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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. Wilson^2)
 
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 E_r = 0 guiding center frame of plasma flow. A threshold island width w_pol 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 w_pol. Detailed measurements of mode propagation in the E_r = 0 frame are also consistent with a polarization current threshold.

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