Control of the Local Gradient and the Minimum Value of the Safety Factor Profile by Using Moving ECCD
S. T. Paruchuri, A. Pajares, T. Rafiq, E. Schuster
Division of Plasma Physics (DPP) Annual Meeting of the American Physical Society (APS)
Pittsburgh, PA, USA (Remote), November 8-12, 2021
Active control of the safety factor is essential for preventing
magnetohydrodynamic instabilities and achieving the needed level of
performance (steady-state operation, high pressure, etc.) in advanced
modes of operation in tokamaks. The total number of actuators, such as
neutral beam injectors and electron cyclotron current drives (ECCDs),
and their power saturation limits define the controller's capability
to track a given target safety factor. The ECCD deposition location has
in general been assumed fixed by safety-factor controllers. A model-based
safety-factor controller is proposed in this work by exploiting the
capability of changing the ECCD position in real time by mirror steering
with the ultimate goal of increasing the tokamak's actuation capabilities.
A dynamical model that includes the effect of the varying ECCD position
on the plasma dynamics is first developed. Then, controllers considering
the ECCD position as a controllable input are designed to tackle two
very challenging control problems associated with the safety factor,
namely the regulation of its minimum value at a time-evolving location
and the regulation of its local gradient around a predefined q value
with a time-evolving location. Finally, the performances of these
controllers are assessed in closed-loop simulations using the Control
Oriented Transport SIMulator (COTSIM) for a DIII-D scenario.
*Supported by the US DOE under DE-SC0010661 and DE-SC0010537.