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 acitivity is carried out in the framework of the European HiPer project (http://www.hiper-laser.org),dedicated to demonstrating laser driven fusion as a future energy source. Last years experiments have been carried out at Prague Asterix laser facility (Fig.2) in collaboration with other european research groups and have been financed by Laserlab-Europe.