Elevated-qmin Steady-State Scenarios on DIII-D: New Controls and Stability Characterization

C.T. Holcomb, B.S. Victor, J.R. Ferron, T.C. Luce, R.J. LaHaye, E. Schuster, W.P. Wehner, F. Turco and W.M. Solomon,

Division of Plasma Physics (DPP) Annual Meeting of the American Physical Society (APS)

Savannah, GA, USA, November 16-20, 2015

Abstract

Fully non-inductive, high performance plasmas with qmin > 1.5 have been sustained on DIII-D at βN > 3.5. Since the formation and sustainment of such plasmas entails operation near multiple stability limits there is a sensitivity to variations in conditions that can lead to different results, i.e. tearing modes. The parameter range for stability at high-βN is not well known, and it is unclear if tearing modes are destabilized by neoclassical (i.e. missing bootstrap current) or classical (i.e. tearing too close to ideal-wall kink limit) βN limits. We present work to improve reproducibility and stability understanding using new controls. Electron cyclotron heating applied at breakdown improves repeatability. Current profile and βN feedback control are used to obtain equilibria that are assessed for stability and steady-state potential. We discuss the range of stable operation found using these tools and tests designed to identify tearing destabilization mechanisms.