Effects of spatial and temporal smoothing on stimulated brillouin scattering in the independent-hot-spot model limit

The influence of laser beam smoothing on stimulated Brillouin backscattering (SBBS) is studied analytically in the limit of the independent hot spot model. It is shown that the temporal beam smoothing can reduce the SBBS reflectivity significantly even though the laser bandwidth is smaller than the growth rate for the average intensity. The explanation of this reduction effect is given in terms of SBBS growth in the statistically significant hot spots. The minimum laser bandwidth corresponding to an important reduction of the reflectivity is thus determined properly. The dependence of this reduction effect on the acoustic damping for a given laser bandwidth is discussed.     

Collective properties of a relativistic electron beam injected into a high intensity optical lattice

Behaviour of a relativistic electron bunch, injected and trapped in a high intensity optical lattice resulting from the interference of two laser beams is studied. The optical lattice modifies the phase space distribution of the electron bunch due to the trapping and compression of the electrons by a ponderomotive force. High-frequency longitudinal beam eigenmodes of the trapped electron bunch are described in the framework of fluid and kinetic models. Such beam oscillations are expected to play a pivotal role in a stimulated Raman scattering of laser beams on the electrons.     

On parametric absorption in laser plasma

A theory for the nonlinear saturation of convective parametric instabilities in inhomogeneous plasmas is proposed. The role of parametric processes in laser light absorption in a dense plasma corona is discussed on the basis of this theory.     

Ponderomotive ion acceleration in dense plasmas at super-high laser intensities

Short laser pulses at super-high intensities such as those proposed in the Extreme Light Infrastructure (ELI) project open new prospects for efficient acceleration of ions in overdense plasmas. A simple analytical model and numerical simulations demonstrate that pulses with intensities exceeding 10²² W/cm² may efficiently accelerate ions to high energies up to a few GeV. Maximum ion energy and the energy spectrum of the accelerated ions can be tuned by an appropriate choice of laser pulse intensity and duration at a given plasma density distribution.     

Numerical simulations of energy transfer in counter-streaming plasmas

Collisionless shock formation is investigated with large scale fully electromagnetic two-dimensional Particle-in-Cell numerical simulations. Two plasmas are colliding in the center of mass reference frame at sub-relativistic velocities. Their interaction leads to collisionless stochastic electron heating, ion slowing down and formation of a shock front. We focus here on the initial stage of evolution where electron heating is due to the Weibel-like micro-instability driven by the high-speed ion flow. A two stage process is described in the detailed analysis of our simulation results. Filament generation, followed by turbulent mixing, constitute the dominant mechanism for energy repartition. The global properties are illustrated by examination of single filament evolution in terms of energy/particle density and fields.     

Observation of the plasma channel dynamics and Coulomb explosion in the interaction of a high-intensity laser pulse with a He gas jet

We report the first interferometric observations of the dynamics of electron-ion cavitation of relativistically self-focused intense 4 TW, 400 fs laser pulse in a He gas jet. The electron density in a channel 1 mm long and 30 μm in diameter drops by a factor of approximately 10 from the maximum value of ∼8×10¹⁹ cm⁻³. A high radial velocity of the plasma expansion, ∼3.8×10⁸ cm/s, corresponding to an ion energy of about 300 keV, is observed. The total energy of fast ions is estimated to be 6% of the laser pulse energy. The high-velocity radial plasma expulsion is explained by a charge separation due to the strong ponderomotive force. This experiment demonstrates a new possibility for direct transmission of a significant portion of the energy of a laser pulse to ions. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 12, 787–792 (25 December 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Strong reduction of the degree of spatial coherence of a laser beam propagating through a preformed plasma

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.