Hydrodynamic Instabilities in Inertial Confinement Fusion Implosions

Sam Miller, Supervised by Valeri Goncharov and Radha Bahukutumbi

Annex Conference room, LLE

In direct-drive inertial confinement fusion, a spherical target, comprised of a deuterium-tritium (DT) gas vapor fill enclosed by a shell with an inner layer of cryogenically frozen DT ice and an outer ablator layer (typically CH plastic), is directly irradiated by multiple high-power (~10¹⁵ W/cm²), short-pulse (nanosecond time scale) laser beams. The performance of these implosions is currently understood to be largely driven by the level of laser illumination symmetry and surface roughness of the target shell. As the target implodes, the shell becomes unstable due to the Rayleigh–Taylor (RT) instability during both the acceleration and deceleration phases. RT instability growth is one of the primary sources of target degradation because it causes the shell to become asymmetric and potentially break apart, significantly reducing target performance, before reaching maximum compression and fusion yield.

Recently completed research focuses on the stability of the fuel–shell material interface in room-temperature plastic target implosions. In these target designs, deceleration-phase RT growth is enhanced by the density discontinuity and finite Atwood number at the fuel–shell interface. The Atwood number of the interface is systematically varied by altering the ratio of deuterium to tritium (D:T) within the DT gas fill. It is shown that the stability of the interface is best characterized by the effective Atwood number, which is primarily determined by radiation heating of the shell and not by the composition of the fuel. Both simulation and experimental data show that yield performance scales with the fraction of D and T present in the fuel and that the observed inferred ion temperature asymmetry (∆T_i = T_i^max - T_i^min), which indicates the presence of long-wavelength modes, has a small sensitivity to the different D:T ratios.

A research proposal will be presented to study a new hydrodynamic instability that has been identified during the early stages of cryogenic implosions. Perturbations created by target defects, like inner shell voids and surface roughness, are propagated via acoustic waves that reverberate within the shell during the initial compression of the shell. The presence of an ablator-ice (CH-DT) interface creates reflected rarefaction and compression waves that can amplify these initial perturbations. The reflected rarefaction wave launched by the interface in picket-pulse designs has been shown to create an acoustic trap for perturbations near the outer edge of the shell which could create instability seeds later in the implosion.

The interplay of shell defects and acoustic wave propagation and its impact on implosion performance, as well as potential mitigation strategies and experimental verification, will be the primary focus of the proposed research.