Development of a ray-based linear gain post-processor to model stimulated Raman scattering in multiple emission directions

Stimulated Raman sidescattering has become a major concern in ignition-scale inertial confinement fusion (ICF) experiments due to its detrimental role in generating suprathermal electrons, which can preheat the fuel capsule and degrade implosion performance. This paper introduces RAMUD (RAman in MUltiple Directions), a numerically efficient, ray-based, linear gain post-processor, which models stimulated Raman scattering using a ray tracing approach, enabling the study of Raman light emitted in both backscattering and sidescattering geometries in a 2D spatial domain. The code comprises two core modules: the first one calculates the refraction of the laser and Raman light, using the standard eikonal ray tracing equation based on user-defined intial scattering angles and plasma parameters typically obtained from radiation-hydrodynamic simulations. The second module employs a path integral method to compute linear convective gains, quantifying the amplification of the scattered light. By processing the outputs of these calculations (Raman wavelength, final propagation angle after refraction, gain values, transmitted power after inverse Bremsstrahlung absorption), RAMUD generates numerical Raman gain spectra that can be directly compared to experimental measurements from ICF experiments. We describe RAMUD's operational principle and test its accuracy against theoretical predictions in a few test cases. We next apply the code to model SRS in a planar direct drive experiment performed at the National Ignition Facility, where intense sidescattering has been reportedly observed. The spectra computed using RAMUD show good agreement with the measured spectra and align closely with the experimental interpretations.