Tunnel ionization within a one-dimensional, undriven plasma sheath

Tunnel ionization of atoms can occur in the presence of strong electric fields, on the order of tens of volts per nanometer, when electrons bound to a neutral atom can tunnel through the potential barrier, resulting in an electron-ion pair. Electric fields of this magnitude are often found in the sheaths of high density, high temperature plasmas of inertial confinement fusion plasmas, or in devices such as field emitter arrays. A simple model of the tunnel ionization process for hydrogen atoms was recently added to Aleph, which is a Particle-in-cell (PIC) Direct Simulation Monte Carlo (DSMC) code developed at Sandia National Labs, thereby expanding its suite of particle-particle and particle-surface interactions to include particle-field interactions. This new functionality has been found to perform well over a large range of electric fields. A tunnel ionization reaction process was applied to a one-dimensional, undriven plasma for which results of the plasma density, plasma potential, and electric fields within the sheath will be presented. Additional simulations which include the effects of warm ions and neutrals, electron-neutral collisions and impact ionization, and the injection of neutral particles were also performed. These simulations show that the addition of electron impact ionization, combined with the injection of additional neutral hydrogen, leads to prolonged sustainment of tunnel ionization reactions in the plasma sheath.