A large amplitude electromagnetic wave propagating in a plasma is known to be subject to severe modulational and Raman instabilities. Previous works were devoted to the weakly relativistic limit and applied mainly to a cold underdense plasma. One extends these works to include the fully relativistic limit for a circularly polarized light for which one derives the dispersion relation in a one-dimensional plasma. The characteristics of the instabilities are also calculated in the case where the plasma is classically overdense, with 1<(ωp/ω0)2<γ, where ωp is the plasma frequency, ω0 is the laser frequency, and γ is the relativistic factor of an electron in the laser field. Particle-in-cell simulations confirm the results of the numerical solutions of the dispersion relation. For (ωp/ω0)2/γ=0.57 the growth rate can be as large as 0.52ω0. The nonlinear stage of the instability results in a strong heating of the electron distribution function. The theory is further extended to the case of an initially hot plasma, for which the dispersion relation of the instabilities is established. Its analytical solution is given in the case of a low density plasma. Particle-in-cell simulations are used to treat the general case. One observes a strong reduction of the growth rate of the instability, which tends to restore the possibility to propagate relativistic waves in plasmas.

This paper is devoted to study the expansion of a beam composed of packets of positively and negatively charged ions generated by alternating extraction and acceleration. This beam is extracted from an ion-ion plasma, i.e. the electron density is negligible compared to the negative ion density. The alternating acceleration of ions is ensured by two grids placed in the ion-ion plasma region. The screen grid in contact with the plasma is biased with a square voltage waveform while the acceleration grid is grounded. A two-dimensional particle-in-cell (2D-PIC) code and an analytical model are used to study the properties of the near-field plume downstream of the acceleration grid. It is shown that the possible operating bias frequency is delimited by an upper limit and a lower one that are in the low MHz range. The simulations show that alternating acceleration with bias frequencies close to the upper frequency limit for the system can achieve high ion exhaust velocities, similar to traditional gridded ion thrusters, and with lower beam divergence than in classical systems. Indeed, ion-ion beam envelope might be reduced to 15° with 70% of ion flux contained within an angle of 3°. Thus, this alternating acceleration method is promising for electric space propulsion.

The effect of the nonlocality of the electron transport due to steep temperature gradients on the Nernst advection of the magnetic field is studied, together with the converse effect of the magnetic field on the electron transport.PACS numbers: 52.50. Jm, 52.25.Fi The aim of this Letter is to show the effect of the nonlocality of electron transport on the advection of the magnetic field, and the feedback effect of the magnetic field on the transport itself. Any thermal transport problem where a moderately large magnetic field exists (Larmor radius > > Debye length), such as in astrophysical plasmas, may be described by the present theory. However, the physical effects will be discussed in the context of laser-created plasmas.Several sources of magnetic field have been reported in the corona of laser-created plasma: current of suprathermal electrons,^ nonlinear effects,^ and thermoelectric effects.-^ On the basis of the usual idea of frozen magnetic field in the plasma expansion, one thought that it would have little effect on the heat transport in the conduction layer. In fact recent papers'* have mentioned that the magnetic field is indeed advected toward high densities by the electrons which carry the heat flux (Nernst effect).On the other hand, in usual experiments, the heat transport is not linear (violation of Spitzer-Harm law). A theoretical treatment of the heat flux in steep temperature gradients has been developed,^ leading to delocalization formulas for the moments of the electron distribution function, in particular for the heat flux,where q^n is the Spitzer-Harm (SH) flux, and >v is a delocalization kernel given in Ref. 5 and by Luciani, Mora, and Virmont.^ This formula has been shown to be in very good agreement with direct numerical solutions of the Fokker-Planck equation. Furthermore, the theoretical basis of such formulas is now clear: A physical system cannot be considered in nearly local equilibrium if it is too inhomogeneous, and the usual Chapman-Enskog expansion, whose divergence is unavoidable, has to be replaced by a set of delocalization formulas.Let us consider a one-dimensional plasma, with the inhomogeneity along x and the magnetic field B along y. We use the high-Z limit, and expand f (x,v) in the first relevant Legendre polynomials:The reduced Fokker-Planck equation reads (l/V3)Vx-yfi=Q,/o, (l/V3)Vxy/o-e/9/o/e>' (3)The notations are y = m^v^ll T^\ T^ is the hot temperature; e = 8Xo^E/V3ro; b = 2V2^BXo/(7'o^g)^'^^; ^o = 47r€ §roV«^(Z + l)^^lnA; Vx = 9/6X; dlL= dx/ 8Xo-We have used a quasistatic hypothesis (see the discussion later on); the distribution function adiabatically adjusts itself to the hydrodynamic quantities density and temperature. The necessary information about these quantities is contained in the linearized collision operator. This is particularly explicit when we use the asymptotic form of the electron-electron collision operator for large velocities,) since the parallel diffusion coefficient D(x) is simply proportional to the electron temperature, D(x) = r(x)/ro....

A new Weibel source due to the inverse bremsstrahlung absorption is presented. It has been shown that in homogeneous plasmas, this mechanism may drive strong collisionless Weibel modes with growth rates of order of ␥ϳ10 11 s Ϫ1 and negligible group velocities. In the laser-produced plasmas, for short laser wavelengths ( L Ͻ1 m) and high laser fluxes (IϾ10 14 W/cm 2 ), this Weibel source is most efficient as the ones due to the heat flux and the plasma expansion. The useful scaling law of the convective e-foldings, with respect to the laser and the plasma parameters, is also derived. ͓S1063-651X͑97͒07206-1͔PACS number͑s͒: 52.40.Nk, 52.35.Qz