Tight regulation of the burn condition in ITER has been proven possible in simulations by the use of robustification techniques even under emulated time-dependent variations in the fuel concentration. One of the most fundamental control problems arising in ITER and future burning-plasma tokamaks is the regulation of the plasma temperature and density to produce a determined amount of fusion power while avoiding possible thermal instabilities. Such problem, known as burn control, requires the development of controllers that integrate all the available actuators. Moreover, the complex burning-plasma dynamics and the uncertain nature of some of its components suggest that nonlinear, robust, burn controllers are necessary. For instance, the deuterium–tritium concentration in the fueling lines may vary over time and the estimation of such variation during operation may be difficult or not even possible. Available actuators for the regulation of the burn condition are auxiliary power modulation, fueling rate modulation, and impurity injection. Also, recent experiments in the DIII-D tokamak have shown that in-vessel coil-current modulation can be used for burn control purposes. The in-vessel coils generate non-axisymmetric magnetic fields that have the capability to decrease the plasma-energy confinement time, which allows for regulation of the plasma energy during positive energy perturbations. In this work, all these actuators are used to design a nonlinear burn controller which is robust to unknown variations in the deuterium–tritium concentration of the fueling lines. Furthermore, fueling rate modulation is not only used to control the plasma density, but also to control the plasma energy, if necessary, by means of isotopic fuel tailoring. The model-based nonlinear controller is synthesized from a zero-dimensional model of the burning-plasma dynamics. A nonlinear simulation study is carried out to illustrate the successful controller performance in ITER-like scenarios in which unknown variations of the deuterium–tritium concentration of the fueling lines are emulated.
MHD control in burning plasmas