Complete Suppression of the M/N=2/1 Neoclassical Tearing Mode Using Radially Localized Electron Cyclotron Current Drive on Diii-D and the Requirements for Iter Page: 4 of 8
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COMPLETE SUPPRESSION OF THE M/N = 2/1 NEOCLASSICAL TEARING
MODE USING RADIALLY LOCALIZED ELECTRON CYCLOTRON DRIVE
N DIII-D AND THE REQUIREMENTS FOR ITERR.J. La Haye, et al.
Complete Suppression of the m/n = 2/1 Neoclassical Tearing Mode
Using Radially Localized Electron Cyclotron Current Drive on
DIII-D and the Requirements for ITER
R.J. La Haye, T.C. Luce, C.C. Petty, D.A. Humphreys, A.W. Hyatt, F.W. Perkins,1
R. Prater, E.J. Strait, and M.R. Wade2
General Atomics, P.O. Box 85608, San Diego, California 92186-5608, USA
1Princeton Plasma Physics Laboratory, Princeton, New Jersey, USA
2Oak Ridge National Laboratory, Oak Ridge, Tennessee USA
Introduction: DIII-D experiments demonstrate the first real-time feedback control of the
relative location of a narrow beam of microwaves to completely suppress and eliminate a
growing tearing mode at the q=2 surface. Long wavelength tearing modes such as the m/n = 2/1
instability are particularly deleterious to tokamak operation. Confinement is seriously degraded
by the island, plasma rotation can cease (mode-lock) and disruption can occur. The neoclassical
tearing mode (NTM) becomes unstable due to the presence of a helically-perturbed bootstrap
current and can be stabilized by replacing the "missing" bootstrap current in the island 0-point
by precisely located co-electron cyclotron current drive (ECCD). The geometry for the ECCD
launch, the second harmonic resonance 2fc and the q=2 surface are shown in Fig. 1. The
optimum position is found when the DIII-D plasma control system (PCS) is put into a "search
and suppress" mode that makes small radial shifts (in about 1 cm steps) in the ECCD location
based on minimizing the Mirnov amplitude.
1. "Hybrid Scenario" Stationary High-Performance Discharges
The suppression of tearing modes in DIII-D was carried out in high performance, long-
pulse discharges with normalized beta [p(%)/Ip(MA)/a(m)BT(T)] PN = 2.7, normalized
confinement time (to that of the L-mode scaling) H89p = 2.6, q95 = 4.3 and fraction of plasma
current non-inductively driven by the bootstrap effect fbs = 0.35 [1]. These discharges are
referred to as a hybrid scenario as they have sufficient bootstrap current to significantlyreduce the volt-sec consumption but not enough for full
non-inductive steady state. The key element is the
relaxation of the current profile to a stationary state with
safety factor on axis qmin > 1, without sawteeth and
fishbones. Under the influence of a small m/n = 3/2
tearing mode, the current relaxes to a stationary state
maintained for up to 40 energy confinement times TE
and >2 resistive diffusion times TR. The confinement
remains well above that of standard H-mode (H89p
2.6 > 2) despite the approximate 2 5% reduction in TE
due to the 3/2 tearing mode. Real-time feedback control
of the energy content is done by regulation of neutral
beam (NBI) power using the diamagnetic flux, and
control of the particle inventory is performed by gas
fueling and active cryopumping based on the CO2
interferometer. The highest performance in ON and
PNH89P is found to be limited by the m/n=2/1 tearing
mode. Requesting a higher diamagnetic flux (and thus
beta) such that the NBI power is increased causes an2tce
ECCD
LaunchFig. 1. Cross-section of the plasma
equilibrium, the ECCD launch of rays
towards the 2fx resonance and the q=2
surface. The rffrequency is 110 GHz.
General Atomics Report GA-A24336 1
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La Haye, R. J.; Luce, T. C.; Petty, C. C.; Humphreys, D. A.; Hyatt, A. W.; Perkins, F. W. et al. Complete Suppression of the M/N=2/1 Neoclassical Tearing Mode Using Radially Localized Electron Cyclotron Current Drive on Diii-D and the Requirements for Iter, article, June 1, 2003; United States. (https://digital.library.unt.edu/ark:/67531/metadc779672/m1/4/: accessed July 15, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.