Significant progress has been made on the DIII-D tokamak in the capability to control key plasma features and using such control to expand the operational limits of stationary and steady-state tokamak operation. Recent experiments have demonstrated the capability to suppress the key plasma instabilities of concern for ITER, including edge localized modes, neoclassical tearing modes, and resistive wall modes. In addition, the ability to regulate the rotation and current density profiles through feedback control has been demonstrated. The use of these control techniques has allowed an expansion of the envelope of viable, stationary tokamak operation, highlighted by the demonstration of sustained (~2 s) operation of !N ~_ 4 (50% above the no-wall stability limit) as well as fully noninductive operation with ! ~_ 3.5%. This development is supported by a vigorous basic physics program, which has provided new insights into turbulence dynamics over a large range in spatial scales, new measurements of the structure of fast-ion instabilities and their effect on the fast ion population, and important information on the transport of carbon and associated tritium co-deposition on plasma facing surfaces.

External kink modes have been identified as one of the major obstacles to achieving high pressure plasmas in toroidal devices. From the beginning of fusion research, it has been well known that a conducting shell can improve the stability if the shell is ideal. A shell with finite resistivity can still stabilize the fast growing ideal magnetohydrodynamic (MHD) mode, however, the external kink mode is converted into the resistive wall mode (RWM) branch, which grows with the shell skin time constant. On the DIII-D device, two schemes have been explored to stabilize the slowly-growing resistive wall mode: one using magnetic feedback and the other utilizing rapid plasma rotation. Recent RWM experiments have revealed that these two schemes are strongly coupled in a synergetic manner and that the stabilization operation functions as a unified scheme. It was found that the magnetic feedback operation can track the residual error field which excites the stable resistive wall mode near the marginally stable condition (Error Field Amplification). The magnetic field applied through the feedback process can reduce the mode amplitude, compensate the error field, and simultaneously maintain the plasma rotation. As a consequence of the higher plasma rotation, a stable path opens up to the ideal-wall limit. Discharges with beta up to twice the no-wall beta limit have been achieved. Experimental observations and stability calculations indicate that these discharges are at the ideal wall kink limit.