We present PHANTAM, a ray-based program developed to simulate target-scale, multi-beam laser-plasma interactions (LPIs). By combining a ray-tracing model with multi-beam convective theory, PHANTAM self-consistently resolves multi-wave coupling and collective effects for both shared light and shared plasma wave modes. Using this code, we performed millimeter-scale 2D and 3D simulations of two-beam-driven Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering (SBS) in inhomogeneous plasmas. The parameters of plasma and incident lasers are relevant to indirect-drive hohlraum experiments. Our results demonstrate that SRS backscatter remains energetically insignificant due to strong Landau damping and resonance detuning (electron density ~0.06 times the critical electron density). In contrast, millimeter-scale convective amplification paths establish SBS as the primary backscattering LPI. Crucially, single-beam backward-scattered SBS light undergoes primary amplification in these non-overlapping regions, and is re-amplified via side-scattering when entering the beam-overlap region, ultimately emerging as the predominant SBS mode.

laser-plasma interaction,Stimulated Raman Scattering (SRS),Stimulated Brillouin Scattering (SBS)