Stability of Elevated-qmin Steady-Sate Scenarios on DIII-D

C.T. Holcomb, B. Victor, J.R. Ferron and T.C. Luce and E. Schuster

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

San Jose, CA, USA, October 31 - November 4, 2016

Abstract

Limits to high performance steady-state operation with qmin>1.4 and βN ≤ 3.5 are identified and explained. Various βN and q-profile histories were produced while testing feedback control of these profiles. Ten pulses had no core MHD at βN =3.4-3.5, with qmin =1.4-1.8, and q95 =5-5.8. These have predicted ideal-wall kink βN limits between 4 and 5. One pulse had an n=1 tearing mode (TM) at βN =3.5 with no clear trigger. Five pulses developed n=1 TMs when βN=2, qmin=2, and q95=4.7. Stability calculations for these latter cases will be shown. In seven other shots, additional NBI power from sources with more normal injection was used, and these had off-axis fishbone (OAFB) modes at βN =3.5. These sources produce more large-radius trapped ions whose precession can drive OAFB. Preliminary analysis suggests a threshold power or voltage exists. In some cases OAFB appear to trigger n=1 TMs. These studies seek to clarify the operational limits of a steady-state scenario for next step devices.