Stability Limits in High Performance, Negative Central Shear Discharges

Exploration of negative central shear equilibria in DIII-D has yielded discharges that transiently achieve $\beta_N\simeq 4$. The discharges exhibit broad current density profiles, leading to a significant separation in the no- and with-wall ideal kink stability limits predicted by MHD theory. As the no-wall limit is approached and exceeded in experiments, performance is often limited by n=1 resistive wall mode (RWM) instabilities that lead to abrupt collapses of the plasma stored energy. In addition, instabilities with n=1 rotating tearing precursors are observed when minimum $q$ value drops below 2. Theoretical calculations predict that magnetic feedback control using the in-vessel coils (internal coils) can provide RWM stabilization to $\beta_N$ values approaching the n=1 ideal-wall limit. In experiments, applying I-coil control indeed facilitates access to increased $\beta_N$ values above the no-wall limit.