When considering intense particle or laser beams propagating in dense plasma or gas, ionization plays an important role. Impact ionization and tunnel ionization may create new plasma electrons, altering the physics of wakefield accelerators, causing blue shifts in laser spectra, creating and modifying instabilities, etc. Here we describe the addition of an impact ionization package into the 3-D, object-oriented, fully parallel PIC code OSIRIS. We apply the simulation tool to simulate the parameters of the upcoming E164 Plasma Wakefield Accelerator experiment at the Stanford Linear Accelerator Center (SLAC). We find that impact ionization is dominated by the plasma electrons moving in the wake rather than the 30 GeV drive beam electrons. Impact ionization leads to a significant number of trapped electrons accelerated from rest in the wake.

In current plasma-based accelerator experiments, very short bunches (100-150 {micro}m for E164 [1] and 10-20 {micro}m for E164X [2] experiment at Stanford Linear Accelerator Center (SLAC)) are used to drive plasma wakes and achieve high accelerating gradients, on the order of 10-100GV/m. The self-fields of such intense bunches can tunnel ionize neutral gases and create the plasma [3,4]. This may completely change the physics of plasma wakes. A 3-D object-oriented fully parallel PIC code OSIRIS [5] is used to simulate various gas types, beam parameters, etc. to support the design of the experiments. The simulation results for real experiment parameters are presented.