Plasma Control in a Tokamak-Type Machine using only Magnetic Measurements

Abstract

The "clean" nuclear energy of tomorrow will probably be produced using Tokamak-type machines, despite the technological difficulties. Thermonuclear fusion based on the Tokamak process is more promising than that based on other types of fusion machines such as the "Stellarator" process or even inertial fusion, a field reserved for the military. For cost-effective energy production, with the Tokamak process, and discharge times of several tens of minutes, followed by downtime, the machine's windings must be made using superconductors. Finally, plasma control must be simple, reliable, and efficient in order to optimize energy exchanges between the plasma and the outside world. For this reason, we propose in this paper to show that the control of the plasma equilibrium and that of the plasma current amplitude are quite feasible, in a simple and efficient way using only magnetic measurements installed inside the vacuum chamber, mainly absolute flux measurements. For machines with or without iron core or with or without magnetic « divertor », the flux deviations, calculated on the presumed plasma contour, are then fed back to the voltages of the poloidal field coils alone through a high-performance control loop. Based on this approach, neither coupling with numerical simulation data nor any knowledge of the plasma profile is necessary in real time. We will show that there is a particular closure matrix of the plasma position control loop that allows high loop gains, decoupled from the vacuum chamber and independent on the presence of a magnetic core. Finally, for machines with "divertor", the control of the plasma and that of the zero-field points become practically independent thanks to a preprograming voltage applied to the poloidal field coils, thus promoting control of free zero-field points.