Kinetic-informed fluid model for cross-field sheaths in multi-ion species plasmas

In low-temperature plasmas, kinetic processes significantly influence sheath dynamics, requiring careful consideration of collisions, instabilities, and external fields for accurate modeling. In the context of cross-field sheaths, prior work has demonstrated that applied B-fields of modest strength (Bz 0.01 T) inhibit the formation of classical sheaths, potentially resulting in zero-potential drops or inverted sheaths. We present findings from 1D3V PIC-MCC simulations of multi-ion species plasmas indicating that the inclusion of a substantial drag term in the electron momentum balance facilitates cross-field transport and enables the development of modified classical sheaths. This drag arises from a combination of electron-neutral collisions and enhanced Coulombic interactions driven by kinetic instabilities. Additionally, we observe significant ion heating and enhanced drag associated with ion-ion two-stream instabilities, which must be accounted for in our analysis. By leveraging the insights gained from the PIC simulations and analyzing the unique electron velocity distribution functions (EVDFs), we develop a simplified fluid model and compare it against a proposed modified Bohm criterion.