Convective nature of the stimulated Raman side scattering in inertial confinement fusion

The absolute growth of stimulated Raman side scattering (SRSS) predicted by previous theories appeared to be surprisingly absent in the recent ignition-scale direct-drive experiments with the absence attributed to different reasons. We present evidence from simulations that the SRSS modes are naturally all convective (i.e., absolute SRSS does not exist at all) in an experimentally relevant regime where a finite-beam-width laser is incident into a nonuniform low-density plasma below a quarter of the critical density. The convective gain demonstrated by our newly proposed formula via numerical fitting monotonically increases with the beam width without saturation, which is significantly different from the prediction of previous convective SRSS theories. The findings have significant implications for the assessment of SRSS in ignition designs.