Laser parametric instability experiments of a 3ω, 15 kJ, 6-ns laser pulse in gas-filled hohlraums at the Ligne d'Intégration Laser facility

Abstract

Experimental investigation of stimulated Raman (SRS) and Brillouin (SBS) scattering have been obtained at the Ligne-d'Intégration-Laser facility (LIL, CEA-Cesta, France). The parametric instabilities (LPI) are driven by firing four laser beamlets (one quad) into millimeter size, gas-filled hohlraum targets. A quad delivers energy on target of 15 kJ at 3ω in a 6-ns shaped laser pulse. The quad is focused by means of 3ω gratings and is optically smoothed with a kinoform phase plate and with smoothing by spectral dispersion-like 2 GHz and/or 14 GHz laser bandwidth. Open- and closed-geometry hohlraums have been used, all being filled with 1-atm, neo-pentane (C5H12) gas. For SRS and SBS studies, the light backscattered into the focusing optics is analyzed with spectral and time resolutions. Near-backscattered light at 3ω and transmitted light at 3ω are also monitored in the open geometry case. Depending on the target geometry (plasma length and hydrodynamic evolution of the plasma), it is shown that, at maximum laser intensity about 9 × 1014 W/cm2, Raman reflectivity noticeably increases up to 30% in 4-mm long plasmas while SBS stays below 10%. Consequently, laser transmission through long plasmas drops to about 10% of incident energy. Adding 14 GHz bandwidth to the laser always reduces LPI reflectivities, although this reduction is not dramatic.

Forward scattering and filamentation of a spatially smoothed laser pulse in the hydrodynamic and kinetic frameworks

“…However, the small mitigation of cross beam energy transfer [26] and backward Brillouin scattering by laser smoothing techniques may presage similar tendencies concerning the forward scattering. The latter can significantly alter the laser beam propagation by increasing the beam opening angle, thereby explaining the small laser beam transmission measured in some experiments [27].…”

Stimulated forward Raman scattering in density rippled magnetized plasma

“…The laser-plasma instabilities often change the structure of the laser energy deposited, causing the implosion's symmetry to change. SRS is one of the most critical instabilities in laser-driven fusion because it disperses the incident laser energy and produces hot electrons [2,3,4,5]. The growth rate of stimulated forward Raman scattering decreases significantly when the lower hybrid wave is localised in the presence of an azimuthal magnetic field, according to Sajjal et al [6].…”