The effect of a collisional presheath on the Bohm criterion

The plasma exit flow speed at the sheath entrance is constrained by the Bohm criterion. The so-called Bohm speed regulates the plasma particle and power exhaust fluxes to the wall, and it is commonly deployed as a boundary condition to exclude the sheath region in modeling. Traditional Bohm criterion analyses invoke equation of state, and thus ignore transport physics of both collisionless and collisional origin. Previously we have found that the electron parallel heat flux has a critical role in setting the Bohm speed. Here we perform both collisional VPIC simulation and theoretical analysis to decipher the subtle and important roles of thermal force and energy exchange between the parallel and perpendicular degrees of freedom. Although thermal force enters the Bohm criterion analysis straightforwardly since it is a gradient of temperature, similar to heat flux, the collisional energy exchange between parallel and perpendicular temperatures is less obvious. The collisionality dependence of electric field at the sheath entrance makes this physics important even when the collisionality is lowered. This work was supported by the U.S. DOE Office of Science through the Tokamak Disruption. Simulation (TDS) SciDAC project.