A Deterministic Collisional Ionization Module for Particle-in-Cell Codes

We present a new collisional ionization module for particle-in-cell (PIC) codes. Our method treats and calculates ionization events deterministically as each particle’s rate is calculated explicitly and deposited onto a grid. This grid of ionization rates is then used to advance ion densities, which allows us to track how much new charge is generated each timestep, so we can create newly ionized electrons accordingly. Additionally, the ionization rate grid, with little modification, keeps track of how much energy is lost per grid cell due to ionization physics. We interpolate this information back onto the particles; this allows for a continuous decrease in the energy of macro-particles as they participate in ionization events and allows for the easy calculation of the new momentum of ionized electrons. Collectively, this particle-to-grid and grid-to-particle information transfer act as a "smoothing" process, reducing noise considerably compared to other current algorithms. This module has been tested for its accuracy and integrated into the PIC code OSIRIS. We present simulations of intense laser interactions with water targets for ion acceleration, exploring the difference that collisional ionization makes to the dynamics.