| Predictive
Capability of MHD Stability Limits in Tokamak Discharges
Development of a fully predictive capability for the MHD
stability limits in high performance tokamak discharges is
critical to the success of the Advanced Tokamak program. The
theory and experimental diagnostic capabilities have now been
developed to the point where detailed predictions can be productively
tested so that competing effects can be isolated and either
eliminated or confirmed. Turnbull et al. have tested discharge
equilibrium reconstruction simulations against observations
for the principal limiting phenomena in the DIII-D experiment:
L-mode negative central shear (NCS) disruptions, H-mode NCS
edge instabilities, and tearing and resistive wall modes (RWMs)
in long pulse discharges.
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Figure
2 Predicted (a) radial ECE profile
and (b) poloidal Mirnov signal for the RWM in DIII-D discharge
96519 compared to measured diagnostic signals. The ECE
profile in (a) shows two times with different toroidal
phase.
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In the case of predominantly ideal MHD instabilities, agreement
between the predictions and experimentally observed stability
limits and thresholds can now be obtained to within several
percent, and the predicted fluctuations and growth rates to
within the estimated experimental errors. Edge instabilities
can be explained by a new model for edge localized modes as
predominantly ideal low to intermediate n modes. Accurate
ideal calculations are critical to demonstrating RWM stabilization
by plasma rotation, and the ideal eigenfunctions provide a
good representation of the RWM structure when the rotation
slows (Figure 2). Ideal eigenfunctions can then be used to
predict stabilization using active feedback.
For non-ideal modes, the agreement is approaching levels
similar to those for the ideal comparisons;  '
calculations, for example, indicate that some discharges
are linearly unstable to classical tearing modes, consistent
with the observed growth of islands in those discharges.
INVESTIGATORS
A. D. Turnbull, M. S. Chu, L. L. Lao, J. R. Ferron, P. B.
Snyder, D. A. Humphreys, R. J. La Haye, T. C. Luce, E. J.
Strait, and T. S. Taylor, General Atomics; D. Brennan, Oak
Ridge Institute for Science Education; A. M. Garofalo and
J. Bialek, Columbia University; I. N. Bogatu, D. H. Edgell,
and J. S. Kim, FARTECH; J. D. Callen and K. Comer, University
of Wisconsin, Madison; M. S. Chance and M. Okabayashi, Princeton
Plasma Physics Laboratory; S. A. Galkin, University of California,
San Diego; B. W. Rice, Lawrence Livermore National Laboratory;
H. R. Wilson, Culham Laboratory, UK Atomic Energy Authority.
PUBLICATION
A. D. Turnbull, D. Brennan, M. S. Chu, L. L. Lao, J. R. Ferron,
A. M. Garofalo, P. B. Snyder, J. Bialek, I. N. Bogatu, J.
D. Callen, M. S. Chance, K. Comer, D. H. Edgell, S. A. Galkin,
D. A. Humphreys, J. S. Kim, R. J. La Haye, T. C. Luce, M.
Okabayashi, B. W. Rice, E. J. Strait, T. S. Taylor, and H.
R. Wilson, “Predictive capability of MHD stability limits
in high performance DIII–D discharges,” Nucl.
Fusion (in press).
URL
http://web.gat.com/theory/
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