The Partially Collisional CPK Algorithm: What is Tis, How it Works, and Recent Applications

Summary form only given. A new type of "smart PIC" algorithm, intended to bridge the gap between Eulerian fluid and kinetic regimes, is now being used for a variety of applications in ICF and weapon effects. The CPK method (complex particle kinetic) concept [Hewett, 2003] uses an ensemble of small, fluid-like macro-particles to represent particle distributions in phase space. These macro-particles are Gaussian-shaped in both position and velocity with internal parameters such as width, thermal velocity, total mass and charge in addition to the traditional PIC like Lagrangian location and drift velocity. Time evolution is modeled by a combination of Lagrangian motion and internal evolution within each individual macro-particle. In regions of "high activity", macro-particles are aggressively fragmented in phase space to probe for emerging kinetic features and aggressively merged, for economy, if interesting features fail to materialize. Exploiting the shape of the CPK particle, fragmentation in both position and velocity space can be accomplished without loss of significant phase space information. Fragmentation preserves the kinetic capabilities of PIC; merging dramatically shrinks the number of particles in non-kinetic or collisional regions. In collision-dominated regimes, merging naturally produces a few Lagrangian particles that act much as nodes in free-Lagrangian hydrodynamics. Recent improvements [Larson, 2003], allow inter-particle interactions between those particles within a mean-free-path, thus naturally including partially-collisional effects that go over to collision-dominated physics, as the mean-free-path becomes less that the particle width. Applications relevant to our principal applications will be presented