Simulations of the collisional Bohm criterion and edge-to-center density ratio over a range of gas pressure.
Abstract not provided.
Beving, Lucas Paul↗
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Simulations of the collisional Bohm criterion and edge-to-center density ratio over a range of gas pressure.
Abstract not provided.
Beving, Lucas Paul↗
How sheath properties change with gas pressure: modeling and simulation
Abstract Particle-in-cell simulations are used to study how neutral pressure influences plasma properties at the sheath edge. The high rate of ion–neutral collisions at pressures above several mTorr are found to cause a decrease in the ion velocity at the sheath edge (collisional Bohm criterion), a decrease in the edge-to-center density ratio ( h l factor), and an increase in the sheath width and sheath potential drop. A comparison with existing analytic models generally indicates favorable agreement, but with some distinctions. One is that models for the h l factor need to be made consistent with the collisional Bohm criterion. With this and similar corrections, a comprehensive fluid-based model of the plasma boundary transition is constructed that compares well with the simulation results.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Transport physics dependence of Bohm speed in presheath–sheath transition
We report the ion exit flow speed at the sheath entrance is constrained by the Bohm criterion, which is used as a boundary condition for simulations that do not resolve the sheath region. Traditional Bohm criterion analysis invokes the equation of state and, thus, ignores transport physics in the sheath transition problem. An expression for the Bohm speed away from the asymptotic limit is derived from a set of anisotropic plasma transport equations. The thermal force, collisional temperature isotropization, and heat flux enter into the evaluation of the Bohm speed. By comparison with kinetic simulation results, this expression is shown to be accurate in the presheath–sheath transition region rather than a single point at the sheath entrance over a broad range of collisionality.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗