Turbulent mixing in spherical implosions

Inertial Confinement Fusion (ICF) is a mode of nuclear fusion, in which high energy density laser pulses are used to implode a fuel capsule (typically Deuterium) to very high temperatures and pressures resulting in ignition. ICF holds the potential for clean and abundant energy, since the process uses fuel that is widely available, while avoiding storage issues associated with fissile materials. However, the yield of the ICF process can be affected due to significant turbulent mixing that can occur during the implosion phase between the fuel material and the ablator surrounding it. The mixing is driven by Rayleigh-Taylor and Richtmyer-Meshkov instabilities, which are observed at the fuel/ablator interface, and lead to mixing between the two, and eventually to yield degradation. The development of these instabilities, and the properties of the resulting turbulent mixing is also affected by the spherical geometry (as shown in the video below), which can distort the turbulent coherent structures.

We have investigated using numerical simulations the development of a turbulent mixing layer occurring at a multi-material interface, in the process of a spherical implosion. Perturbations were imposed at the surface, and grew during the implosion through a combination of Rayleigh-Taylor, Richtmyer-Meshkov instabilities as well as Bell-Plesset effects. At large convergence ratios, our simulations show highly asymmetric growth of the mixing layer, with significant levels of anisotropy.

Figure: Isosurfaces of mass fraction at the interface between heavy and light fluids in a spherical implosion.

Implosion-driven turbulent mixing between different materials.

Related Publications:

  1. I. Boureima, P. Ramaprabhu and N. Attal, “Properties of the turbulent mixing layer in a spherical implosion”, J. Fluids Engg., 140(5), 050905 (2017). https://doi.org/10.1115/1.4038401
  2. C.C. Joggerst, A. Nelson et al., “Cross-code comparisons of mixing during the implosion of dense cylindrical and spherical shells”, J. Comp. Phys., 275, 154 – 173 (2014). https://doi.org/10.1016/j.jcp.2014.06.037