Design and implementation of a model-based hierarchical architecture for plasma shape control in the TCV tokamak

Plasma shape control is a fundamental tool for effective tokamak operation, allowing improved plasma performance and the pursuit of challenging scientific objectives. In this framework, the Tokamak á Configuration Variable (TCV) of the École Polytechnique Fédérale de Lausanne (EPFL), with its large set of independent poloidal field (PF) coils, provides one of the best possible testbeds for such a control system. This article describes the design, implementation, and experimental validation of a novel shape control system for TCV. In the proposed design, the existing magnetic control architecture of TCV, which takes care of vertically stabilizing the plasma and controlling its position, current, and the scenario currents flowing in the PF coils, is left unmodified. The proposed shape controller acts on the reference signals fed to this control loop, leveraging it to fulfill the primary magnetic control objectives while driving the plasma shape toward the desired reference. This design choice results in greater compatibility between the existing magnetic controllers and the new shape controller, minimizing the effort required to deploy the proposed controller over the wide range of accessible TCV plasma shapes. The design and tuning procedures of this novel shape controller using linearized free-boundary magnetic control simulations have been integrated with the shot preparation tools at the Swiss Plasma Center of the EPFL, with the aim of providing the machine operators with a flexible and robust tool that can be easily tuned and validated in simulation in-between shots. The effectiveness of the proposed approach is demonstrated through experiments performed on a variety of plasma configurations.