April 15-17

Abstract Details

files Add files

Abstracts

Author: Stephen C. Jardin
Requested Type: Pre-Selected Invited
Submitted: 2019-02-21 07:18:17

Co-authors: I. Krebs, N. Ferraro

Contact Info:
Princeton Plasma Physics Laboratory
P.O. Box 451
Princeton, NJ   08543
United States

Abstract Text:
The ubiquitous sawtooth phenomena in tokamaks are so-named because the central temperature rises slowly and falls rapidly, similar to the blades of a saw. First discovered in 1974 [1], it has so far eluded a theoretical explanation that is widely accepted and consistent with experimental observations. We propose here a new explanation for sawtooth phenomena in auxiliary heated tokamaks that is motivated by our recent understanding of flux pumping in tokamaks [2, 3]. In this theory, the role of the m=1 mode is to generate a central dynamo voltage which regulates the central safety factor, q0, to be very near but slightly above unity with very low central shear. As the temperature and density profiles peak, they abruptly become unstable to centrally localized non-resonant ideal MHD modes with poloidal and toroidal mode numbers (m,n) with m=n > 1. It is these higher order modes interacting with each other that cause the sudden crash of the temperature profile, due to rapid E x B convection, not magnetic reconnection. Long time 3D MHD simulations of multiple cycles using M3D-C1 demonstrate this phenomenon, which appears to be consistent with many experimental observations: that q0 changes very little during the crash and is near 1.0 [4]; that the crash can be very abrupt and fast, occurring on an ideal MHD time scale [5]; that rapid impurity penetration can occur during the crash implying strong convection [5], and this also possibly offers an explanations of how (1,1) impurity snakes can survive many sawtooth oscillations. Important elements of these simulations are that they use high toroidal mode number resolution, and that they use inductive current drive (not purely RF).
[1] S. von Goeler, et al. Phys. Rev. Lett. 33, 1201 (1974)
[2] S. Jardin, et al. Phys. Rev. Lett. 21 21501 (2015)
[3] I. Krebs, et al. Phys. Plasmas 24 102511 (2017)
[4] Y. Nam, et al. Nucl. Fusion 58 066009 (2018)
[5] J. A. Wesson, et al. Phys. Rev. Lett. 79 5018 (1997)

Comments: