Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session TI02: Fundamental/Low Temperature Plasmas
9:30 AM–12:30 PM,
Thursday, October 20, 2022
Room: Ballroom 100 B
Chair: Zhehui Wang, LANL
Abstract: TI02.00005 : Bohm Criterion of Plasma Sheaths away from Asymptotic Limits*
11:30 AM–12:00 PM
Presenter:
Yuzhi Li
(Virginia Tech)
Authors:
Yuzhi Li
(Virginia Tech)
Bhuvana Srinivasan
(Virginia Tech)
Yanzeng Zhang
(Los Alamos National Laboratory)
Xianzhu Tang
(Los Alamos Natl Lab)
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 set a lower bound for the plasma
exit flow speed at the sheath entrance.
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. The
established Bohm criterion analysis are also 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.
Our analysis can be readily extended for more complicated plasmas,
such as a high-recycling divertor where a strong hydrogen
recirculation loop exists. The resulting Bohm speed is consistent with
the underlying plasma transport model, where atomic processes such as
ionization and ion-neutral friction are considered.
*Work supported by OFES and OASCR under theory and SciDAC program.
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