Bohm criterion and plasma particle/power exhaust to and recycling at the wall

Highlights: • Sheath boundary condition is used in tokamak edge modeling to constrain the particle and power fluxes into the divertor. • These fluxes are drivers for wall feedback into the plasma via particle and power recycling. • The Bohm criterion, which sets a lower bound, the so-called Bohm speed, for the ion exit flow at the sheath entrance is reexamined. • It is found that ideal and nonideal Bohm speed, along with anisotropy, significantly complicate what the correct sheath boundary condition is and how they can be implemented. - Abstract: The plasma particle and power exhaust to the divertor surface drives both particle and power recycling at the surface, which in return constrains the plasma density and temperature at the target and their profile further upstream. Both particle and power exhaust fluxes are mediated by the plasma sheath next to the divertor surface. In particular, the Bohm criterion constrains the ion exit flow speed, which enters directly into the particle flux and the kinetic flow energy component of the ion power flux, and indirectly into the electron power flux through the sheath potential drop. Here we give an overview on how the Bohm speed is set in a general plasma and how it enters power exhaust and power recycling at the divertor surface, and the implication on the correct implementation of sheath boundary conditions in numerical codes. The cases of ideal and non-ideal Bohm speed are distinguished as a result of the physics discussion.