Z. M. Sheng

Weakly relativistic one-dimensional laser pulse envelope solitons in a warm plasma

A class of exact one-dimensional solutions of coupled nonlinear equations describing the propagation of a weakly relativistic circularly polarized electromagnetic pulse in a warm, collisionless and unbounded plasma is presented. The solutions investigated are in the form of a slowly moving dark or bright envelope soliton with a propagation velocity comparable to the thermal speed of the particles. For such a slowly propagating entity, the modulational envelope is strongly modified by the effects arising due to ion inertia as well as by the thermal effects of both ions and electrons. Different regions of existence of dark and bright solitons have been identified. The analysis carried out here is restricted to nearly quasi-neutral dynamics where the second derivative term in the Poisson equation plays a subsidiary role. Under this approximation, the eigenvalue problem has continuum solutions and one can establish the nonlinear relationship between the group velocity of the soliton and the amplitude and frequency of the light pulse.

Stimulated photon cascade and condensate in a relativistic laser-plasma interaction

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.

Direct observation of ultrafast surface transport of laser-driven fast electrons in a solid target

We demonstrate rapid spread of surface ionization on a glass target excited by an intense, ultrashort laser pulse at an intensity of 3 × 1017 W cm−2. Time- and space-resolved reflectivity of the target surface indicates that the initial plasma region created by the pump pulse expands at c/7. The measured quasi-static megagauss magnetic field is found to expand in a manner very similar to that of surface ionization. Two-dimensional particle-in-cell simulations reproduce measurements of surface ionization and magnetic fields. Both the experiment and simulation convincingly demonstrate the role of self-induced electric and magnetic fields in confining fast electrons along the target-vacuum interface.

Measurement of hot electron transport in overdense plasma VIA self induced giant magnetic pulses

Spatial and temporal resolved ultrashort(8ps) multimegagauss(65 MG) magnetic field has been measured in plasma produced on Al-coated BK-7 glass by the interaction of a relativististic intensity laser(4x1018W/cm2, 30 fs) using pump-probe polarimetry. The 2D profile of magnetic field is captured using a CCD camera. Mapping of this magnetic field maps the transport of relativistic electrons in the plasma. The magnetic field profiles indicate filamentary behavior (Weibel-like instability). Particle in cell simulation are used to explain the result obtained.

Anisotropic filamentation and modulation of ultra-intense linearly polarized laser light in overdense plasma

3D particle-in-cell simulations of the interaction of an ultra-intense linearly-polarized laser light with an over- dense plasma are presented. Intense laser radiation is shown to be unstable against modulation both in the direction of the laser propagation direction and in the direction perpendicular to the polarization direction. Growth rate of the instability has a maximum of the order of 0.1(omega) 0 when laser frequency (omega) 0 is of the order of the plasma frequency modified due to the relativistic increase of electron mass in the laser field. As a result the laser breaks up into clumps with the size of the relativistic collision-less skin depth. Analytical description of the instability is also presented. Dependence of the growth rate on the laser intensity and wavenumber of perturbations is discussed.

Laser Envelope Solitons in Plasmas

Modulated light pulses in which the modulation envelope propagates as an isolated solitary plasma wave are an interesting class of exact one dimensional nonlinear solutions of the relativistic cold plasma model. They have been investigated in great detail in recent years due to their potential applications in various intense laser plasma interaction scenarios including plasma based particle and photon acceleration schemes, fast ignition method of laser fusion and radiation dynamics around a pulsar. We review some of the interesting properties of these solitons and discuss a few fundamental issues related to their existence, spectral properties and the influence of ion dynamics and finite temperature effects. We also present a new class of solitary wave solutions that exhibit an oscillatory structure in the amplitude of the electrostatic potential.