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

Abstract

We report results and modelling of an experiment performed at the 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 10 16 W/cm 2 impinging on a hot (T > 1 keV), inhomogeneous and long scalelength preformed plasma. Measurements show a significant SRS backscattering (∼ 4 − 20% of laser energy) driven at low plasma densities and no signatures of 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 gets 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), well reproduced by numerical simulations.

On the role of bandwidth in pump and seed light waves for stimulated Raman scattering in inhomogeneous plasmas

“…The results obtained in our study are relevant for dynamics inside laser hot spots, so, e.g., of optically smoothed laser beams [57][58][59] . Laser hot spots usually cover a wide range of peak intensity values, corresponding in our study to the pump laser intensity, expressed via an initial value of the Rosenbluth gain G R .…”

Role of hot electrons in shock ignition constrained by experiment at the National Ignition Facility

“…At the experimental intensities, TPD is expected to play a larger role and may increase the observed hot electron temperature and reduce the hot electron energy deposited. The quarter critical surface is the region where absolute backscattered SRS occurs and so it is used as the launch site for the hot electrons in our simulations, however there is evidence that SRS occurs at a range of densities below quarter critical as well 27,32,36,40 . Given the experimental evidence and PIC simulations 33 , the most probable values for the hot electron population were used in each case, but these are scaled from the NIF experiment and different laser intensities may generate a different hot-electron distribution.…”

“…In addition to difficulties with theory and simulation at SI relevant conditions, there is not a consensus in observation from experiment on hot electron characteristics. It is possible to use planar or conical geometry targets on lower energy facilities to increase laser intensity and make plasma conditions more closely map to those expected during SI implosions [34][35][36][37][38][39][40] . This is done to investigate the LPI and hot electrons that are generated at > 10 15 W/cm 2 intensities, long density scale lengths ≃ 500µm and electron temperatures ≃ 3keV predicted to occur in SI plasmas 18,27 .…”

Frequency chirp effects on stimulated Raman scattering in inhomogeneous plasmas

“…Those plasmas are mostly inhomogeneous in ICF experiments. In typical experiments related to direct-drive ICF and shock ignition, SRS excitation has been reported mostly for plasma densities in the range from 4% to 10% n cr with gradient scale lengths of several hundred laser wave lengths and at temperatures of a few keV 1,2 . In the presence of a density gradient, the amplification of the scattered light may be limited by the mismatch of the resonance condition.…”

“…This effect is more significant in multi-dimensional geometry, in particular for optically smoothed laser beams with speckles 25 . The EPW driven by intense speckles can be continuously amplified until leaving the speckles unless other nonlinear saturation mechanisms come into play, mostly due to kinetic effects 1,[26][27][28] . Generally, the time the EPW wave trains stay inside a speckle, ∆t ∼ πλ 0 F 2 /v L , could be longer than the time for e-fold growth, 1/γ 0 , associated with the SRS instability growth rate γ 0 , where λ 0 is the laser wavelength, v L is the group velocity of the EPW, and F is the f-number of the laser.…”