Abstract Details
Abstracts
Author: Ben Zhu
Requested Type: Pre-Selected Invited
Submitted: 2017-03-16 15:41:57
Co-authors: M.Francisquez, B.N.Rogers
Contact Info:
Dartmouth College
6127 Wilder Laboratory
Hanover, NH 03755
USA
Abstract Text:
The Kelvin-Helmholtz instability (KHI) is believed to play an important role at the tokamak edge — in the poloidal direction, it is potentially excited by the strong edge flows in the H-mode pedestal, while in the radial direction, it breaks filaments of gradient-driven instability, generates zonal flows and saturates turbulence. We study the KHI and its (well known) susceptibility to stabilization by a component of the magnetic field along the flow using a reduced, electromagnetic 2D fluid model. Our key finding is that there is a substantial parameter regime in which the mode is linearly unstable, but the nonlinear evolution exhibits only benign Alfvenic oscillatory behavior - that is, the mode is linearly unstable, but nonlinearly stable or meta stable. The oscillations of the nonlinearly metastable mode become progressively larger and more chaotic as the strength of the magnetic field along the flow is made weaker, driving the system further away from the marginal linear stability boundary. In some limits the free energy of the linear instability is harmlessly transferred to stable modes, while in some cases the energy cascades to small scale flows and magnetic fields with reconnection eventually taking place. We describe a coupled three mode analysis that, in agreement with the simulations, shows the system remains dynamically stable with Alfvenic oscillations near the marginal linear stability boundary while the behavior becomes progressively more chaotic and eventually turbulent as the magnetic field parallel to the flow is weakened further below marginal stability.
Comments: