Design and nonlinear validation of the ITER magnetic control system

Plasma magnetic control is an important aspect for the operation of a tokamak, especially when performance need to be pushed. Typically a model-based approach is used, starting from a linearized model, describing the electromagnetic interactions between the plasma and surrounding conductive structures. The design of the controller is carried out on the basis of this model, taking into account the requirements, that typically concern the maximum currents and voltages on the poloidal field coils, the maximum available power and the minimum tolerable distance between the plasma and the first wall. During a single experiment, the plasma goes through different phases and hence different controllers need to be designed. With reference to the ITER tokamak, this paper describes the procedure adopted to design a control system able to operate a whole plasma pulse, along with the switching strategies among the various controllers. These controllers are designed on the basis of the linearized models, but then the overall control system behavior is validated through nonlinear simulations, in challenging situations, such as the transition between different operational conditions, or in the presence of critical disturbances.