[en] The process of second harmonic generation of an intense short pulse laser in a plasma is resonantly enhanced by the application of a magnetic wiggler. The wiggler of suitable wave number k-vector₀ provides necessary momentum to second harmonic photons to make harmonic generation a resonant process. The laser imparts an oscillatory velocity to electrons and exerts a longitudinal ponderomotive force on them at (2ω₁,2k-vector₁), where ω₁ and k-vector₁ are the frequency and the wave number of the laser, respectively. As the electrons acquire oscillatory velocity at the second harmonic, the wiggler magnetic field beats with it to produce a transverse second harmonic current at (2ω₁,2k-vector₁+k-vector₀), driving the second harmonic electromagnetic radiation. However, the group velocity of the second harmonic wave is greater than that of the fundamental wave, hence, the generated pulse slips out of the main laser pulse and its amplitude saturates

[en] In the presence of a magnetic wiggler of suitable period, a Gaussian laser beam resonantly generates a second harmonic in a plasma. The phase matching conditions for the process are satisfied for a specific value of the wiggler period. The self-focusing of the fundamental pulse enhances the intensity of the second-harmonic pulse. The harmonic undergoes periodic focusing in the plasma channel created by the fundamental wave. The normalized second-harmonic amplitude varies periodically with distance with successive maxima acquiring higher values

[en] A Gaussian laser beam propagating through collisional plasma heats the electrons non-uniformaly, creating a depressed density duct via ambipolar diffusion. As the beam propagates through the self-made duct its size shrinks due to self-focusing, and marginal rays begin to play a role. This effect is incorporated by expanding the eikonal up to fourth power in r. The radial intensity profile of the laser becomes less steep than Gaussian

[en] Ponderomotive acceleration of electrons by a short laser pulse undergoing relativistic self focusing in a plasma is investigated. The saturation in nonlinear plasma permittivity causes periodic self focusing of the laser. The periodicity lengths are different for different axial segments of the pulse. As a result pulse shape is distorted. An electron initially on the laser axis and at the front of the self focusing pulse gains energy from the pulse until it is run over by the pulse peak. By the time electron reaches to the tail, if pulse begins diverging, the deceleration of the electron is slower and the electron is left with net energy gain. The electrons slightly off the laser axis see a radial ponderomotive force too. Initially when they are accelerated by the pulse front the acceleration is strong as they are closer to axis. When they see the tail of the pulse (after being run by the pulse) they are farther from the axis and the retardation ponderomotive force is weaker. Thus there is net energy gain. (author)

[en] An analytical model is developed for third harmonic generation (THG) efficiency from a high-density inhomogeneous plasma produced by laser irradiation of a thin metallic film. The laser suffers strong reflection from the critical layer. The superposition of forward and backward waves creates a quasistatic density ripple of wave number 2k and a second harmonic density ripple at 2ω, 4k, where ω and k are the frequency and wave number of the laser. The density ripple couples with the oscillatory electron velocity at ω, k to produce a nonlinear current at 3ω, 5k driving a resonant third harmonic radiation in the region where 5k=k₃ and k₃ is the wave number at 3ω frequency. As the density scale length of the plasma is increased, the efficiency of THG increases. The same behavior is reproduced in two particle in cell simulation by launching a laser in a rippled density underdense plasma of phase-matched density and a ripple period of half-laser wavelength.

[en] A theoretical model of avalanche breakdown of air by a Gaussian laser beam and frequency upshift is developed. The laser beam, below the threshold for tunnel ionization, heats the seed electrons to high energy and initiates avalanche ionization of the air. The ensuing plasma density profile that has maximum on axis and falls off radially causes refraction divergence of the beam. The temporal evolution of plasma density causes self-phase modulation of the laser, causing frequency broadening and spectral emission in the visible.

[en] A Gaussian laser beam, propagating as an eigenmode through a low density plasma channel in the presence of an axial magnetic field, undergoes stimulated Raman back scattering, producing an upper hybrid wave and a radially localized electromagnetic sideband wave. The channel may be self created by the laser due to ponderomotive force or by a pre-pulse. The radial width of sideband is ≅a and Langmuir wave of extent b≅(aλ_D)^1/2<< a. The nonlocal effect arising, due to self generated magnetic field, modifies the electron response to these Eigen modes, reduces the region of nonlocal interaction and hence the growth rate. The growth rate decreases with the pump wave amplitude and it maximum for back scattering. A nonlocal theory of stimulated Raman back-scattering of a laser, propagating through a plasma channel in the presence of an axial magnetic field, is developed. The laser excites a forward propagating upper hybrid mode, that is strongly localized radially, and a backward propagating electromagnetic wave-sideband. The growth rate significantly decreases with the magnetic field.

[en] 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-vector) and a noncollinear sideband electromagnetic wave (ω^',k-vector^'). The driver velocity v-vector_0- couples to the sideband wave to exert a ponderomotive force at (ω,k-vector) on the electrons, driving the ion acoustic wave. The density perturbation of ion acoustic wave beats with v-vector_0- to produce a nonlinear current at (ω^',k-vector^'), 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-vector) and a backscattered TM mode (ω^',k-vector_z^'). 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.

[en] Ponderomotive acceleration of electrons by a short laser pulse undergoing relativistic self-focusing in a plasma is investigated. The saturation in nonlinear plasma permittivity causes periodic self-focusing of the laser. The periodicity lengths are different for different axial segments of the pulse. As a result, pulse shape is distorted. An electron initially on the laser axis and at the front of the self-focusing pulse gains energy from the pulse until it is run over by the pulse peak. By the time electron reaches the tail, if pulse begins diverging, the deceleration of the electron is slower and the electron is left with net energy gain. The electrons slightly off the laser axis see a radial ponderomotive force too. Initially, when they are accelerated by the pulse front the acceleration is strong as they are closer to the axis. When they see the tail of the pulse (after being run by the pulse), they are farther from the axis and the retardation ponderomotive force is weaker. Thus, there is net energy gain.

[en] 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