Langmuir decay instability for localized Langmuir wave packets in two-dimensional inhomogeneous plasmas

The Langmuir decay instability (LDI) is a key saturation mechanism for stimulated Raman scattering (SRS) in inertial confinement fusion plasmas. However, a quantitative characterization of the two-dimensional angular spectrum of LDI in such inhomogeneous environments, particularly its dependence on the pump-wave incidence angle, remains limited. Here, we combine a k-space theoretical model with two-dimensional particle-in-cell simulations to study a localized Langmuir wave packet in a linear density gradient. Our results show that although the density gradient suppresses the overall strength of LDI, the backscattering channel (φ ≈ 180°) remains the dominant decay mode across all tested pump incidence angles. These results help clarify the multidimensional evolution of LDI in inhomogeneous plasmas and may be useful for developing more predictive models of SRS saturation in laser-fusion schemes.