Kevin Glize (UKRI STFC): Collectively driven stimulated Raman scattering from picosecond laser pulses.
Laser-plasma instabilities which occur in millimetric, typical in ICF experiments, are still a preoccupying topic because they lead to reflectivity levels higher than predicted. In this context, this work focuses on stimulated Raman scattering (SRS) through a multi-scale analysis: mono, bi and multi speckle scales.
This picosecond experimental study highlights the difference between isolated and collective speckle behaviours in SRS development. A weak speckle, stable if isolated, may become unstable under the influence of local plasma modications generated by an intense, SRS unstable, nearby speckle: kinetic perturbations with suprathermal electrons and/or wavecoupling with electromagnetic and electrostatic seeds. From a set of experimental campaigns, featuring a highly spatially- and temporally-resolved Thomson-scattering diagnostic and backward Raman imaging, we evidenced and characterized this collective behaviour. An experimental difference between these two kinds of perturbations was observed which lead to differentiate their contributions.
In parallel, 2D PIC simulations using CALDER were performed to interpret bispeckle experimental results and in particular to understand the influence from both kinds of perturbations.