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Total-f gyrokinetic study of bootstrap current in edge pedestal and a novel analytic formula

Author: Robert Hager
Requested Type: Consider for Invited
Submitted: 2015-01-19 18:00:10

Co-authors: C.-S. Chang

Contact Info:
Princeton Plasma Physics Laboratory
100 Stellarator Road
Princeton, New Jersey   08540
USA

Abstract Text:
We utilized the global, gyrokinetic neoclassical total-f particle code XGCa to establish better understanding of the physics of the bootstrap current in the H-mode edge pedestal and to develop a novel analytical formula that provides accurate prediction. Such a formula is urgently needed because most existing formulas are based on approximations that are easily violated in the edge pedestal.
To work out the limits of such local neoclassical approaches, we also used the local neoclassical code NEO [Belli et al., PPCF 54, 015015 (2012)].
In the first step, we performed a cross verification of XGCa, XGC0, and NEO in the local regime, which helped to identify and fix an inaccuracy in XGC0’s collision operator. XGC0 is a neoclassical guiding center code that was used by Koh et al. [Phys. Plasmas 19, 072505 (2012)]. This inaccuracy was found to be responsible for the enhanced bootstrap current in the collisional NSTX pedestal reported by Koh et al. With this correction XGCa, XGC0, and NEO show excellent agreement in the local regime, where NEO’s perturbative formalism is valid. Differences to the Sauter formula [Phys. Plasmas 6, 2834 (1999)] are found even in the local regime for electron collisionality corresponding to the plateau and collisional regimes. When the pedestal width becomes comparable to the ion orbit width, bootstrap currents obtained with XGCa are usually smaller than the currents obtained with NEO.
These deviations of the Sauter model and NEO from XGCa are found to be due to the collisional suppression of the passing particle contribution to the bootstrap current and the non-local effect of the equilibrium radial electric field.
The new understanding of edge pedestal physics and the results of numerous XGCa simulations using geometries of various present-day tokamaks have been used to develop an improved bootstrap current formula which reproduces XGCa results much better that the Sauter formula, and which is much more accurate than the Koh et al. formula.

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