Plasma current, shape, and position control in ITER

A linear model for feedback control of the plasma position and shape in the International Thermonuclear Experimental Reactor (ITER) is discussed. A model of the poloidal field (PF) system and of the disturbances is first derived. The main task of the control system is to avoid any contact of the hot plasma with the wall during the long duration of the burn phase. For this purpose, the control variables are specified as six gaps between the plasma separatrix and the first wall, including divertor channels. The structure model includes PF coils, vacuum vessel, first wall, backplate, and divertor fins, and it refers to the TAC-4 outline design ITER geometry. A multivariable controller is designed using the optimal linear quadratic approach. The simulation of the closed-loop system shows how the plasma shape is recovered: Step gap variations of 15 cm and poloidal beta drops of 0.2 are considered as disturbances. The performance parameters are voltages and currents in the PF coils and gap recovery time; voltage saturation of the actuators is also taken into account.