Advancing Sheath Physics Using Particle-In-Cell Simulations

This talk will summarize recent advances in sheath physics that were enabled by particle-in-cell simulations conducted in collaboration with the Plasma Research Facility at Sandia National Laboratory. Three results are emphasized: (1) Ions in low pressure discharges can be heated substantially by electron-ion energy relaxation when it is enhanced by ion-acoustic instabilities excited near sheaths. Observations from PIC simulations indicate that when the electron-to-ion temperature ratio exceeds approximately 28, the electron-ion energy equilibration rate dramatically increases to such an extent that the temperature ratio at the sheath edge cannot substantially exceed this threshold. Because ion-acoustic waves are reflected from the sheath, ion heating extends into the bulk plasma as well. (2) The second result concerns how sheath and presheath properties change as neutral gas pressure is increased. Many fluid-based models have been proposed to describe this. Tests using particle-in-cell simulations show general agreement with the trends of the models, but also show inconsistencies in how they treat different properties, such as the collisional Bohm criterion, edge-to-center density ratio, sheath width, and sheath potential drop. A new fluid model is proposed to consistently include each of these properties and is tested with the simulations. (3) The third result shows that a new type of electron plasma wave instability can be excited by an ambipolar electric field, as in the presheath near a sheath. This instability is fundamentally different than commonly understood electrostatic instabilities in that it does not satisfy the Penrose criterion. This is possible because of the steady background electric field. The instability wavelength is tens of Debye lengths with a growth rate that is proportional to the electric field strength. This result many have broad implications for electron scattering in the presence of ambipolar electric fields.