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
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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.