Warm wave breaking of nonlinear plasma waves with arbitrary phase velocities

Wave breaking field of relativistically intense electrostatic waves in electronegative plasma with super-thermal electrons

The wave breaking limit of relativistically intense electrostatic waves in an unmagnetised electronegative plasma, where electrons are alleged to attach onto neutral atoms or molecules and thus forming a significant amount of negative ions, has been studied analytically. A nonlinear theory has been developed, using one-dimensional (1D) relativistic multi-fluid model in order to study the roles of super-thermal electrons, negative ion species and the Lorentz factor, on the dynamics of the wave. A generalised kappa-type distribution function has been chosen for the velocities of the electrons, to couple the densities of the fluids. By assuming the travelling wave solution, the equation of motion for the evolution of the wave in a stationary wave frame has been derived and numerical solutions have been presented. Studies have been further extended, using standard Sagdeev pseudopotential method, to discover the maximum electric field amplitude sustained by these waves. The dependence of wave breaking limit on the different input parameters such as the Lorentz factor, electron temperature, spectral index of the electron velocity distribution and on the fraction and the mass ratio of the negative to positive ion species has been shown explicitly. The wavelength of these waves has been calculated for a wide range of input parameters and its dependence on aforementioned plasma parameters have been studied in detail. These results are relevant to understand particle acceleration and relativistic wave breaking phenomena in high intensity laser plasma experiments and space environments where the secondary ion species and super-thermal electrons exist.

Vlasov simulations of thermal plasma waves with relativistic phase velocity in a Lorentz boosted frame

For certain classes of relativistic plasma problems, performing numerical calculations in a Lorentz boosted frame can be even more advantageous for gridded momentum-space-time (e.g., Vlasov) problems than has been demonstrated for position space-time problems and result in a potential reduction in the number of calculations needed by a factor ∼γ_{b}^{6}. In this study, the Lorentz boosted frame technique was applied to the problem of warm wave-breaking limits of plasma waves with relativistic phase velocity. The numerical results are consistent with analytic conclusions. By appropriate normalization and for sufficiently warm plasma, the dynamics for the Vlasov equation in different Lorentz frames were found to be independent of γ_{p}.

Nonlinear pulse propagation and phase velocity of laser-driven plasma waves

Laser evolution and plasma wave excitation by a relativistically intense short-pulse laser in underdense plasma are investigated in the broad pulse limit, including the effects of pulse steepening, frequency redshifting, and energy depletion. The nonlinear plasma wave phase velocity is shown to be significantly lower than the laser group velocity and further decreases as the pulse propagates owing to laser evolution. This lowers the thresholds for trapping and wave breaking and reduces the energy gain and efficiency of laser-plasma accelerators that use a uniform plasma profile.

Relativistic warm plasma theory of nonlinear laser-driven electron plasma waves

A relativistic, warm fluid model of a nonequilibrium, collisionless plasma is developed and applied to examine nonlinear Langmuir waves excited by relativistically intense, short-pulse lasers. Closure of the covariant fluid theory is obtained via an asymptotic expansion assuming a nonrelativistic plasma temperature. The momentum spread is calculated in the presence of an intense laser field and shown to be intrinsically anisotropic. Coupling between the transverse and longitudinal momentum variances is enabled by the laser field. A generalized dispersion relation is derived for Langmuir waves in a thermal plasma in the presence of an intense laser field. Including thermal fluctuations in three-velocity-space dimensions, the properties of the nonlinear electron plasma wave, such as the plasma temperature evolution and nonlinear wavelength, are examined and the maximum amplitude of the nonlinear oscillation is derived. The presence of a relativistically intense laser pulse is shown to strongly influence the maximum plasma wave amplitude for nonrelativistic phase velocities owing to the coupling between the longitudinal and transverse momentum variances.

Unphysical kinetic effects in particle-in-cell modeling of laser wakefield accelerators

Unphysical heating and macroparticle trapping that arise in the numerical modeling of laser wakefield accelerators using particle-in-cell codes are investigated. A dark current free laser wakefield accelerator stage, in which no trapping of background plasma electrons into the plasma wave should occur, and a highly nonlinear cavitated wake with self-trapping, are modeled. Numerical errors can lead to errors in the macroparticle orbits in both phase and momentum. These errors grow as a function of distance behind the drive laser and can be large enough to result in unphysical trapping in the plasma wake. The resulting numerical heating in intense short-pulse laser-plasma interactions grows much faster and to a higher level than the known numerical grid heating of an initially warm plasma in an undriven system. The amount of heating, at least in the region immediately behind the laser pulse, can, in general, be decreased by decreasing the grid size, increasing the number of particles per cell, or using smoother interpolation methods. The effect of numerical heating on macroparticle trapping is less severe in a highly nonlinear cavitated wake, since trapping occurs in the first plasma wave period immediately behind the laser pulse.