Multi-species plasma transport in 1D direct-drive ICF simulations

Abstract

A multi-species plasma ion transport model has been added to the adaptive mesh refinement radiation hydrodynamics code, xRage, to include kinetic transport effects when the particle distributions are near Maxwellian, with deviations proportional to a Knudsen number smaller than one. The model is first verified against self-similar solutions reported previously for the pressure equilibrium case, and next shown to be relatively insensitive to the choice of equation of state for the ions. Simulations are then used to examine Inertial Confinement Fusion dynamics in a 1D spherical geometry characteristic of an Omega implosion with a plastic (CH) shell containing a deuterium-tritium (DT) fuel, and with an added heavy ion impurity, argon. Even in this simplified 1D geometry, several interesting results are apparent. Ion stratification occurs similarly to that reported previously in purely kinetic simulations. The hydrogen in the plastic shell is transported radially inward, carried with the main drive shock, and thus migrates away from the C ions. The fuel D and T ions show the expected stratification with an increase in the lighter species concentration during the shock implosion and a reversal, with heavier species concentrations enhanced after shock expansion from the center. This stratification during burn yields different burn weighted ion temperatures, Ti, for the reactions, Ti[DD] < Ti[DT] < Ti[TT], consistent in their ordering with experiments. The mix widths per ion, measured where concentrations fall to 10% of their interfacial value, are evaluated as a function of time, and these are seen to be significant (of order 10 μm) even at early times, well before the main shock converges and before the shell deceleration. The 1D geometry may be a reasonable approximation for this early time mix and implies that this transport may play a role in reducing or modifying the instabilities driven by initial perturbations, ablation, and Rayleigh-Taylor unstable deceleration. An apparent depletion of the heavier ions seen at the incoming ion shock front warrants further investigation.