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Title: Bootstrap current for the edge pedestal plasma in a diverted tokamak geometry

Abstract

The edge bootstrap current plays a critical role in the equilibrium and stability of the steep edge pedestal plasma. The pedestal plasma has an unconventional and difficult neoclassical property, as compared with the core plasma. It has a narrow passing particle region in velocity space that can be easily modified or destroyed by Coulomb collisions. At the same time, the edge pedestal plasma has steep pressure and electrostatic potential gradients whose scale-lengths are comparable with the ion banana width, and includes a magnetic separatrix surface, across which the topological properties of the magnetic field and particle orbits change abruptly. A drift-kinetic particle code XGC0, equipped with a mass-momentum-energy conserving collision operator, is used to study the edge bootstrap current in a realistic diverted magnetic field geometry with a self-consistent radial electric field. When the edge electrons are in the weakly collisional banana regime, surprisingly, the present kinetic simulation confirms that the existing analytic expressions [represented by O. Sauter et al., Phys. Plasmas 6, 2834 (1999)] are still valid in this unconventional region, except in a thin radial layer in contact with the magnetic separatrix. The agreement arises from the dominance of the electron contribution to the bootstrap current compared withmore » ion contribution and from a reasonable separation of the trapped-passing dynamics without a strong collisional mixing. However, when the pedestal electrons are in plateau-collisional regime, there is significant deviation of numerical results from the existing analytic formulas, mainly due to large effective collisionality of the passing and the boundary layer trapped particles in edge region. In a conventional aspect ratio tokamak, the edge bootstrap current from kinetic simulation can be significantly less than that from the Sauter formula if the electron collisionality is high. On the other hand, when the aspect ratio is close to unity, the collisional edge bootstrap current can be significantly greater than that from the Sauter formula. Rapid toroidal rotation of the magnetic field lines at the high field side of a tight aspect-ratio tokamak is believed to be the cause of the different behavior. A new analytic fitting formula, as a simple modification to the Sauter formula, is obtained to bring the analytic expression to a better agreement with the edge kinetic simulation results.« less

Authors:
;  [1]; ; ;  [2];  [3]
  1. Korea Advanced Institute of Science and Technology, Department of Physics, Daejeon 305-701 (Korea, Republic of)
  2. Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543 (United States)
  3. Courant Institute of Mathematical Sciences, New York University, New York, New York 10012 (United States)
Publication Date:
OSTI Identifier:
22072568
Resource Type:
Journal Article
Journal Name:
Physics of Plasmas
Additional Journal Information:
Journal Volume: 19; Journal Issue: 7; Other Information: (c) 2012 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 1070-664X
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; ASPECT RATIO; BANANA REGIME; BOOTSTRAP CURRENT; BOUNDARY LAYERS; COLLISIONS; ELECTRIC FIELDS; ELECTRONS; ELECTROSTATICS; IONS; MAGNETIC FIELD CONFIGURATIONS; MAGNETIC SURFACES; PARTICLES; PLASMA; PLASMA SIMULATION; TOKAMAK DEVICES; TOPOLOGY

Citation Formats

Koh, S, Choe, W, Chang, C S, Ku, S, Menard, J E, and Weitzner, H. Bootstrap current for the edge pedestal plasma in a diverted tokamak geometry. United States: N. p., 2012. Web. doi:10.1063/1.4736953.
Koh, S, Choe, W, Chang, C S, Ku, S, Menard, J E, & Weitzner, H. Bootstrap current for the edge pedestal plasma in a diverted tokamak geometry. United States. doi:10.1063/1.4736953.
Koh, S, Choe, W, Chang, C S, Ku, S, Menard, J E, and Weitzner, H. Sun . "Bootstrap current for the edge pedestal plasma in a diverted tokamak geometry". United States. doi:10.1063/1.4736953.
@article{osti_22072568,
title = {Bootstrap current for the edge pedestal plasma in a diverted tokamak geometry},
author = {Koh, S and Choe, W and Chang, C S and Ku, S and Menard, J E and Weitzner, H},
abstractNote = {The edge bootstrap current plays a critical role in the equilibrium and stability of the steep edge pedestal plasma. The pedestal plasma has an unconventional and difficult neoclassical property, as compared with the core plasma. It has a narrow passing particle region in velocity space that can be easily modified or destroyed by Coulomb collisions. At the same time, the edge pedestal plasma has steep pressure and electrostatic potential gradients whose scale-lengths are comparable with the ion banana width, and includes a magnetic separatrix surface, across which the topological properties of the magnetic field and particle orbits change abruptly. A drift-kinetic particle code XGC0, equipped with a mass-momentum-energy conserving collision operator, is used to study the edge bootstrap current in a realistic diverted magnetic field geometry with a self-consistent radial electric field. When the edge electrons are in the weakly collisional banana regime, surprisingly, the present kinetic simulation confirms that the existing analytic expressions [represented by O. Sauter et al., Phys. Plasmas 6, 2834 (1999)] are still valid in this unconventional region, except in a thin radial layer in contact with the magnetic separatrix. The agreement arises from the dominance of the electron contribution to the bootstrap current compared with ion contribution and from a reasonable separation of the trapped-passing dynamics without a strong collisional mixing. However, when the pedestal electrons are in plateau-collisional regime, there is significant deviation of numerical results from the existing analytic formulas, mainly due to large effective collisionality of the passing and the boundary layer trapped particles in edge region. In a conventional aspect ratio tokamak, the edge bootstrap current from kinetic simulation can be significantly less than that from the Sauter formula if the electron collisionality is high. On the other hand, when the aspect ratio is close to unity, the collisional edge bootstrap current can be significantly greater than that from the Sauter formula. Rapid toroidal rotation of the magnetic field lines at the high field side of a tight aspect-ratio tokamak is believed to be the cause of the different behavior. A new analytic fitting formula, as a simple modification to the Sauter formula, is obtained to bring the analytic expression to a better agreement with the edge kinetic simulation results.},
doi = {10.1063/1.4736953},
journal = {Physics of Plasmas},
issn = {1070-664X},
number = 7,
volume = 19,
place = {United States},
year = {2012},
month = {7}
}