One of the main concerns of Inertial Confinement Fusion is the impact of laser parametric instabilities growing during the interaction of the laser pulse with the long-scale plasma corona; the issue is even more serious in the Shock Ignition scheme, where the intensity of the laser spike (~10$^{16]$ W/cm$^2}$) producing the igniting shock wave is an order of magnitude higher than the intensity needed in the classical direct-drive scheme. Among laser-plasma instabilities, Stimulated Brillouin Scattering (SBS) and Cross-Beam Energy Transfer (CBET) can reduce the efficiency of fuel compression and produce an imbalance of the laser beam coupling. The growth of Stimulated Raman Scattering (SRS) and Two Plasmon Decay (TPD), on the other hand, can result in the generation of large fluxes of suprathermal hot electrons (HEs), that can be absorbed by the cold fuel, enhancing its entropy and preventing ignition. The non-linear character of these instabilities makes the extrapolation of analytical models quite inaccurate; a more detailed understanding of these processes therefore calls for experiments dedicated at investigating Laser Plasma Interaction (LPI) in conditions as close as possible to those envisaged in SI. The final aim of this research is identifying the proper ways to tune or mitigate the effects of such instabilities to reach an effective compression and ignition of the fuel.
Here, we will summarize recent experimental results obtained in campaigns at Gekko XII, PALS and VULCAN laser facilities, carried out in the framework of the EuroFusion research program and supported by Laserlab Europe. Experiments were aimed at understanding the role of plasma and laser parameters on the growth of SRS and TPD as well as identifying the mechanisms originating HEs in conditions relevant for SI. The regime of interaction in these experiments is significantly different, with laser intensity ranging from I$\lambda$$^{2}$ ~2·10$^{14}$ W$\mu$m$^{2}$/cm$^{2}$ to ~2·10$^{16}$ W$\mu$m$^{2}$/cm$^{2}$, plasma density scalelength rising from 50 to 400 $\mu$m, laser wavelength going from the UV to the IR range, laser irradiation in single- and multi-beam configuration, plasma temperature from 1 keV to 5 keV, allowing a good understanding of the role played by the different experimental parameters. We will finally discuss the new directions of our research, showing also preliminary results of recent experiments aimed in particular at investigating the effects of laser bandwidth/chirp on LPI and the role played by Side-SRS in Shock Ignition ICF conditions.
Zs. Major