Synthesis and simulation of a two-level magnetic control system for tokamak-reactor plasma

Abstract

Synthesis and simulation of a hierarchical (two-level) magnetic system for controlling a tokamakreactor plasma throughout the entire divertor discharge stage, including the plasma current ramp-up phase, are carried out. The plasma vertical velocity is stabilized about zero by using a proportional controller in a scalar control loop. The gain of the controller—the coefficient that ensures the required stability margins—is found by using a second-order linear model constructed by solving the identification problem on the basis of numerical experiments carried out with the DINA plasmophysical computer code. The internal cascade (the lower level of the system) for tracking the scenario currents in the poloidal magnetic field coils is synthesized by using the complete dynamic channel decoupling method. The external cascade (the upper level of the system) for tracking the plasma current and shape is synthesized by using the method of pseudoseparation of the control channels and the multidimensional diagonal proportional-integral controller, with proportional, integrating, and double integrating units connected in parallel in each channel. In the hierarchical control system, the lower level (the internal cascade) is subordinated to the upper level (the external cascade). The external cascade acts on the internal one by the signals that set the required currents in the coils of the central solenoid and of the poloidal magnetic field in order to ensure the required plasma current and shape in accordance with the output signals from the plant, which are transmitted through the vector feedback channel. The lower level is aimed exclusively at tracking the reference inputs by tracking the currents in the control coils. An operating mode of the system under the conditions of current saturation in the control coils is proposed and implemented. Results are presented from numerical simulations of the two-level (cascade) control system for reference scenario no. 2 of the ITER database (www.iter.org) with the DINA nonlinear code.