A new shape controller for extremely shaped plasmas in JET

This paper deals with the design of a new plasma current and shape controller for the Joint European Torus (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. 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 into a square problem. Moreover, the singular value decomposition approach makes it possible to solve this modified problem by means of a number of SISO problems.