Improvements to collisional ionization models for particle-in-cell codes

A novel electron-impact ionization algorithm has been designed for use in particle-in-cell codes. This improved model uses a combination of modified-Bell and relativistic-binary-encounter-Bethe cross sections for greater accuracy and samples the secondary electron kinetic energies from a differential cross section. The algorithm also conserves the number of real-particle ionization events for arbitrary weighted macro-particles, while ionization schemes in existing algorithms are shown to break this condition. Further limitations of existing models have been explored, and the improved model is described within this framework. Benchmarks have been provided to demonstrate the accuracy of this new model.