Bohm criterion and plasma particle/power exhaust to and recycling at the wall
Abstract
The plasma particle and power exhaust to the divertor surface drives both particle and power recycling at the surface, which in return constrains the plasma density and temperature at the target and their profile further upstream. Both particle and power exhaust fluxes are mediated by the plasma sheath next to the divertor surface. In particular, the Bohm criterion constrains the ion exit flow speed, which enters directly into the particle flux and the kinetic flow energy component of the ion power flux, and indirectly into the electron power flux through the sheath potential drop. Here we give an overview on how the Bohm speed is set in a general plasma and how it enters power exhaust and power recycling at the divertor surface, and the implication on the correct implementation of sheath boundary conditions in numerical codes. The cases of ideal and nonideal Bohm speed are distinguished as a result of the physics discussion.
 Authors:
 Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
 Publication Date:
 Research Org.:
 Los Alamos National Lab. (LANL), Los Alamos, NM (United States)
 Sponsoring Org.:
 USDOE Office of Science (SC). Advanced Scientific Computing Research (ASCR) (SC21); USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC24)
 OSTI Identifier:
 1361705
 Alternate Identifier(s):
 OSTI ID: 1369183
 Report Number(s):
 LAUR1624857
Journal ID: ISSN 23521791
 Grant/Contract Number:
 AC5206NA25396
 Resource Type:
 Journal Article: Published Article
 Journal Name:
 Nuclear Materials and Energy
 Additional Journal Information:
 Conference: International Conference on Plasma Surface Interactions in Controlled Fusion Devices , Rome (Italy), 30 May 2016; Journal ID: ISSN 23521791
 Publisher:
 Elsevier
 Country of Publication:
 United States
 Language:
 English
 Subject:
 70 PLASMA PHYSICS AND FUSION TECHNOLOGY; Magnetic Fusion Energy; Bohm criterion; Ideal Bohm speed; Nonideal Bohm speed; Plasma particle exhaust; Plasma power exhaust; Power recycling; Ion reflection
Citation Formats
Tang, Xianzhu, and Guo, Zehua. Bohm criterion and plasma particle/power exhaust to and recycling at the wall. United States: N. p., 2017.
Web. doi:10.1016/j.nme.2017.05.011.
Tang, Xianzhu, & Guo, Zehua. Bohm criterion and plasma particle/power exhaust to and recycling at the wall. United States. doi:10.1016/j.nme.2017.05.011.
Tang, Xianzhu, and Guo, Zehua. 2017.
"Bohm criterion and plasma particle/power exhaust to and recycling at the wall". United States.
doi:10.1016/j.nme.2017.05.011.
@article{osti_1361705,
title = {Bohm criterion and plasma particle/power exhaust to and recycling at the wall},
author = {Tang, Xianzhu and Guo, Zehua},
abstractNote = {The plasma particle and power exhaust to the divertor surface drives both particle and power recycling at the surface, which in return constrains the plasma density and temperature at the target and their profile further upstream. Both particle and power exhaust fluxes are mediated by the plasma sheath next to the divertor surface. In particular, the Bohm criterion constrains the ion exit flow speed, which enters directly into the particle flux and the kinetic flow energy component of the ion power flux, and indirectly into the electron power flux through the sheath potential drop. Here we give an overview on how the Bohm speed is set in a general plasma and how it enters power exhaust and power recycling at the divertor surface, and the implication on the correct implementation of sheath boundary conditions in numerical codes. The cases of ideal and nonideal Bohm speed are distinguished as a result of the physics discussion.},
doi = {10.1016/j.nme.2017.05.011},
journal = {Nuclear Materials and Energy},
number = ,
volume = ,
place = {United States},
year = 2017,
month = 6
}

The plasma particle and power exhaust to the divertor surface drives both particle and power recycling at the surface, which in return constrains the plasma density and temperature at the target and their profile further upstream. Both particle and power exhaust fluxes are mediated by the plasma sheath next to the divertor surface. In particular, the Bohm criterion constrains the ion exit flow speed, which enters directly into the particle flux and the kinetic flow energy component of the ion power flux, and indirectly into the electron power flux through the sheath potential drop. Here we give an overview onmore »

Effect of the electron energy distribution function in plasma on the Bohm criterion and on the drop voltage through the sheath: Case of microwave expanding plasma
This work is devoted to the study of the sheath expansion above a charged species collector immersed into the plasma in the general case of the electron energy distribution function (EEDF) (not necessarily a Boltzmann distributed function). In the first part, considering the general form of the electron kinetic energy distribution function (EEDF) in the plasma and assuming a one dimensional system, we propose an equation describing the limit value of the ion velocity at the sheath edge. Then, we extend this equation to the three dimensional problem. These results are discussed assuming Boltzmann distribution for the one dimensional systemmore » 
Fulfillment of the kinetic Bohm criterion in a quasineutral particleincell model
Quasineutral particleincell models of ions must fulfill the kinetic Bohm criterion, in its inequality form, at the domain boundary in order to match correctly with solutions of the Debye sheaths tied to the walls. The simple, fluid form of the Bohm criterion is shown to be a bad approximation of the exact, kinetic form when the ion velocity distribution function has a significant dispersion and involves different charge numbers. The fulfillment of the Bohm criterion is measured by a weighting algorithm at the boundary, but linear weighting algorithms have difficulties to reproduce the nonlinear behavior around the sheath edge. Amore » 
The BohmChodura plasma sheath criterion
The ion flow velocity entering the Debye sheath is constrained to satisfy the Bohm criterion: velocity must be equal to or greater than sonic. The ion flow velocity entering the magnetic presheath (which arises upstream of the Debye sheath when a magnetic field, oblique to the solid surface is present) is constrained to satisfy the Chodura criterion: velocityparallel to {bold B} must be equal to or greater than sonic. The Bohm criterion, in its marginal form where {ital v}={ital c}{sub {ital s}}, is associated with a ``gentle entry`` into the sheath with {Delta}{ital n}{sub {ital e}}={Delta}{ital n}{sub {ital i}}, whilemore »