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Author: Xianzhu Tang
Requested Type: Pre-Selected Invited
Submitted: 2016-02-15 12:50:16

Co-authors: Zehua Guo

Contact Info:
Los Alamos National Laboratory
Los Alamos, NM   87545

Abstract Text:
The exhaust of plasma particle and energy to a material surface is
regulated by a non-neutral sheath~[1]. This is generally known as the sheath boundary condition for plasma exhaust, which provides the driver for plasma-surface interaction.

Sheath theory relates the plasma particle and energy exhaust fluxes to the plasma density, temperature, and flow at the sheath entrance. The particle exhaust flux is the product of plasma density and flow speed. The flow speed is constrained by the Bohm criterion to a local sound speed. For a warm ion plasma, the highest local parallel sound speed in the literature for Bohm criterion analysis is $c_{sparallel}equiv
sqrt{k_Bleft(T_{eparallel}+3T_{iparallel}right)/m_i},$ with
$T_{eparallel}$ and $T_{iparallel}$ the electron and ion parallel
temperature. We find~[5] from kinetic simulations that plasma exit
flow speed robustly exceeds $c_{sparallel}$ at the sheath entrance. To understand the origin of this discrepancy, we analyze both the flow acceleration in the quasineutral upstream plasma, using the extended CGL formulation~[3], and the Bohm criterion using a more complete physics model for the non-neutral plasma that takes into account both the sheath heat flux $q_n equiv int m {w}_parallel^3 fd^3{bf v}$ with ${bf w}$ the particle peculiar velocity, and the electron temperature variation. We find that contrary to the prevailing view, which is based on analyses that ignore the electron temperature variation and the parallel heat flux, the Bohm criterion produces a Bohm speed that is robustly greater than $c_{sparallel}.$ This suggests a greater particle exhaust flux for given plasma density and temperature at the sheath entrance.

Work supported by DOE FES.

[1] P.C. Stangeby, {em The Plasma Boundary of Magnetic Fusion Devices}
(Taylor & Francis, 2000).\
[2] X.Z. Tang & Z. Guo, Phys. Plasmas {bf 22}, 100703 (2015).\
[3] Z. Guo, X.Z. Tang, & C. McDevitt, PoP {bf 21}, 102512 (2014)