Fast and Robust Integration of N-body Systems

Systems of particles interacting via potential fields range from plasma to biological to gravitational applications. Calculation of forces by direct summation of particle contributions becomes prohibitively expensive for large particle ensembles. Traditional particle-mesh methods remedy this problem by solving Poisson's equation. On non-uniform meshes, however, this approach may produce spurious self-forces (Vay et al. 2004). Alternatively, tree-code algorithms (Barnes & Hut 1986) group particles into nested spatial clusters and estimate particle-cluster forces in a hierarchical multi-pole approximation. All existing algorithms employ explicit time-stepping integrators. We propose a novel algorithm for simulating N-body systems: an Event-driven Particle-Mesh (EPM) method. It combines Particle-in-Cell (PIC) gather/scatter techniques with direct evaluation (summation) of forces and performs asynchronous calculations via discrete-event simulation (DES) (Karimabadi et al 2005; Omelchenko & Karimabadi 2006, 2007). DES preserves causality from first principles and selects particle updates in self-adaptive order. Ill addition to ensuring stability and accuracy this eliminates CPU overhead associated with inactive parts of the phase space. In this paper we describe a uniform-mesh implementation of the new algorithm and validate it by simulating the interaction of a powerful laser with a neutral plasma cluster.