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(EXP3/05) Polarization Current and Neoclassical Tearing Mode Threshold in Tokamaks: Comparison of Experiment with Theory

R. J. La Haye1), R. J. Buttery2), S. Günter3), G. T. A. Huysmans4), M. Maraschek3), F. Waelbroeck5), and H. R. Wilson2)
 
1) General Atomics, San Diego, California USA
2) Euratom/UKAEA Fusion Association, Culham Science Center, Abingdon, United Kingdom
3) Max Planck Institute fur Plasmaphysik, Garching, Federal Republic of Germany
4) JET Joint Undertaking, Abingdon, United Kingdom (now at CEA, Cadarache, France)
5) Institute for Fusion Studies, Austin, Texas USA

Abstract.  Neoclassical tearing mode islands are one of the main causes of reduced performance at high $ \beta_{\theta}^{}$ in standard ELMy sawtoothing H-mode. The leading candidate for the threshold is the helical polarization/inertial current which arises from mode propagation at frequency $ \omega$ in the Er = 0 guiding center frame of plasma flow. A threshold island width wpol is predicted, which is proportional to the ion banana width $ \epsilon^{1/2}_{}$$ \rho_{\theta\mathrm{i}}^{}$ and also depends on $ \omega$. The polarization current is predicted to be stabilizing only for 0 $ \leq$ $ \omega$ $ \leq$ $ \omega_{\mathrm{i}*}^{}$, the ion diamagnetic drift frequency, and yields a minimum $ \beta_{\theta}^{}$ (below which the helically perturbed bootstrap current is too small to excite NTMs) that gives critical $ \beta_{\mathrm{N}}^{}$ scaling linearly with $ \rho_{\mathrm{i}*}^{}$. A database compiled from the tokamaks ASDEX Upgrade (AUG), DIII-D and JET shows such a $ \beta_{\mathrm{Ncrit}}^{}$ $ \propto$ $ \rho_{\mathrm{i}*}^{}$ is indeed observed for the m/n=3/2 NTM induced by a sawtooth crash. Typically, unstable seed island widths that grow are observed to be of the order wpol. Detailed measurements of mode propagation in the Er = 0 frame are also consistent with a polarization current threshold.

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IAEA 2001