Fig. 1. (a)–(c) Images of focal spots obtained in configurations corresponding to minimum (RPP2-P), intermediate (RPP1), and maximum (RPP2-S) SSRS growth. (d) Scheme of convective SSRS driven in an inhomogeneous plasma by a laser beam of size D. L_res and Δz are the length and thickness, respectively, of the resonance region, λ_SSRS is the SSRS light wavelength, and θ_out is the exit angle from the plasma. Here, the scattered light (λ_SSRS) builds up in the resonance region of length L_res and thickness Δz, leaving the plasma at an angle θ_out. (e) Setup of Octopus fiber-holder.

Fig. 2. (a) longitudinal profiles of electron density (solid lines) and temperature (dashed lines) obtained by the 2D CHIC code for the irradiation of a thick and a 2 μm foil target with circular laser spot (RPP1) at a time 100 ps after the laser peak. The blue rectangle represents the density region where SRS is driven. (b) Radial isodensity profiles at n = 0.10n_c along the s-direction obtained by the 3D code Troll for the irradiation of a thick target with RPP1, RPP2P, and RPP2S geometries at a time 100 ps after the laser peak. (c) Ray-tracing simulation of light scattered along the s-direction at n = 0.12n_c for RPP2P geometry, calculated on the density map obtained by the 3D Troll code at a time of 100 ps after the laser peak, showing the resonance thickness Δz and length Lres′.

Fig. 3. Angularly resolved spectrum obtained by focusing a laser pulse (I = 1.2 × 10¹⁶ W/cm²) onto a circular focal spot (RPP1) on a thick target. The signal is plotted on a logarithmic scale, and the polar angle is zero in the backscattering direction. The black dots indicate the λ_SSRS–θ_out relation expected for a 1D plasma density profile, while the dashed blue line is the experimental relation. The vertical orange line indicates the ω₀/2 wavelength.

Fig. 4. SRS scattered energy measured in the P- and S-planes in different shots. The left and right columns show the results from shots with thick and 2 μm foil targets, respectively, while the rows correspond to shots with RPP2-P, RPP1, and RPP2-S, moving from top to bottom. Thus, the density scale length of the plasma increases from left to right, while the spot extent in the S-direction increases from top to bottom. The uncertainty, represented by the error bars, is 20% of the calculated energies.

Fig. 5. (a) Time-integrated spectrum and (b) time-resolved intensity of BSRS and ω₀/2 measured by the FAB spectrometer.

Fig. 6. Angularly resolved spectra from thick targets obtained with the elliptical focal spot in RPP2-P (top) and RPP2-S (bottom) configurations. In both shots, the peak laser intensity is comparable to that of Fig. 3. The marks have the same meaning as in Fig. 3.

Fig. 7. (a) n-BSRS spectra at θ = 23° extracted from the P-planes of Fig. 3 (RPP1) and Fig. 6 (RPP2-P). (b) Average intensities of n-BSRS and SSRS obtained in RPP1 shots for different targets. The values, not comparable with each other, were calculated by integrating the spectrum at θ = 23° in the P-plane and by summing up the SSRS signal at all angles in the S-plane, respectively. BSRS reflectivities measured in the FAB cone are also reported (green triangles). The vertical lines represents the standard deviation of the statistical ensembles.

Fig. 8. Angularly resolved spectrum from a 2 μm Parylene-N target obtained using the circular focal spot (RPP1) with peak laser intensity I = 0.9 × 10¹⁶ W/cm². The vertical orange line indicates the ω₀/2 wavelength.