We propose a mechanism that leads to efficient acceleration of electrons in plasma by two counterpropagating laser pulses. It is triggered by stochastic motion of electrons when the laser fields exceed some threshold amplitudes, as found in single-electron dynamics. It is further confirmed in particle-in-cell simulations. In vacuum or tenuous plasma, electron acceleration in the case with two colliding laser pulses can be much more efficient than with one laser pulse only. In plasma at moderate densities, such as a few percent of the critical density, the amplitude of the Raman-backscattered wave is high enough to serve as the second counterpropagating pulse to trigger the electron stochastic motion. As a result, even with one intense laser pulse only, electrons can be heated up to a temperature much higher than the corresponding laser ponderomotive potential.
The propagation of a linearly polarized relativistic laser pulse in an underdense plasma is studied by fluid-Maxwell and particle-in-cell simulations. A nonlinear interplay between backward and forward stimulated Raman scattering instabilities produces a strong spatial modulation of the light pulse and the down cascade in its frequency spectrum. The Raman cascade saturates by a unique photon condensation at the bottom of the light spectra near the electron plasma frequency, related to strong depletion and possible break-up of the laser beam. In the final stage of the cascade-into-condensate mechanism, the depleted downshifted laser pulse is gradually transformed into a train of ultra-short relativistic light solitons.
Existence and stability of one-dimensional electromagnetic solitons formed in a relativistic interaction of a linearly polarized laser light with an underdense cold plasma are discussed. In a weakly relativistic model, the original equation of the nonlinear Schrödinger type, with local and nonlocal cubic nonlinearities, is derived. Standing electromagnetic soliton solutions are analytically shown to be stable in agreement with the model simulation. A difference in soliton stability for linear and circular polarization is discussed. Finally, by fully relativistic fluid–Maxwell simulations, a family of large relativistic solitons is revealed, while analytical estimates for the maximum amplitude and the soliton eigenfrequency come close to simulation results.
Measurements of three-halves harmonic radiation (3 0 /2) produced by femtosecond, Ti:sapphire laser pulses (р2ϫ10 17 W/cm 2 ) in long density scale length plasmas generated from solid aluminum targets are presented. The 3 0 /2 emission yield shows excellent agreement with theories of the two-plasmon decay instability in the predominantly linear regime.
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