April 4-6

Abstract Details

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Author: Li Yuzhi
Requested Type: Consider for Invited
Submitted: 2022-03-04 23:21:28

Co-authors: B.Srinivasan, Y.Zhang, X.Z.Tang

Contact Info:
Virginia Tech
215 Randolph Hall, 460 Old Tur
Blacksburg, VA   24060
United States

Abstract Text:
Sheath theory has a central place in plasma physics as its original
formulation coincided with the recognition of plasma physics as a
sub-field in physics and it applies to any plasma bounded by a
material boundary. One of the most celebrated findings in sheath
theory is the so-called Bohm criterion that predicts a threshold, the
so-called Bohm speed, which would provide a lower bound for the plasma exit flow speed at the sheath entrance. The 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 quasi-neutral plasma modeling. Traditionally, evaluation of Bohm speed from the Bohm criterion invokes drastic simplification of plasma transport that ignores the transport physics in the plasma-sheath transition problem. These and even more sophisticated kinetic Bohm criterion analysis are all performed in the asymptotic limit of vanishing Debye length, and hence their applicability becomes suspect in a realistic plasma. Here, we drive an expression for the Bohm speed from a set of anisotropic plasma transport equations. The thermal force, temperature isotropization and heat flux enter into the evaluation of the Bohm speed. Away from the asymptotic limit, it is shown from the simulation results that there exists a plasma-sheath transition region, where the quasi neutrality is weakly perturbed, rather than a single sheath entrance in the asymptotic limit, so a Bohm speed is predicted for the entire transition region. By comparison with kinetic simulation results, the Bohm speed model in our work is shown to be accurate in the sheath transition region over a broad range of collisionality. This, to our knowledge, is the first time that a predictive formula for Bohm speed has been shown to be quantitatively accurate in the intermediate plasma regime, which is away from the known limiting cases and the asymptotic limit.

Ref: Y. Li et al PRL 128, 085002 (2022)