Suprathermal electron generation in a coronal plasma and associated preheating of Direct Drive fusion targets

摘要

Parametric instabilities taking place in a coronal plasma, like stimulated Raman scattering (SRS) and two-plasmon decay, can couple a fraction of the incident laser light to electron plasma waves. These waves can eventually reach a high amplitude, break, and produce hot electrons. Hydrodynamics calculations show that our direct-drive target design is significantly threatened by these hot electrons, as a uniform preheating by 1% of the incident laser power is enough to reduce the gain below 1. We have used a 1D and 2D Particle-In-Cell code to study laser-plasma instabilities in the corona surrounding the direct-drive target, and in particular the yield, temperature, and collimation of suprathermal electrons as a function of plasma and laser parameters. We observe that the 2D "speckle" geometry leads to hot electrons that are significantly colder than in 1D.