Investigation of stimulated Raman backscatter for lasers with orbital angular momentum*

We investigate how light with orbital angular momentum (OAM) scatters from stimulated Raman scattering (SRS) differently than light without OAM for inertial confinement fusion parameters. We use the codes pF3D and OSIRIS and study how pumps scatter from thermal noise and from backward propagating seeds with selected degrees of OAM behave differently for kλ_D ≈ 0.3. For pF3D simulations it is found that when the pump power is held fixed that SRS reflectivity is reduced for pumps with OAM. This is caused by the reduction in peak intensity for a OAM mode that has the same power. To lowest order, the reflection of a Gaussian pulse can be predicted based on assuming each transverse slice grows based on 1D linear theory. When the spot size for the OAM mode is reduced to match the peak intensity then SRS reflectivity is similar to the case without OAM. The growth of well-categorized seeds also agrees with the 1D analysis. OAM diagnostics for the pump, scattered light wave, and plasma wave show the angular momentum is conserved and the relative OAM between modes changes in space and time. Preliminary results from OSIRIS are also presented.