V. K. Tripathi

Self-focusing of electromagnetic waves in a magnetoplasma

The steady state self-focusing of a Gaussian electromagnetic beam in a magneto-plasma has been studied. On a short time scale, a non-linearity in the dielectric constant of a plasma appears due to the ponderomotive force. This force in the case of the extraordinary mode has opposite signs forω>ωc andω<ωc, whereωc is the electron cyclotron frequency. The self-focusing due to this effect is predicted at frequencies except forωc/2<ω<ωc. The focusing of the ordinary mode is adversely affected by the magnetic field. On a larger time scale, the non-uniform heating of electrons by the beam and the resulting redistribution of the electron density is a source of non-linearity. This non-local non-linearity is several orders of magnitude higher than the ponderomotive non-linearity. We predict self-focusing of the extraordinary mode only above the gyroresonance (ω>ωc), while the ordinary mode can be focused at all frequencies.

Excitation of electromagnetic waves by an axial electron beam in a slow wave structure

An axial electron beam passing through a slow wave structure is unstable to an electromagnetic perturbation whose phase velocity equals the velocity of the beam. This phenomenon of Cerenkov emission is the basis of all traveling wave tubes. In this paper a simple treatment of excitation of electromagnetic waves in a slow wave structure, viz., two parallel planes partially loaded with a dielectric is presented. When the effects of a guide field or a background plasma are ignored, the dispersion relation is quite simple. It is in the form of a coupling between a TM mode of a stationary electron plasma‐filled slow wave structure and the beam mode. In the presence of a guide magnetic field, the electron motion is predominantly one‐dimensional and the growth rate of instability is considerably modified. At relatively higher gas pressures, when a background plasma is created by the beam, the mode structure is considerably modified, leading to a reduction in the growth rate, hence to the efficiency of the device.

Laser induced electron acceleration in a tapered magnetic wiggler

The acceleration of electrons by a laser pulse, in the presence of a magnetic wiggler, in vacuum and plasma is studied. The vector potentials of the laser pulse and magnetic wiggler are taken as AL=−xA0 sin(ωt−kz)exp⌊−(t−(z−zL)/vg)2/τL2⌋ and Aw=xA0w sin[kwz/(1+αz)], respectively, where α is the tapering parameter. For a specific value of k/kw, the inverse free-electron laser resonance condition is satisfied and energy gained by the electron increases. The resonance condition is sensitive to the electron energy and the electron density of the medium. It can be maintained for longer duration for a suitably tapered wiggler period and the electron can gain much higher energy. The wiggler period increases with initial electron energy and with the decrease in plasma density. Energy gained by the electron decreases with plasma density.

Cross-focusing of Extraordinary and Ordinary Modes in a Magnetoplasma

This paper presents an investigation of the self-focusing of a high power electromagnetic beam in a plasma/semiconductor in the presence of a static magnetic field when the extraordinary and ordinary modes are present simultaneously. The non-uniform heating and consequent redistribution of carriers by the Gaussian beam has been taken as the source of non-linearity in the plasma and parabolic semiconductor (e.g. Ge). In the non-parabolic semiconductor (e.g. n -InSb) the dependence of an effective electron mass on temperature has been taken as the mechanism of non-linearity. The non-linearity leads to the mutual coupling of the two modes; they support the self-focusing of each other. In a particular case where one mode is weak and the second is propagating in a self-made uniform waveguide, the weak mode propagates in an oscillatory waveguide. When both the modes have initially equal intensity, the rates of self-focusing of the two modes are different and this leads to the beam being elliptically polarized. ...

Ponderomotive acceleration of electrons by a laser pulse in magnetized plasma

Electron acceleration by a circularly polarized Gaussian laser pulse in magnetized plasma is investigated in the limit of frozen refractive index. The electron acceleration depends on the ratio of laser frequency to electron cyclotron frequency, amplitude of the laser pulse and plasma density. Near Doppler shifted cyclotron resonance the electron acquires maximum energy. In this scheme, 0.10 MeV electrons can be effectively accelerated to 1–100 MeV using moderate intensity laser pulse.

Relativistic second-harmonic generation of a laser from underdense plasmas

A high intensity laser obliquely incident on a vacuum-plasma interface produces second-harmonic radiation in the reflected component. The efficiency of second-harmonic generation increases with the angle of incidence, up to critical angle of incidence (our model is not valid beyond critical angle of incidence). The efficiency also depends on electron density, showing a maximum at ωp2∕ω2≅0.7, where ωp and ω are relativistic plasma frequency and laser frequency, respectively. The efficiency of second-harmonic generation increases sharply with laser intensity in the nonrelativistic regime and saturates at higher intensities. The intensity of the second harmonic is proportional to square of the laser intensity at low pump laser intensities and tends to proportional to laser intensity in the strong relativistic regime.

Electromagnetic eigenmodes of collisional and collisionless plasmas and their stability to stimulated Brillouin scattering

Nonlinear electromagnetic eigenmodes of collisional and collisionless plasmas, when the temporal extent of the modes is longer than the ambipolar diffusion time, have been investigated. The nonlinearity in a collisionless plasma arises through ponderomotive force, whereas in collisional plasmas Ohmic nonlinearity prevails. The mode structure in both cases, representing a balance between the nonlinearity-induced self-convergence and diffraction-induced divergence, closely resembles Gaussian form. The spot size of the mode decreases with the increasing axial amplitude of the laser, attains a minimum, and then rises very gradually. The modes are susceptible to stimulated Brillouin backscattering. The growth rate of the Brillouin process initially increases with mode amplitude, attains a maximum, and then decreases. The reduction in the growth rate is caused by strong electron evacuation from the axial region by the ponderomotive force and thermal pressure gradient force created by nonuniform Ohmic heating.

Nonlinear electromagnetic plasma eigenmodes and their stability to stimulated Raman scattering

Transverse mode structure of nonlinear laser eigenmodes in underdense and overdense plasmas has been obtained by numerically solving the wave equation under relativistic and ponderomotive nonlinearities. The mode structure closely resembles a Lorentzian with half width scaling inversely as the axial intensity of the laser. The threshold condition for laser penetration in an overdense plasma turns out to be γ0≡(1+a02∕2)1∕2⩾2n0∕ncr−1, where n0 is the equilibrium electron density, ncr is the critical density at laser frequency, and γ0 is the electron Lorentz factor due to the laser of normalized axial intensity a02. The nonlinear laser eigenmode, in a low density plasma, is unstable to stimulated Raman backscattering off a copropagating space charge reactive quasimode. The growth rate increases with laser intensity as a0 rises up to a0∼1. Beyond this value, growth rate decreases with a0, due to the enhancement of electron mass and depletion of electrons from the axial region. Geometrical effects also reduce ...

Laser third-harmonic generation in clustered plasmas

An intense short pulse laser propagating through a gas jet comprised of clusters, quickly converts the clusters into plasma balls and causes oscillations of the electron cloud in the direction of the electric field of the laser. In each wave period, electrons spend a significant amount of their time outside the clusters, leaving behind a net positive charge and the clusters undergo Coulomb explosion and expand. During the expansion when the cluster electrons plasma frequency equals times the laser frequency, , the electron quiver velocity is resonantly enhanced. The nonlinear electron response gives rise to the generation of a third-harmonic. At a typical laser intensity of IL ? 1015?W?cm?2 at 1??m wavelength, the normalized third-harmonic amplitude of the pump wave in a deuterium cluster of radius 100??, and density ~1014?cm?3, turns out to be 1.7?10?3.

Filamentation instability of an electromagnetic wave in an expanding plasma

A laser beam propagating at an angle to the density gradient in an expanding plasma is susceptible to filamentation instability. For a linear density profile and self consistent temperature and expansion velocity profiles, the amplitude of the perturbation grows as an Airy function with the distance of propagation. The characteristic growth length decreases with the size of perturbation and the ratio of expansion velocity to sound velocity. It increases the angle laser k vector makes with the density gradient. Moreover, the growth length does not always increase with θ0: it depends on vb(0). The spatial growth of perturbation of larger size starts occurring deeper into the plasma.