Real-Time Profile Control for Advanced Tokamak Operation on JET
Real-time simultaneous control of several radially distributed magnetic and kinetic plasma parameters (such as the safety factor, q(x), and gyro-normalized temperature gradient, rTe*(x), respectively) is being investigated on JET, in view of developing integrated control of advanced tokamak scenarios and internal transport barriers suitable for ITER. This paper describes the new model-based optimal profile controller which has been tested during the last experimental campaign. The controller aims to use the combination of heating and current drive systems - and optionally the Poloidal Field (PF) system - in an optimal way to regulate the evolution of several parameters. In the first part of the paper, a technique for the experimental identification of a dynamic plasma model is described, taking into account the physical structure and couplings of the transport equations, but making no quantitative assumptions on the transport coefficients or on their dependences. To cope with the high dimensionality of the state space and the large ratio between the various time scales involved, the model identification procedure and controller design both make use of the theory of singularly perturbed systems by means of a multiple-time-scale approximation. The second part of the paper deals with the control theory and algorithm. Conventional optimal control is recovered in the limiting case where the ratio of the plasma confinement time to the resistive diffusion time vanishes. Closed-loop simulations of the new controller have been performed in preparation for experiments, and some results are shown. Finally, in the last part of the paper, first experimental results of current profile control obtained during the last 2007 JET campaign are presented and discussed.