=2πr02/λ is the Rayleigh length, r0 and λ are the spot size and wavelength of the laser pulse respectively.

Modulational instability and associated rogue structures of slow magnetosonic wave in Hall magnetohydrodynamic plasmas

The modulational instability and associated rogue structures of a slow magnetosonic wave are investigated for a Hall magnetohydrodynamic plasma. Nonlinear Schrodinger equation is obtained by using the multiple scale method, which shows a modulationally unstable slow magnetosonic mode evolving into bright wavepackets. The dispersive effects induced by the Hall electron current increase with the increase in plasma β and become weaker as the angle of propagation increases. The growth rate of the modulational instability also increases with the increase in plasma β. The growth rate is greatest for the parallel propagation and drops to zero for perpendicular propagation. The envelope wavepacket of a slow magnetosonic is widened with less oscillations as plasma β increases. But the wavepacket becomes slightly narrower and more oscillatory as the angle of propagation increases. Further a non-stationary envelope solution of the Peregrine soliton is analyzed for rogue waves. The Peregrine soliton contracts temporally and expands spatially with increase in plasma β. However, the width of a slow magnetosonic Peregrine soliton decreases both temporally and spatially with increase of the propagation angle.

Stimulated Raman forward scattering of laser in a pre-formed plasma channel

Stimulated Raman forward scattering (SRFS) of an intense short pulse laser in a plasma channel formed by two pre-laser pulses is investigated. The density nonuniformity of a plasma channel increases the focusing of main laser pulse. Main laser pulse excites a plasma wave and two electromagnetic sideband waves. Laser and the sidebands exert an axial ponderomotive force on electrons driving the plasma wave. The nonlinear currents arise at sideband frequencies. The density perturbation due to plasma wave beats with the oscillatory velocity due to pump to drive the sidebands. The normalized growth rate of SRFS increases with the density nonuniformity of a plasma channel. However, in the presence of a deep plasma channel the focusing is ineffective to laser intensity, but the growth rate increases with the intensity of main laser pulse.

Large amplitude inertial compressional Alfvénic shock and solitary waves, and acceleration of ions in magnetohydrodynamic plasmas

Large amplitude inertial compressional Alfvenic shock and solitary waves in magnetohydrodynamic plasmas are investigated. Dispersive effect caused by non-ideal electron inertia currents perpendicular to the ambient magnetic field can balance the nonlinear steepening of waves leading to the formation of a soliton. A Sagdeev-potential formalism is employed to derive an energy-balance like equation. The range of allowed values of the soliton speed, M (Mach number), plasma β (ratio of the plasma thermal pressure to the pressure in the confining magnetic field), and electron inertia, wherein solitary waves may exist, are determined. Depth of the potential increases with increasing the Mach number and plasma β, however decreases with the increasing electron inertia. The height of soliton increases with increasing in Mach number and decreases with plasma β. And with increasing electron inertial length, the width of soliton increases. The electron-ion collisional dissipation results a dissipative inertial compressional Alfven wave, which can produce a shock like structure and can efficiently accelerate ions to the order of the local Alfven velocity. The shock height increases with the increasing collision frequency, but shock height decreases with increasing plasma β.

Excitation of the beta-induced Alfvén eigenmode by a plasma flow around the magnetic island

It is well known that the rotation of a magnetic island in the reference frame of plasma guiding centers generates parallel electron current outside the island, which is induced by the geodesic curvature of a magnetic field (Smolyakov et al 2007 Phys. Rev. Lett. 99 055002). It is shown in the present work that the surface part of this current located at the island separatrix can drive a pair of counter-propagating, tearing-parity, beta-induced Alfven eigenmodes, which have the same helicity as that of the magnetic island and form a standing wave in the island frame. These Alfvenic modes can accompany tearing activity in tokamak discharges without energetic particles.

Magnetic field effects on two plasmon decay in a plasma

A high amplitude right circularly polarized (RCP) electromagnetic wave in a magnetized plasma parametrically decays into a pair of electrostatic waves. The prominent decay wave pairs are (i) two lower hybrid waves, (ii) two upper hybrid waves, and (iii) a lower hybrid wave and an upper hybrid wave. The phase matching conditions for these processes are satisfied over a wide range of plasma density, no longer restricting the parametric instability near the quarter critical density. However in the radial density profile produced by the pump wave with Gaussian distribution of intensity, only lower hybrid waves may get localized. The growth rate of each decay process is resonantly enhanced when the pump frequency is close to the electron cyclotron frequency. The growth rates for different channels maximize at different values of angles between the wave vectors of the decay wave and the static magnetic field.