Relativistic nonlinear effects in plasmas

In a long subcritical density plasma, an ultrashort ultraintense laser pulse can self-organize into a fast but sub-relativistic propagating structure consisting of the modulated laser light and a large number of trapped electrons from the plasma. Upon impact of the structure with a solid foil target placed in the latter, the remaining laser light is reflected, but the dense and hot trapped electrons pass through the foil, together with the impact-generated target-frontsurface electrons to form a dense hot electron cloud at the back of the target suitable for enhancing target normal sheath acceleration of the target-backsurface ions. The accelerated ions are well collimated and of high charge and energy densities, with peak energies a full order of magnitude higher than that from target normal sheath acceleration without the subcritical density plasma. In the latter case, the space-charge field accelerating the ions is limited since they are formed only by the target-frontsurface electrons during the very short instant of laser reflection.

Target normal sheath acceleration of foil ions by laser-trapped hot electrons from a long subcritical-density preplasma

Ponderomotive profile steepening and resonance absorption of a flowing stratified laser produced plasma are investigated self‐consistently when both cases of linear polarization with respect to the plane of incidence (s and p polarization) are combined. This analysis enables a representation of ‘‘mixed’’ polarized light, desirable for application in advanced codes modeling laser plasmas. Profile steepening forms (i) a layer around the turning point of the light, and, embedded therein, (ii) a thinner sublayer at the critical surface, where plasma waves are excited. The structure and the density transition of both layers are determined as a function of dimensionless parameters, i.e., the angle of incidence and the light intensities corresponding to both polarizations (related to plasma temperature and light wavelength). The plasma flow changes from sub‐ to supersonic in the steepened region; the sound speed depends on the intensity of the longitudinal electric field. In contrast to the purely polarized case...

Self‐consistent plasma profile steepening in presence of laser light with mixed polarization

The density variation of plasmas produced by irradiation with laser beams on spherical targets has been studied in the steady state. This study used an adiabatic plasma model with self‐generated magnetic fields included in a phenomenological manner. The density variation with a radial coordinate is found to have a steep rise through the critical density, which reaches a plateau in the overdense region, then a density minimum and finally, a rise. This variation has been compared with experimentally observed values in earlier works and is found to be in fairly good agreement. The variation of plateau density with the intensity of the laser is also compared with the experimentally observed variation.

Density profile steepening due to self-generated magnetic fields in plasmas produced by laser irradiation of spherical targets

A systematic analysis of the stationary propagation of nonlinearly coupled electromagnetic and ion-acoustic waves in an unmagnetized plasma via the ponderomotive force is carried out. For small but finite amplitudes, the governing equations have a Hamiltonian structure, but with a kinetic energy term that is not positive definite. The Hamiltonian is similar to the well-known Henon–Heiles Hamiltonian of nonlinear dynamics, and is completely integrable in three regimes of the allowed parameter space. The corresponding second invariants of motion are also explicitly obtained. The integrable parameter regimes correspond to supersonic values of the Mach number, which characterizes the propagation speed of the coupled waves. On the other hand, in the sub- as well as near-sonic regimes, the coupled mode equations admit different types of exact analytical solutions, which represent nonlinear localized eigenstates of the electromagnetic field trapped in the density cavity due to the ponderomotive potential. While the density cavity has always a single-dip structure, for larger amplitudes it can support higher-order modes having a larger number of nodes in the electromagnetic field. In particular, we show the existence of a new type of localized electromagnetic wave whose field intensity has a triple-hump structure. For typical parameter values, the triple-hump solitons propagate with larger Mach numbers that are closer to the sonic limit than the single- as well as the double-hump solitons, but carry a lesser amount of the electromagnetic field energy. A comparison between the different types of solutions is carried out. The possibility of the existence of trapped electromagnetic modes having a larger number of humps is also discussed.

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Integrable parameter regimes and stationary states of nonlinearly coupled electromagnetic and ion-acoustic waves

Physical processes which occur at the critical surface such as resonance absorption and scattering of the incident laser light are very sensitive to the exact form of the laser induced density profile. The presence of ‘‘overdense bumps’’ in density profiles has been seen experimentally and calculated theoretically using a simple isothermal model. These structures are important, in that they could, in an experimental situation, reflect a significant amount of light before it reaches the critical surface and considerably affect the physics occuring within a laser pellet target. A generalization of a previous calculation by using an arbitrary equation of state (density response of the electrons to the electrostatic potential φ) of the form ρ=ρ(φ) is obtained. In particular, two models for the electronic response (one including the effects of the presence of trapped electrons within the overdense shock structures) calculating the resulting effect on the produced density profiles are investigated. Qualitatively, it is found that the presence of overdense structures and their one‐dimensional stability within an underdense plasma is unaffected by the more general electronic response. Conditions for the existence of these structures are discussed and a detailed comparison with full particle in cell (PIC) code simulations is made, emphasizing the trapped electron model where excellent agreement is obtained.

C. H. Aldrich

Papers

Laser‐induced profile modification: Effect of electron response