Parametric instabilities of intense lasers from interaction with relativistic hot plasmas

Laser hosing in relativistically hot plasmas

Electron response in an intense laser is studied in the regime where the electron temperature is relativistic. Equations for laser envelope and plasma density evolution, both in the electron plasma wave and ion acoustic wave regimes, are rederived from the relativistic fluid equations to include relativistic plasma temperature effect. These equations are used to study short-pulse and long-pulse laser hosing instabilities using a variational method approach. The analysis shows that relativistic electron temperatures reduce the hosing growth rates and shift the fastest-growing modes to longer wavelengths. These results resolve a long-standing discrepancy between previous nonrelativistic theory and simulations or experiments on hosing.

Time-dependent closure relations for relativistic collisionless fluid equations

Linear fluid equations for relativistic and collisionless plasmas are derived. Closure relations for the fluid equations are analytically computed from the relativistic Vlasov equation in the Fourier space (ω,k), where ω and k are the conjugate variables of time t and space x variables, respectively. The mathematical method used is based on the projection operator techniques and the continued fraction mathematical tools. The generalized heat flux and stress tensor are calculated for arbitrary parameter ω/kc where c is the speed of light, and for arbitrary relativistic parameter z=mc²/T , where m is the particle rest mass and T, the plasma temperature in energy units.

Efficient acceleration of electrons with counterpropagating intense laser pulses in vacuum and underdense plasma

We propose that efficient acceleration of electrons in vacuum and underdense plasmas by an intense laser pulse can be triggered in the presence of another counterpropagating or intersecting laser pulse. This mechanism works when the laser fields exceed some threshold amplitudes for stochastic motion of electrons, as found in single-electron dynamics. Particle-in-cell simulations confirm that electron heating and acceleration in the case with two counterpropagating laser pulses can be much more efficient than with one laser pulse only. Two different diagnoses show that the increased heating and acceleration are caused mainly by direct laser acceleration rather than by plasma waves. In plasma at moderate densities such as a few percent of the critical density and when the underdense plasma region is large enough, the Raman backscattered and side-scattered waves can grow to a sufficiently high level to serve as the second counterpropagating or intersecting pulse and trigger the electron stochastic motion. As a result, even with a single intense laser pulse only in plasma, electrons can be accelerated to an energy level much higher than the corresponding laser ponderomotive potential.

Stimulated electron-acoustic-wave scattering in a laser plasma

Intense laser-plasma interaction can be a source of various electronic instabilities. Recently, stimulated backscattering from a trapped electron-acoustic wave (SEAS) [Montgomery et al., Phys. Rev. Lett. 87, 155001 (2001)] was proposed to reinterpret spectra previously attributed to stimulated Raman scattering (SRS) from unrealistically low densities. By particle simulations in a uniform plasma layer, which is overdense for ordinary SRS, strong reflection by SEAS at the electron plasma frequency is found. Transient SEAS reflectivity pulsations are followed by strong relativistic heating of electrons. Physical conditions are explained by three-wave parametric coupling between laser light, standing backscattered wave and slow electron-acoustic wave. Regions in which SEAS reflection can dominate over SRS are singled out.

Anisotropic filamentation instability of intense laser beams in plasmas near the critical density

The relativistic filamentation instability (RFI) of linearly polarized intense laser beams in plasmas near the critical density is investigated. It is found that the RFI is anisotropic to transverse perturbations in this case; a homogeneous laser beam evolves to a stratified structure parallel to the laser polarization direction, as demonstrated recently with three-dimensional particle-in-cell simulations by Nishihara et al. [Proc. SPIE 3886, 90 (2000)]. A weakly relativistic theory is developed for plasmas near the critical density. It shows that the anisotropy of the RFI results from a suppression of the instability in the laser polarization direction due to the electrostatic response. The anisotropic RFI is also analyzed based on an envelope equation for the laser beam. Finally, the envelope equation is solved numerically, and anisotropic filamentation and self-focusing are illustrated.

Transparency of an overdense plasma layer

The transition from opacity to transparency of overdense plasma layers for the propagation of high-intensity laser pulses of circular polarization is investigated theoretically and by simulation. We discuss in detail the regions where stationary solutions exist and therefore the plasma layer is opaque. We find that the opacity depends not only on the electron density and the laser intensity but also on the thickness of the plasma layer. We also discuss the overlap region where both transparency and opacity are possible and how it depends on the history. Ion acceleration is studied in both the opacity region and the transparency region.