Description
In the context of inertial confinement fusion energy (IFE), it is necessary to mitigate laser– plasma instabilities to avoid drive degradation and to preserve irradiation symmetry. This is achieved through laser beam smoothing, which typically involves spectral broadening and
frequency dispersion to reduce spatial and temporal laser coherences. Hot spots in the speckle pattern act as seeds for laser–plasma instabilities. However, IFE laser facility designs may require a larger chamber radius, leading to a significant increase in the f-number of individual laser beams and, consequently, to larger and longer hot spots in the speckle pattern.
In this work, the impact of increasing the f-number on stimulated Brillouin scattering (SBS) is studied using the 3D HERA platform [1]. First, the SBS response of a Gaussian beam with different apertures is investigated and compared with theoretical predictions [2]. Then, spatially incoherent beams composed of thousands of hot spots, whose size depends on the f-number, are considered. We study the influence of the f-number in a homogeneous plasma and examine the limits of a scaling law identified in a recent experiment [3] carried out at the LULI2000 facility, which relies solely on laser parameters. Finally, we consider a more realistic plasma and discuss the domain of validity of this approach and its limitations.
References
1 F. Ballereau, P., Casanova, M., Duboc, F. et al. Simulation of the Paraxial Laser Propagation Coupled with Hydrodynamics in 3D Geometry. J Sci Comput 33, 1–24, 2007 .
2 L. Divol et P. Mounaix : Analytical calculation of the amplification and angular divergence of the stimulated backscattered light from a gaussian hot spot. Phys. Rev. E, 58:2461–2470, Aug 1998.
3 J. G. Moreau et al., Stimulated Brillouin scattering dependence on polarization state, speckle shape, and polarization smoothing implementation. Phys. Plasmas 32, 032102, 2025.