Abstract Details
Abstracts
Author: Christopher B Smiet
Requested Type: Pre-Selected Invited
Submitted: 2019-02-22 15:37:06
Co-authors: S. Hudson, G. Kramer
Contact Info:
Princeton Plasma Physics Laboratory
321 Witherspoon street
Princeton, New Jersey 08542
US
Abstract Text:
The topological changes that occur in a tokamak during a sawtooth oscillation are strongly constrained by the fact that the magnetic field constitutes a 2,5 degree of freedom Hamiltonian system. It's structure is exposed by repeatedly mapping a poloidal cross-section to itself along the magnetic field lines to produce a Poincar'e plot, where the topologically stable fixed points of this mapping correspond with x- and o-points. We perform 3d numerical simulations of a sawtoothing tokamak discharge and calculate the location and Greene's Residue of these fixed points. The field evolves through a series of bifurcations where fixed points are split or combined according to rules that conserve topological index and the cycle culminates in the annihilation of the magnetic axis with an x-point from the broken (1,1) rational surface. The subsequent topological changes after the crash are not caused by tearing of any intact resonant surface, but by the lowering safety factor shifting the field into renonance with present nonaxisymmetric flux. As a consequence the (1,1) rational surface need not be healed for a sawtooth cycle to occur. The developed topological framework explains the Kadomtsev-like process in the simulation but the topological constraints are applicable to any model for the sawtooth.
Comments:
