Description
The hydrodynamic stability of ablation flows is a key issue in laser-driven inertial confinement fusion (ICF) where a sufficiently symmetric implosion of a spherical capsule is expected to achieve thermonuclear burn. For the early stage of an implosion, or shock-transit phase, illumination asymmetries and initial external surface roughness have been identified for some time as critical perturbation seeds [1]. More recently, experimental and simulation results have pointed out that ablator internal perturbations could account for degraded capsule performances, emphasizing the need for a comprehensive understanding of associated perturbation growth mechanisms [2]. Simulating the effects of density perturbations buried at various depths inside a capsule shell has highlighted the importance of acoustic-wave and vortex interactions with the ablation front during the shock-transit phase [3]. In the present work, we apply a method of non-modal linear stability analysis for determining optimal density perturbations within a capsule ablator [4]. The chosen base flow is relevant to the radiative ablation of a capsule shell during the transit of the first shock-wave within the ablator [5]. Optimal longitudinal distributions of linear density perturbations that maximize the ablation front distortion at different times prior to the shock breakout are obtained for transverse wavelengths between 1 and 100 µm and longitudinal wavelengths in the full range 0.1-100 µm. Ablation front sensitivity is maximum for the longest transverse wavelengths with entropy perturbation amplification by the ablation front as the dominant destabilizing mechanism. With decreasing transverse wavelengths, the flow selects density perturbation patterns of shorter longitudinal wavelengths in accordance with the wave-emission properties of the shock front and the damped-oscillatory response of the ablation front. For the shortest transverse wavelengths, localized density spots triggering early entropy-vorticity deposition and late acoustic emission by the shock front emerge as the dominant pattern.
References
[1] Emery, M. H., et al., Phys. Fluids B 3, 2640 (1981). Goncharov, V. N., Phys. Rev. Lett. 82, 2091(1999).
[2] Ali, S. J., et al., Phys. Rev. E 98, 033204 (2018). Haines, B. M., et al., Phys. Plasmas 26, 012707 (2019).
[3] Miller, S. C., Goncharov, V. N., Phys. Plasmas 29, 082701 (2022).
[4] Thiriet, J.-G., et al., Comput. Fluids 302, 106799 (2025).
[4] Clarisse, J.-M., et al., J. Fluid Mech. 848, 219-255 (2018).