Nonlinear Burn Control and Optimal Actuator Allocation of ITER Plasmas with Uncertain Parameters and Actuator Dynamics

V. Graber, E. Schuster

11th ITER International School on “ITER Plasma Scenarios and Control”

San Diego, California, USA, July 25-29, 2022

In ITER, achieving controlled fusion, where the burning plasma regime is sustained in steady state, will demonstrate the feasibility of commercial fusion power. To drive deuterium-tritium plasmas to high-fusion, steady-state regimes, burn controllers use external heating and fueling sources to regulate its temperature and density. The following actuators can supply the external heating and fueling necessary for burn control in ITER: the ion cyclotron system, the electron cyclotron system, two neutral beam injectors, the pellet injection system, and the gas fueling system. This suite of actuators must be properly managed so that their output precisely matches the burn control requests for external ion heating, electron heating, deuterium fueling, and tritium fueling. Actuator allocators can achieve an optimal mapping between the actuators' outputs and the burn control requests despite the constraints of the actuators. In this work, a model-based, nonlinear burn controller and an optimal actuator allocator, which work in tandem to regulate the plasma, are presented. A model-based approach to burn control design is attractive because it directly incorporates the nonlinear, coupled dynamics of the burning plasma system into the design. The actuator allocator is designed to handle the time delays that will be experienced when using ITER's actuators. For example, the injection of pellets requires a long flight time through guide tubes, and the injected neutral beam particles take an appreciable amount of time to thermalize (slow down) in the plasma. Both the burn controller and actuator allocator employ adaptive estimation techniques to overcome the existence of uncertainties (i.e., not precisely known quantities). These uncertainties include constant plasma parameters that characterize complex phenomena such as the particle recycling caused by plasma-wall interactions. Furthermore, the actuator allocator's adaptive estimation is also designed to handle the following time-varying, state-dependent parameters: the fueling efficiency of the pellet injection, the thermalization delay of the neutral beam particles, and the uneven distribution of the neutral beam heating between the plasma ion and electron populations. The capability of the nonlinear burn controller and the optimal actuator allocator is evaluated in a simulation study.

*Supported by the US DOE under DE-SC0010661.