PPPL-4795
Bootstrap Current for the Edge Pedestal Plasma in a Diverted Tokamak Geometry
Authors: S. Koh, C.S. Chang, S. Ku, J.E. Menard, H. Weitzner and W. Choe
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 driftkinetic
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.
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Submitted to: Physics of Plasmas (March 2012)
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