Sherwood 2015

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approvedabstract.pdf2015-01-16 14:11:24Linda Sugiyama

Role of mode coupling in instabilities of a toroidal plasma*

Author: Linda E. Sugiyama
Requested Type: Consider for Invited
Submitted: 2015-01-16 11:32:27


Contact Info:
M.I.T., Laboratory for Nuclear Science
Rm. 26-557, 77 Massachusetts A
Cambridge , MA   02139-4

Abstract Text:
In magnetically confined fusion plasmas,
predicting the mode numbers of plasma edge instabilities
such as ELMs and inter-ELM modes is an
important unsolved theoretical problem that is closely related to edge stability.
MHD linear eigenmodes typically have a spectrum of toroidal harmonics
with slowly varying growth rates, where the most unstable mode numbers are often
much larger than those observed in experiment.
In contrast, MHD nonlinear numerical simulations, started from the
full range of toroidal harmonics, find moderate mode numbers and spatial
structures similar to experiment.
Several types of mode coupling exist:
toroidal coupling due to R-R_o = r cos(theta),
nonlinear f*g coupling of toroidal harmonics n''= n'+n,
radial overlap of modes with different m,n due to the rapidly varying safety factor
q near the edge, and near-Hamiltonian stochasticity of the perturbed magnetic field
in the presence of an X-point on the plasma separatrix.
Study of an interior mode, the 1/1 internal kink, finds that compressibility
is important. Particular JxB terms that are higher order in inverse aspect ratio
change the linear growth rate scaling and allow a fast, non-resistive
sawtooth crash driven by growing nonlinearity, even at small r/R.
These terms are also important for edge instabilities,
modified somewhat by the higher overall mode numbers and larger r/R.
They most strongly affect the lowest harmonics n=1,2, etc.,
that are created by nonlinear mode beating of the dominant intermediate-n harmonics
and often grow to comparably large amplitudes.
The effects are demonstrated for large ELMs and a smaller KSTAR inter-ELM mode
that saturates without a crash, using the M3D code.
Reduced MHD lacks these terms and sees only partial effects,
including a slow sawtooth crash.

*Work supported by the U.S. D.O.E. OFES and SciDAC programs
and the National Research Foundation of Korea.


March 16-18, 2015
The Courant Institute, New York University