Controlling extremely shaped plasmas in the JET tokamak

This paper deals with the design of a new plasma current and shape controller for the JET tokamak. This new controller is aimed at improving the performance of the present controller so as to allow the control of extremely shaped plasmas with higher values of elongation and triangularity. The design approach makes use of a linearized mathematical model linking the voltages applied to the active coils with a number of geometrical descriptors characterizing the plasma shape. To describe accurately the plasma shape a number of geometrical descriptors, greater than the number of available active circuits, is needed. Hence the control problem under investigation is characterized by the fact that the number of parameters to be controlled is larger than the number of control inputs. To overcome this problem a singular value analysis is carried out to identify the principal directions of the algebraic mapping between coil currents and geometrical descriptors. These principal directions are then assumed as controller outputs so as the original multivariable (rectangular) control problem is transformed in a standard square problem. Moreover the singular value decomposition approach, makes it possible to solve this modified problem by means of a number of SISO problems. Each of this SISO control problem has been solved designing a suitable PID controller.