• High Power Laser Science and Engineering
  • Vol. 9, Issue 4, 04000e60 (2021)
G. Cristoforetti1、*, S. Hüller2, P. Koester1, L. Antonelli3, S. Atzeni4, F. Baffigi1, D. Batani5, C. Baird6, N. Booth6, M. Galimberti6, K. Glize7, A. Héron2, M. Khan3, P. Loiseau8、9, D. Mancelli5, M. Notley6, P. Oliveira6, O. Renner10, M. Smid11, A. Schiavi4, G. Tran8, N. C. Woolsey3, and L. A. Gizzi1
Author Affiliations
  • 1Intense Laser Irradiation Laboratory, INO-CNR, 56124 Pisa, Italy
  • 2Centre de Physique Théorique CPHT, CNRS, IP Paris, Ecole Polytechnique, 91128 Palaiseau, France
  • 3York Plasma Institute, Department of Physics, University of York, YorkYO10 5DD, UK
  • 4Dipartimento SBAI, Università di Roma ‘La Sapienza’, 00161 Roma, Italy
  • 5Université de Bordeaux, CNRS, CEA, CELIA, 33405Talence, France
  • 6STFC Rutherford Appleton Lab, Central Laser Facility, DidcotSN2 1SZ, UK
  • 7Key Laboratory for Laser Plasmas (MOE), Shanghai Jiao Tong University, Shanghai200240, China
  • 8CEA, DAM, DIF, 91297Arpajon, France
  • 9Université Paris-Saclay, CEA, LMCE, 91680Bruyères-le-Châtel, France
  • 10Institute of Physics, ELI Beamlines, Institute of Plasma Physics, Czech Academy of Sciences, 18221Prague, Czech Republic
  • 11Helmholtz-Zentrum Dresden-Rossendorf, 01328Dresden, Germany
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    We report results and modelling of an experiment performed at the Target Area West Vulcan laser facility, aimed at investigating laser–plasma interaction in conditions that are of interest for the shock ignition scheme in inertial confinement fusion (ICF), that is, laser intensity higher than ${10}^{16}$ $\mathrm{W}/{\mathrm{cm}}^2$ impinging on a hot ($T>1$ keV), inhomogeneous and long scalelength pre-formed plasma. Measurements show a significant stimulated Raman scattering (SRS) backscattering ($\sim 4\%{-}20\%$ of laser energy) driven at low plasma densities and no signatures of two-plasmon decay (TPD)/SRS driven at the quarter critical density region. Results are satisfactorily reproduced by an analytical model accounting for the convective SRS growth in independent laser speckles, in conditions where the reflectivity is dominated by the contribution from the most intense speckles, where SRS becomes saturated. Analytical and kinetic simulations well reproduce the onset of SRS at low plasma densities in a regime strongly affected by non-linear Landau damping and by filamentation of the most intense laser speckles. The absence of TPD/SRS at higher densities is explained by pump depletion and plasma smoothing driven by filamentation. The prevalence of laser coupling in the low-density profile justifies the low temperature measured for hot electrons ($7\!{-}\!12$ keV), which is well reproduced by numerical simulations.

    1 Introduction

    A very recent experiment[1] at the Lawrence Livermore National Laboratory (LLNL) National Ignition Facility (NIF) resulted in fusion energy yield of about 1.3 MJ, largely in excess of the fuel energy, and about 70% of the laser pulse energy. LLNL’s scientists deem this is the threshold of fusion ignition. The above experiment was conducted using the indirect-drive (ID) approach[2]. However, the direct-drive (DD) approach[3] may have advantages compared with the ID approach[4,5].