Experimental investigation of Stimulated Raman scattering in planar direct-drive experiments at the Laser Mégajoule facility

The large amplification of SRS electromagnetic waves almost transverse to the density gradient evidenced in experiments performed in hot long scale-length plasmas in the last ten years cast doubt on the primary origin for hot-electrons production in the context of direct-drive Inertial Confinement Fusion (ICF). This side-scatter SRS occurs in inhomogeneous plasmas and gives rise to SRS light emitted at large oblique angles from the density gradient that experiences significant collisional absorption and refraction before exiting the plasma thus making it difficult to observe in experiments.

Experiments were performed on the Laser Mégajoule to investigate SRS, its hot electrons production and to quantify the associated energy losses in hot, long scalelengths plasmas relevant of direct-drive ICF. Planar targets were irradiated by 40 laser beams distributed along two cones (~ 33° and 49°) with a 2pi/5 azimuthal symmetry and focused over mm scale spot sizes. The planar expansion of the plasma was evidenced by time-resolved x-ray imaging of its self-emission while absolutely calibrated measurements confirmed the electron temperature. The NBI diagnostic measured the angular dependence of the SRS scattered light, combined with a time-dependent analysis over a wide angular range. Time-resolved SRS spectra were measured at the 33° polar angle.

This unique angular SRS diagnostics available on the LMJ are exploited to determine the relative importance (energetically) between convective and absolute SRS backscatter, convective and absolute SRS side scatter, and multiple collective beam processes.