Comparison Between Electrostatic PP and PIC Simulations of Electron Bunch Expansion

With the great development of parallel computing techniques, the particle-particle (PP) model has been successfully applied in a number of plasma applications. Compared to particle-mesh (PM) models, such as the widely used particle-in-cell (PIC) method, PP has the advantages of high accuracy in solving Coulomb interactions. In this article, it is shown that PP is also advantageous for simulating nonneutral plasmas, such as electron bunch expansion in vacuum. The numerical effects of the macroparticle weight and the time step length are investigated for the PP model, by which accurate and convergent results can be obtained with less effort. On the contrary, PIC needs to simulate the same problem with extremely large effort. It is found that the simulation accuracy does not grow with reduced cell size monotonically. In a long run, PIC must apply large enough domain to cover all the expanding particles and avoid nonphysical effects caused by imperfect infinite boundary condition, which may result in too heavy computation and make PIC infeasible.