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Recently achieved high intensities of short laser pulses open new prospects in their application to hole boring in inhomogeneous overdense plasmas and for ignition in precompressed DT fusion targets. A simple analytical model and numerical simulations demonstrate that pulses with intensities exceeding 10;{22} W/cm;{2} may penetrate deeply into the plasma as a result of efficient ponderomotive acceleration of ions in the forward direction. The penetration depth as big as hundreds of microns depends on the laser fluence, which has to exceed a few tens of GJ/cm;{2}. The fast ions, accelerated at the bottom of the channel with an efficiency of more than 20%, show a high directionality and may heat the precompressed target core to fusion conditions.

High-intensity laser-matter interaction is an efficient method for high-current relativistic electron beam production. At current densities exceeding a several kA microm{-2} , the beam propagation is maintained by an almost complete current neutralization by the target electrons. In such a geometry of two oppositely directed flows, beam instabilities can develop, depending on the target and the beam parameters. The present paper proposes an analytical description of the filamentation instability of an electron beam propagating through an insulator target. It is shown that the collisionless and resistive instabilities enter into competition with the ionization instability. This latter process is dominant in insulator targets where the field ionization by the fast beam provides free electrons for the neutralization current.

A new terahertz (THz) source in air based on the bifilamentation of femtosecond laser pulses is reported. This THz radiation is 1 order of magnitude more intense than the transition-Cherenkov THz emission from femtosecond laser filaments reported recently and shows different angular and polarization properties. We attribute it to the emission from a bimodal transmission line created by two plasma filaments.

We attribute a strong forward directed THz emission from femtosecond laser filaments in air to a transition-Cherenkov emission from the plasma space charge moving behind the ionization front at light velocity. Distant targets can be easily irradiated by this new source of THz radiation.

Recent experiments demonstrate an efficient transformation of high intensity laser pulse into a relativistic electron beam with a very high current density exceeding 10(12) A cm(-2). The propagation of such a beam inside the target is possible if its current is neutralized. This phenomenon is not well understood, especially in dielectric targets. In this paper, we study the propagation of high current density electron beam in a plastic target using a particle-in-cell simulation code. The code includes both ionization of the plastic and collisions of newborn electrons. The numerical results are compared with a relatively simple analytical model and a reasonable agreement is found. The temporal evolution of the beam velocity distribution, the spatial density profile, and the propagation velocity of the ionization front are analyzed and their dependencies on the beam density and energy are discussed. The beam energy losses are mainly due to the target ionization induced by the self-generated electric field and the return current. For the highest beam density, a two-stream instability is observed to develop in the plasma behind the ionization front and it contributes to the beam energy losses.

Extremely high pressures (approximately 10 TPa) and temperatures (5 x 10(5) K) have been produced using a single laser pulse (100 nJ, 800 nm, 200 fs) focused inside a sapphire crystal. The laser pulse creates an intensity over 10(14) W/cm2 converting material within the absorbing volume of approximately 0.2 microm3 into plasma in a few fs. A pressure of approximately 10 TPa, far exceeding the strength of any material, is created generating strong shock and rarefaction waves. This results in the formation of a nanovoid surrounded by a shell of shock-affected material inside undamaged crystal. Analysis of the size of the void and the shock-affected zone versus the deposited energy shows that the experimental results can be understood on the basis of conservation laws and be modeled by plasma hydrodynamics. Matter subjected to record heating and cooling rates of 10(18) K/s can, thus, be studied in a well-controlled laboratory environment.

The transport of an intense electron beam produced by ultrahigh intensity laser pulses through metals and insulators has been studied by high resolution imaging of the optical emission from the targets. In metals, the emission is mainly due to coherent transition radiation, while in plastic, it is due to the Cerenkov effect and it is orders of magnitude larger. It is also observed that in the case of insulators the fast-electron beam undergoes strong filamentation and the number of filaments increases with the target thickness. This filamented behavior in insulators is due to the instability of the ionization front related to the electric field ionization process. The filamentary structures characteristic growth rate and characteristic transversal scale are in agreement with analytical predictions.

Full particle-in-cell simulations of Brillouin backscattering in the high-intensity regime are presented. The final state of the strongly nonlinear evolution of the ion-acoustic wave packet consists of its collapse and the formation of a density cavity which is supported by large-amplitude localized electromagnetic fields. The cavitation manifests itself in large oscillations of the reflectivity which is terminated by a low-level saturation with strong kinetic effects. This newly discovered scenario demonstrates the importance of fully kinetic descriptions of Brillouin backscattering for high intensities.

The electron kinetic effects are shown to play an important role in the nonlinear evolution of a driven ion-acoustic wave. The numerical simulation results obtained (i) with a hybrid code, in which the electrons behave as a fluid and the ions are described along the particle-in-cell (PIC) method, are compared with those obtained (ii) with a full-PIC code, in which the kinetic effects on both species are retained. The electron kinetic effects interplay with the usual fluid-type nonlinearity to give rise to a broadband spectrum of ion-acoustic waves saturated at a low level, even in the case of a strong excitation. This low asymptotic level might solve the long-standing problem of the small stimulated Brillouin scattering reflectivity observed in laser-plasma interaction experiments.

A strong reduction of the spatial coherence of a laser beam after its propagation through a plasma has been measured using a Fresnel biprism interferometer. The laser beam was diffraction limited; the coherence width was reduced from 40 mm in vacuum down to a few mm with the plasma. Numerical results based on a paraxial model exhibit a coherence degree close to the experimental one; they also prove the importance of taking into account the nonlocal transport effects in numerical simulations for such plasma conditions.

The electrostatic field at the solid-vacuum interface generated by two electron populations with different thermal energies, each following a Boltzmann distribution, is analytically derived from the Poisson equation and studied in terms of plasma parameters. In particular, the effect of the pressure of each of the two populations on the amplitude of the electric field and on its spatial extension is described. In order to evaluate the cold electron temperature, an analytical model for the Ohmic heating of the background electron population by laser generated fast electrons is developed and the consequences on ion detachment, ionization, and acceleration processes in laser-solid experiments are discussed. The efficiency of ion acceleration is shown to be controlled by the heating rate of the background electrons.

Spatiotemporal smoothing of large-scale laser intensity fluctuations is observed for a laser beam focused into underdense helium plasmas. This smoothing is found to be severely enhanced when focusing the laser beam into a helium gas jet. In contrast to other experiments with preformed plasmas, the average and the peak laser intensities are well below the threshold for ponderomotive self-focusing. The coherence characteristics of the transmitted light are measured for various electron densities, and the smoothing effect is explained by multiple scattering of laser light on self-induced density perturbations.

Thomson scattering has been used to investigate the nonlinear evolution of electron plasma waves (EPWs) generated by stimulated Raman scattering (SRS). Two complementary diagnostics demonstrate the occurrence of the cascade of Langmuir decay instabilities (LDI). The EPW wave-number spectrum displays an asymmetric broadening towards small wave numbers, interpreted as a signature of the secondary EPWs produced in the LDI cascade. The number of cascade steps is in agreement with the broadening of the associated ion-acoustic-waves' spectra. The total energy transferred in the EPWs cascade is found to be either less than or of the same order of magnitude as the energy of the primary EPW.

A physical model is proposed for description of electron kinetics driven by a powerful electromagnetic pulse in the Earth's atmosphere. The model is based on a numerical solution to the Boltzmann kinetic equation for two groups of electrons. Slow electrons (with energies below a few keV) are described in a two-term approximation assuming a weak anisotropy of the electron distribution function. Fast electrons (with energies above a few keV) are described by a modified macroparticle method, taking into account the electron acceleration in the electric field, energy losses in the continuous deceleration approximation, and the multiple pitch angle scattering. The model is applied to a problem of the electric discharge in a nitrogen, which is preionized by an external gamma-ray source. It is shown that the runaway electrons have an important effect on the energy distribution of free electrons, and on the avalanche ionization rate. This mechanism might explain the observation of multiple lightning discharges observed in the Ivy-Mike thermonuclear test in the early 1950's.

Using interferometry, we investigate the dynamics of interaction of a relativistically intense 4-TW, 400-fs laser pulse with a He gas jet. We observe a stable plasma channel 1 mm long and less than 30 microm in diameter, with a radial gradient of electron density approximately 5 x 10(22) cm(-4) and with an on-axis electron density approximately ten times less than its maximum value of 8 x 10(19) cm(-3). A high radial velocity of the surrounding gas ionization of approximately 3.8 x 10(8) cm/s has been observed after the channel formation, and it is attributed to the fast ions expelled from the laser channel and propagating radially outward. We developed a kinetic model which describes the plasma channel formation and the subsequent ambient gas excitation and ionization. Comparing the model predictions with the interferometric data, we reconstructed the axial profile of laser channel and on-axis laser intensity. The estimated maximum energy of accelerated ions is about 500 keV, and the total energy of the fast ions is 5% of the laser pulse energy.