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Title: Analysis of equilibrium and turbulent fluxes across the separatrix in a gyrokinetic simulation

The SOL width is a parameter of paramount importance in modern tokamaks as it controls the power density deposited at the divertor plates, critical for plasma-facing material survivability. An understanding of the parameters controlling it has consequently long been sought. Prior to Chang et al. (2017 NF 57 11), studies of the tokamak edge have been mostly confined to reduced fluid models and simplified geometries, leaving out important pieces of physics. Here, we analyze the results of a DIII-D simulation performed with the full-f gyrokinetic code XGC1 which includes both turbulence and neoclassical effects in realistic divertor geometry. More specifically, we calculate the particle and heat E x B fluxes along the separatrix, discriminating between equilibrium and turbulent contributions. We find that the density SOL width is impacted almost exclusively by the turbulent electron flux. In this simulation, the level of edge turbulence is regulated by a mechanism we are only beginning to understand: grad-B-drifts and ion X-point losses at the top and bottom of the machine, along with ion banana orbits at the low field side (LFS), result in a complex poloidal potential structure at the separatrix which is the cause of the E x B drift pattern thatmore » we observe. Turbulence is being suppressed by the shear flows that this potential generates. At the same time, turbulence, along with increased edge collisionality and electron inertia, can influence the shape of the potential structure by making the electrons non-adiabatic. Furthermore, being the only means through which the electrons can lose confinement, it needs to be in a balance with the original direct ion orbit losses to maintain charge neutrality.« less
Authors:
ORCiD logo [1] ; ORCiD logo [2] ;  [1] ;  [3] ;  [3] ; ORCiD logo [3] ; ORCiD logo [3]
  1. Univ. of Colorado, Boulder, CO (United States)
  2. Lodestar Research Corp., Boulder, CO (United States)
  3. Princeton Univ., Princeton, NJ (United States). Princeton Plasma Physics Lab.
Publication Date:
Report Number(s):
DOE-ER/54392-90; PPPL/SO15882C-1; LRC-18-175
Journal ID: ISSN 1070-664X
Grant/Contract Number:
FG02-97ER54392; FG02- 97ER54392; SO15882-C
Type:
Accepted Manuscript
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 25; Journal Issue: 7; Related Information: http://dx.doi.org/10.5281/zenodo.1230040; Journal ID: ISSN 1070-664X
Publisher:
American Institute of Physics (AIP)
Research Org:
Lodestar Research Corp., Boulder, CO (United States)
Sponsoring Org:
USDOE Office of Science (SC), Fusion Energy Sciences (FES) (SC-24)
Contributing Orgs:
Princeton Plasma Physics Laboratory
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; fusion; tokamak; scrape-off-layer; turbulence; gyro-kinetic; simulation
OSTI Identifier:
1466035
Alternate Identifier(s):
OSTI ID: 1461242

Keramidas Charidakos, I., Myra, J. R., Parker, S., Ku, S., Churchill, R. M., Hager, R., and Chang, C. S.. Analysis of equilibrium and turbulent fluxes across the separatrix in a gyrokinetic simulation. United States: N. p., Web. doi:10.1063/1.5037723.
Keramidas Charidakos, I., Myra, J. R., Parker, S., Ku, S., Churchill, R. M., Hager, R., & Chang, C. S.. Analysis of equilibrium and turbulent fluxes across the separatrix in a gyrokinetic simulation. United States. doi:10.1063/1.5037723.
Keramidas Charidakos, I., Myra, J. R., Parker, S., Ku, S., Churchill, R. M., Hager, R., and Chang, C. S.. 2018. "Analysis of equilibrium and turbulent fluxes across the separatrix in a gyrokinetic simulation". United States. doi:10.1063/1.5037723.
@article{osti_1466035,
title = {Analysis of equilibrium and turbulent fluxes across the separatrix in a gyrokinetic simulation},
author = {Keramidas Charidakos, I. and Myra, J. R. and Parker, S. and Ku, S. and Churchill, R. M. and Hager, R. and Chang, C. S.},
abstractNote = {The SOL width is a parameter of paramount importance in modern tokamaks as it controls the power density deposited at the divertor plates, critical for plasma-facing material survivability. An understanding of the parameters controlling it has consequently long been sought. Prior to Chang et al. (2017 NF 57 11), studies of the tokamak edge have been mostly confined to reduced fluid models and simplified geometries, leaving out important pieces of physics. Here, we analyze the results of a DIII-D simulation performed with the full-f gyrokinetic code XGC1 which includes both turbulence and neoclassical effects in realistic divertor geometry. More specifically, we calculate the particle and heat E x B fluxes along the separatrix, discriminating between equilibrium and turbulent contributions. We find that the density SOL width is impacted almost exclusively by the turbulent electron flux. In this simulation, the level of edge turbulence is regulated by a mechanism we are only beginning to understand: grad-B-drifts and ion X-point losses at the top and bottom of the machine, along with ion banana orbits at the low field side (LFS), result in a complex poloidal potential structure at the separatrix which is the cause of the E x B drift pattern that we observe. Turbulence is being suppressed by the shear flows that this potential generates. At the same time, turbulence, along with increased edge collisionality and electron inertia, can influence the shape of the potential structure by making the electrons non-adiabatic. Furthermore, being the only means through which the electrons can lose confinement, it needs to be in a balance with the original direct ion orbit losses to maintain charge neutrality.},
doi = {10.1063/1.5037723},
journal = {Physics of Plasmas},
number = 7,
volume = 25,
place = {United States},
year = {2018},
month = {7}
}