First-Principles Model-based Closed-loop Control of the Current Profile Dynamic Evolution on DIII-D
IAEA Fusion Energy Conference
San Diego, California, USA, October 8-13, 2012
| 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.