Stimulated Brillouin scattering reduction induced by self-focusing for a single laser speckle interacting with an expanding plasma

Abstract

Relativistic self-focusing and its effect on stimulated Raman and stimulated Brillouin scattering in laser plasma interaction Phys. Plasmas 8, 3419 (2001); 10.1063/1.1377048 Stimulated Brillouin and Raman scattering from a randomized laser beam in large inhomogeneous collisional plasmas. II. Model description and comparison with experiments Phys. Plasmas 8, 1636 (2001); 10.1063/1.1357218 Effect of pump depletion and self-focusing on stimulated Brillouin scattering process in laser-plasma interactions Phys. Plasmas 6, 927 (1999); 10.1063/1.873332Effect of the speckle self-focusing on the stationary stimulated Brillouin scattering reflectivity from a randomized laser beam in an inhomogeneous plasma The origin of the low level of stimulated Brillouin scattering (SBS) observed in laser-plasma experiments carried out with a single laser speckle is investigated by means of three-dimensional simulations and modeling in the limit when the laser beam power P is well above the critical power for ponderomotive self-focusing We find that the order of magnitude of the time averaged reflectivities, together with the temporal and spatial SBS localization observed in our simulations, are correctly reproduced by our modeling. It is observed that, after a short transient stage, SBS reaches a significant level only (i) as long as the incident laser pulse is increasing in amplitude and (ii) in a single self-focused speckle located in the low-density front part of the plasma. In order to describe self-focusing in an inhomogeneous expanding plasma, we have derived a new Lagrangian density describing this process. Using then a variational approach, our model reproduces the position and the peak intensity of the self-focusing hot spot in the front part of the plasma density profile as well as the local density depletion in this hot spot. The knowledge of these parameters then makes it possible to estimate the spatial amplification of SBS as a function of the laser beam power and consequently to explain the experimentally observed SBS reflectivity, considerably reduced with respect to standard theory in the regime of large laser beam power. V C 2014 AIP Publishing LLC.

Observation and modelling of stimulated Raman scattering driven by an optically smoothed laser beam in experimental conditions relevant for shock ignition

“…Let us mention that SBS is also seen in our PIC simulations with mobile ions, but starts from an unnaturally high noise level because of the limited number of particles; in contrast to SRS, however, SBS is mitigated by the ponderomotively depleted density profile [64] .…”

“…In these speckles, a significant density depletion is expected to give rise to a reduction or even a suppression of stimulated scattering in the center of the hot spot, as shown in Ref. [64].…”

“…In these speckles, a significant density depletion is expected to give rise to a reduction or even a suppression of stimulated scattering in the centre of the hot spot, as shown in Ref. [64]. In the simulations carried out with the EMI2d code, we focused on the micro-physics around an intense speckle with v osc /c = 0.1 corresponding to a peak intensity of 5 × 10 16 W/cm 2 for the conditions considered.…”

Observation and modelling of Stimulated Raman Scattering driven by an optically smoothed laser beam in experimental conditions relevant for Shock Ignition

“…In these speckles, a significant density depletion is expected to give rise to a reduction or even a suppression of stimulated scattering in the center of the hot spot, as shown in Ref. [56].…”

Modeling of energy transfer between two crossing smoothed laser beams in a plasma with flow profile

“…We have computed the energy transfer with two approaches, namely a time-dependent paraxial code, coupled to fluid equations for the plasma fluid, with the code Harmony [9,10] and with the paraxial geometrical optics approach, based on a stationary CBET model [2,11,12]. The amplification in power, T = P probe,out /P probe,in , of the probe beam at the right-hand-side boundary ("out") with respect to its incident power ("in") is shown in Fig.…”