  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/cm2 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.
