Abstract
Abstract 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.
Highlights
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
We report results and modelling of an experiment performed at the Target Area West (TAW) Vulcan laser facility, aimed at investigating laser-plasma interaction in conditions which are of interest for the Shock Ignition scheme to Inertial Confinement Fusion, i.e. laser intensity higher than 1016 W/cm2 impinging on a hot (T > 1 keV), inhomogeneous and long scalelength preformed plasma
Light backscattered at λ ≈ 527 nm consisted of 15 − 35 % of laser energy, with no clear dependence, in the explored range, on laser intensity or time delay between heating and interaction beams; this value fell to 7 − 8 % when the heating beams were not used
Summary
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. The above experiment was conducted using the indirect-drive (ID) approach[2]. The efficiency of laser energy coupling with the plasma corona is significantly larger, requiring a lower laser energy for achieving fuel ignition. The ID approach is intrinsically non symmetric, with laser beams overlapping at the entrance of the hohlraum and propagating over long plasmas before irradiating the internal hohlraum surface; this produces undesired plasma instabilities (e.g. Crossed Beam Energy Transfer(CBET)) and suprathermal or hot electrons (HE), on one side, and a non uniform X-ray irradiation of the capsule, on the other. A symmetric irradiation scheme appears a necessary precondition for reducing long-scale implosion asymmetries and for achieving a higher control of laser plasma interaction
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