Rotation in the Plasma Flow Frame of Marginal Neoclassical Tearing Mode Islands in DIII-D and NSTX

Small island effects inhibit the pervasive occurrence of neoclassical tearing modes. $m/n=2/1$ or 3/1 islands are reduced (by decreasing beta and thus the destabilizing bootstrap current density) to the self-stabilization size (``marginal point") in DIII-D and NSTX [1]. Non-zero island propagation in the plasma flow frame can produce a stabilizing polarization current provided it occurs in the direction of (but does not exceed) the ion diamagnetic drift [2]. This ``polarization threshold" naturally scales as a characteristic island size of several times the ion banana width, which is consistent with experiment. Comparison of measurements in \mbox{DIII-D} and NSTX discharges of similar cross-section (but different aspect ratio) with polarization current theory will be presented.\par \vskip6pt \noindent [1] R.J. La~Haye, et al., Proc.\ 38th EPS Conf.\ on Plasma Phys., Strasbourg, France, 2011, P2.088.\par \noindent [2] K.~Imada and H.R.\ Wilson, Proc.\ 38th EPS Conf.\ on Plasma Phys., Strasbourg, France, 2011, O3.116.