Receding-horizon optimal control of the current profile evolution during the ramp-up phase of a tokamak discharge

The control of the toroidal current density spatial profile in tokamak plasmas will be absolutely critical in future commercial-grade reactors to enable high fusion gain, non-inductive sustainment of the plasma current for steady-state operation, and magnetohydrodynamic (MHD) instability-free performance. The evolution in time of the current profile is related to the evolution of the poloidal magnetic flux, which is modeled in normalized cylindrical coordinates using a partial differential equation (PDE) usually referred to as the magnetic flux diffusion equation. The control objective during the ramp-up phase is to drive an arbitrary initial profile to approximately match, in a short time windows during the early flattop phase, a predefined target profile that will be maintained during the subsequent phases of the discharge. Thus, such a matching problem can be treated as an optimal control problem for a PDE system. A distinctive characteristic of the current profile control problem in tokamaks is that it admits interior, boundary and diffusivity actuation. A receding-horizon control scheme is proposed in this work to exploit this unique characteristic and to solve the associated open-loop finite-time optimal control problem using different optimization techniques. The efficiency of the proposed scheme is shown in simulations.

Research Highlights
► A simplified dynamic model describing the evolution of the poloidal flux, and therefore the ı profile, during the inductive phase of the discharge has been introduced.
► Using this model, a closed-loop, multi-parameter, receding-horizon, optimal controller has been proposed to match a desired ı profile within a predefined time window during the flattop phase of the tokamak discharge.
► The proposed controller satisfactorily rejects system disturbances due to its feedback nature.