First-Principles Model-based Closed-loop Control of the Current Profile Dynamic Evolution on DIII-D
J. Barton, M.D. Boyer, W. Shi, W.P. Wehner, E. Schuster, T.C. Luce, J.R. Ferron, M.L. Walker, D.A. Humphreys, B.G. Penaflor and R.D. Johnson
IAEA Fusion Energy Conference
San Diego, California, USA, October 8-13, 2012
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Abstract
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Recent experiments performed at DIII-D represent the first successful
application of first-principles-driven, model-based, closed-loop
magnetic profile control in a tokamak. For ITER to be capable of
operating in advanced tokamak operating regimes, characterized by a
high fusion gain, good plasma confinement, magnetohydrodynamic stability,
and a noninductively driven plasma current, for extended periods of time,
several challenging plasma control problems still need to be solved.
For instance, setting up a suitable toroidal current density profile,
which will require active closed-loop profile control, is key for one
possible advanced operating scenario characterized by noninductive
sustainment of the plasma current and steady-state operation. The current
profile evolution in tokamaks is related to the poloidal magnetic flux
profile evolution, which is modeled in normalized cylindrical coordinates
using a nonlinear partial di↵erential equation referred to as the
magnetic diffusion equation. For control design purposes, the magnetic
diffusion equation has been combined with empirical correlations
obtained from physical observations and experimental data from DIII-D
for the electron temperature, plasma resistivity, and noninductive
current drive to develop a control-oriented model of the system valid
for low confinement mode discharges. This first-principles-driven model
was used to synthesize a combined feedforward + feedback control scheme
to drive the current profile to a desired target profile. Static and
dynamic plasma response models were integrated into the design of the
feedback controllers, and as a result, the model-based controllers know
in which direction to actuate the system to generate a desired response
in the profile evolution thanks to the embedded physics. Therefore, the
need for trial-and-error tuning of the control scheme is eliminated,
which is desirable for application on ITER where discharges are at a
premium. A general framework for real-time feedforward + feedback
control of magnetic and kinetic plasma profiles was implemented in the
DIII-D Plasma Control System, and experimental results are presented to
demonstrate the ability of the first-principles-driven, model-based
feedback controllers to control the current profile evolution. These
results demonstrate the applicability of this control scheme to active,
closed-loop current profile control in advanced operating scenarios on
ITER.
