Investigation of Hydrodynamic Stability of High Aspect Ratio Targets in Laser Implosion Experiments

Abstract

With the OCTAL laser facility, spherical implosions have been performed with large aspect ratio targets (R/ΔR up to 240), up to 300 µm in diameter, filled with equimolar DT at different pressures (1–20 bars). The one- dimensional simulations seem to fit rather well with all the experimental results (emission and X-ray shadowgraphy) except for the neutron yield. Hydrodynamics of the implosion and shock collapse time appear to be drastically sensitive to the preheat energy level. In particular, the reflected outgoing shock gives rise to an important perturbation at the pusher-fuel interface.

The stability is investigated by studying the growth of perturbations due to geometrical and illumination defects in the linear phase with the perturbation code PERTUS, The formulation used is based on a decomposition into spherical harmonics of the first-order flow quantities, whose evolution is computed independently for each mode.

The low and intermediate harmonic order (ℓ<100) mode growth is such that the linear phase assumption is no longer valid well before the completion of the implosion. A matching of the numerical results to the experimental data is attempted and points to the presence between the core and the shell of a mixing layer which could cause a significant reduction of the neutron yield.