May 1-3

Log in

Abstract Details

files Add files

status:file name:submitted:by:
approvedbohm.pdf2017-03-17 16:18:44Xianzhu Tang

Abstracts

Author: Xianzhu Tang
Requested Type: Pre-Selected Invited
Submitted: 2017-03-17 16:18:04

Co-authors: Zehua Guo

Contact Info:
Los Alamos National Laboratory
Theoretical Division
Los Alamos, NM   87545
U.S.A.

Abstract Text:
The competition between parallel and perpendicular transport sets the
width of the scrape-off layer for plasma particle and energy exhaust.
Transport along the magnetic field is dominated by parallel streaming,
which is a fast process constrained by ambipolarity. This
becomes most transparent at the divertor surface,
where the non-neutral sheath regulates the exhaust
flux in terms of the boundary plasma density, temperature, and flow.
The exit flow speed is constrained by the Bohm criterion to a
local sound speed, which is set by the plasma
temperature. 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.

Direct kinetic-Maxwell simulations [1] reveal that the plasma exit
flow robustly exceeds $c_{sparallel}$ at the sheath entrance,
contradicting a foundational plasma theory prediction. Based on the extended CGL
formulation~[2], we have performed a new analysis that
yields
begin{equation}
u_{Bohm}equiv sqrt{ frac{3k_Bleft(T_{iparallel} + Zbeta
T_{eparallel}right)}{m_i} -
Zbetafrac{m_e}{m_i}frac{j^2}{left(e n_eright)^2} }
end{equation}
where $j$ is the net
current density to the wall and $Z$ the ion change
number. The most important and interesting quantity is the heat flux
factor $beta,$ which depends on the parallel heat flux~[3].

With this new formulation, we are able to explain a wide range of
plasma exit flow speeds as functions of plasma collisionality and
current into the wall, and resolve a number of observed contradictions
between simulations and traditional theory.

This work was supported by OFES.

[1] X.Z. Tang & Z. Guo, PoP {bf 22}, 100703 (2015);
[2] Z. Guo, X.Z. Tang, & C. McDevitt, PoP {bf 21}, 102512 (2014);
[3] X.Z. Tang & Z. Guo, PoP {bf 23}, 120701 (2016)

Comments: