The Stabilizing Effect of Flow Shear on m/n=3/2 Magnetic Island Width in DIII-D

It is found that flow shear has a stabilizing effect on $m/n=3/2$ neoclassical tearing mode (NTM) islands through a more negative classical tearing stability index $\Delta^\prime$ in the DIII-D [1] tokamak. The heating neutral beams are mixed between co- and counter-directions to vary the torque on the plasma and thus the plasma flow (rotation) and flow shear. This is done ``shot to shot" in the presence of a ``saturated" $m/n=3/2$ NTM while slowly raising the plasma. A heuristic model for the stabilizing effect of flow shear on $\Delta^\prime$ is shown to explain how flow shear acts to reduce NTM island size and obviate the effect of higher beta and concomitant destabilizing helically perturbed bootstrap current.\par \vskip6pt \noindent [1] J.L. Luxon, Nucl.\ Fusion {\bf 42}, 614 (2002).