Abstract

The failure of the National Ignition Campaign at the NIF to demonstrate ignition has motivated a revision of the fidelity notions for our current ICF simulation capabilities, as well as a probe for possible missing physics. It is now understood that, during the implosion process, fuel in ICF capsules (particularly at the Omega facility, but perhaps also at the NIF) traverse weakly collisional regimes that may allow so-called kinetic behaviors not described by the standard radiation hydrodynamic models currently used to model ICF implosions. Such behaviors may include (among others) fuel stratification at shocks, kinetic mix at interfaces, and nonlocal heat and momentum transport, which can significantly affect compression, heating, and ultimately fusion reactivity.

Modeling such kinetic behaviors at any level of fidelity, however, demands a quantum leap in algorithmic complexity from standard rad-hydro models. Kinetic physics in semi-collisional plasmas is governed by a high-dimensional (3D+3V+time) set of the multispecies Vlasov-Fokker-Planck equations supporting multiple, very disparate time and length scales. The Fokker-Planck collision operator is nonlinear and nonlocal, and features strict conservation properties in the continuum that must be numerically enforced for long-term accuracy. 

Even in a reduced dimensionality system (1D-2V, spherically symmetric), a naive numerical treatment of this set of equations for ICF simulation is impractical, demanding evolving circa 10^12 degrees of freedom over 10^10 time steps, which is significantly beyond envisioned exascale computing capabilities. In this talk, I will describe a novel multiscale simulation code, iFP, which has been designed for long-term kinetic ICF capsule simulations. iFP is time-implicit and adaptive in both physical and velocity space. It has been specifically designed to deal with spatio-temporal temperature disparities such as those present in ICF capsules, and as a result it is able to simulate ICF implosions at a tiny fraction of the cost of the naive estimates (10^6 degrees of freedom over 10^5 time steps), well within current computational capabiities. iFP has been thoroughly verified in planar and spherical geometries, and we have begun exercising it for the simulation of spherical ICF implosions.