Laser-plasma interaction in a regime relevant for laser-fusion via shock-ignition
Shock ignition (SI) is a promising two-step approach to Inertial Confinement Fusion (ICF), where a strong converging shock wave is launched at the end of the compression phase to ignite the fuel. Both compression of the DT pellet and the shock wave can be produced by a single tailored laser pulse (Fig.1), consisting of a ns long peak at moderate intensities lower than 10<sup>15</sup> W cm<sup>-2</sup> followed by a short intense spike (300-500 ps) at I≈10<sup>15</sup>–10<sup>16</sup> W cm<sup>-2</sup>.The main advantage of SI approach is the expected high gain, enabling ignition at moderate laser energies, which are already available at LMJ and NIF facilities.
The success of SI concept is mainly determined by the coupling of laser spike with the compressed corona, where an efficient laser absorption, able to generate a few-hundreds Megabar shock wave at the ablation layer, is needed. However, the physics of laser-plasma interaction at laser intensities typical of SI spike, is highly non-linear and still largely unknown.
Our activity is focussed to investigate the laser-plasma coupling in this regime, whereparametric instabilities as Stimulated Brillouin Scattering (SBS), Stimulated Raman Scattering (SRS) and Two-Plasmon Decay (TPD), often interplaying, can non-linearly grow and laser filamentation can further affect their relevance. These processes can significantly degrade laser-plasma coupling producing a strong reflection of light (SBS and SRS) and generate suprathermal electrons, whose effect on the shock wave ignition is currently under investigation. An exhaustive knoweldge of the effects influencing laser-plasma coupling is therefore necessary to design laser and experimental systems necessary for the SI.The activity was first carried out in the context of the European HiPER project (http://www.hiper-laser.org), currently concluded, dedicated to demonstrating the potential of laser-induced fusion as a future energy source. Subsequently the ILIL working group on inertial fusion was one of the promoters of a new European project EUROFUSION, focused on the role of fast electrons to obtain pressures on the target of the order of hundreds of Mbar, and on the importance of uniformity of the laser pulse. In recent years the experiments have been conducted at the Prague Asterix Laser laboratory (Fig. 2) in collaboration with other European research groups and have been funded by LaserLab Europe. The experimental activity of the Megajoule (LMJ) laser in Bordeaux, in France, has also started in 2019, always on the same themes.