Improved Reproducibility of Plasma Discharges via Physics-model-based q-profile Feedback Control in DIII-D
E. Schuster, W. Wehner, J. Barton, M. Boyer,T. Luce, J. Ferron, C. Holcomb, M. Walker, D. Humphreys, W. Solomon, B. Penaflor and R. Johnson
26th IAEA Fusion Energy Conference
Kyoto, Japan, October 17-22, 2016
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Abstract
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Recent experiments on DIII-D demonstrate the potential of
physics-model-based q-profile control to improve reproducibility of
plasma discharges. A combined feedforward + feedback control scheme
is employed to optimize the current ramp-up phase by consistently
achieving target q profiles (Target 1: qmin = 1.3, q95 = 4.4; Target 2:
qmin = 1.65, q95 = 5.0; Target 3: qmin = 2.1, q95 = 6.2) at prescribed
times during the plasma formation phase (Target 1: t = 1.5 s;
Target 2: t = 1.3 s; Target 3: t = 1.0 s). At the core of the control
scheme is a nonlinear, first-principles-driven, physics-based,
control-oriented model of the plasma dynamics valid for low
confinement (L-mode) scenarios. To prevent undesired L-H transitions,
a constraint on the maximum allowable total auxiliary power is
imposed in addition to the maximum powers for the individual heating
and current-drive sources. Experimental results are presented to
demonstrate the effectiveness of the combined feedforward+feedback
control scheme to consistently achieve the desired target profiles at
the predefined times. These results also show how the addition of
feedback control significantly improves upon the feedforward-only
control solution by reducing the matching error and also how the
feedback controller is able to reduce the matching error as the
constraint on the maximum allowable total auxiliary power is relaxed
while keeping the plasma in L-mode.
