Physics of Unlocked Tearing Modes and Disruption Avoidance by Feedback-based Electromagnetic Torque Injection

Recent DIII-D experiments show that electromagnetic torque injection with non-axi-symmetric coils has the potential to sustain an H-mode edge and high core confinement, even in the presence of a finite size island. As reported in the last IAEA conference [1], we demonstrated a promising scheme to avoid tearing mode locking and possible disruptions by utilizing feedback-controlled electromagnetic torque injection. Here, we explore the physics of the unlocked tearing mode state. Remarkably, in some cases the feedback control sustains the plasma with core and edge pressure gradients similar to those before the onset of the tearing mode, although with slightly reduced confinement. Although the maximum value of the rotation profile is an order of magnitude smaller than before the mode onset and near zero at rational surfaces, a significant rotation shear at the rational surfaces may help to limit the island size. Preliminary resistive MHD analysis with MARS-K[2,3] suggests neoclassical toroidal viscosity torque plays an important role for formation of flow shear. An advantage of this approach is that it can produce the flow shear at integer surfaces by resonance, together with other torque formations. Another possible contributing factor to the favorable performance may be the automatic compensation of error fields (“dynamic error field correction”), as the consequence reducing the stable kink response, by the same feedback system that controls the tearing mode.