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Phase-Locking of Multi-Helicity Neoclassical Tearing Modes in Tokamak Plasmas

Author: Richard Fitzpatrick
Requested Type: Consider for Invited
Submitted: 2015-01-15 15:03:38

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Contact Info:
University of Texas at Austin
Robert Lee Moore Hall
Austin, TX   78712
United States

Abstract Text:
Conventional high-beta tokamak plasmas are characterized by a single, relatively benign, neoclassical tearing mode (NTM)-typically, the m=3, n=2 mode. The
so-called hybrid scenario combines comparatively high q_95 operation with
improved confinement compared with the conventional H_98,y2 scaling law.
If this kind of scenario could be reproduced on ITER then it would enable high-Q
operation at reduced plasma current. Somewhat unusually, hybrid discharges
tend to exhibit simultaneous NTMs with different mode numbers. For example,
2,1 and 3,2 NTMs have been observed simultaneously in both DIII-D and JET
hybrid discharges. In all cases, the different modes are
eventually observed to phase-lock to one another, giving rise to a significant
flattening, or even a reversal, of the core toroidal plasma rotation profile.
This behavior is highly undesirable because the loss of core plasma rotation is
known to have a deleterious effect on plasma stability (because it facilitates
locked mode formation).
We present a simple cylindrical model of the phase-locking of two or more
NTMs with different poloidal and toroidal mode numbers in a tokamak plasma.
Such locking takes place via a combination of nonlinear three-wave coupling
and conventional toroidal coupling: e.g., 2,1+2,1=4,2 and 3,2+1,0=4,2,
where the 1,0 perturbation corresponds to the Shafranov shift of the
equilibrium flux-surfaces.
In accordance with experimental observations, the model predicts that there
is a bifurcation to a phase-locked state when the frequency mismatch is
reduced to half of its original value. Furthermore, the phase-locked state is characterized by the permanent alignment of the X-points of the NTM
island chains on the outboard mid-plane, and a modified toroidal angular
velocity profile interior to the outermost coupled rational surface that is
such that the core plasma rotation is flattened or even inverted.

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