Diffusion-dominated mixing in moderate convergence implosions

Abstract

High-Z material mixed into the fuel degrades inertial fusion implosions and can prevent ignition. Mix is often assumed to be dominated by hydrodynamic instabilities, but we report Omega data, using shells with ∼150nm deuterated layers to gain unprecedented resolution, which give strong evidence that the dominant mix mechanism is diffusion for these moderate temperature (≲6 keV) and convergence (∼12) implosions. Small-scale instability-driven or turbulent mix is negligible.

Sudden diffusion of turbulent mixing layers in weakly coupled plasmas under compression

“…Yet, for materials directly injected into the hot spot at early times, such as meteors from the filling tube [14], diffusion can occur very rapidly throughout the hot spot, depending on the temperature or density at the end of compression. This work also supports the hypothesis of mixing caused by physical diffusion in recent experiments, although not aimed at achieving ignition, performed at the Omega facility [16,17]. In addition, summing the plasma diffusion with the turbulent diffusion evaluated from turbulence models without low-Reynolds-number corrections appear an efficient strategy to quantify mixing at interfaces as the relaminarization process has a small impact on the layer dynamics.…”

“…All the characteristics relative to the simulations are reported in Table VI. With respect to the case discussed by Zylstra et al [17] in the simulations ZS a,b , the initial interface between DT and CH is more diffused, due to computational constraints.…”

“…Moreover, plasma transport coefficients have been shown to play a significant role in the mixing zone dynamics (see Vold et al [15]). Situations where mixing is only produced by molecular diffusive processes may also occur [16,17], although in Omega capsules not designed for ignition. Actually, the question of how the mixing zone dynamics evolve with nonuniformities and strong temporal variations of plasma transport coefficients [18], due to temperature and mass fraction variations, is still pending.…”

One-dimensional hydrodynamic simulations of low convergence ratio direct-drive inertial confinement fusion implosions

“…Ten calibration shots performed in a direct-drive configuration on OMEGA for a study of diffusion-dominated mixing were simulated [59]. The capsules used in these shots consisted of…”

“…Ten calibration shots performed in a direct-drive configuration on OMEGA for a study of diffusion-dominated mixing were simulated [59]. The capsules used in these shots consisted of different HT vapour mixtures, surrounded by either pure CH or CH with 2% D shells (these shells were homogeneous, and did not have a layer structure such as those presented in the paper referenced).…”