Extremum seeking applied to the plasma control system of the Frascati Tokamak Upgrade

In this paper we will report on the experimental results arising from the implementation of extremum seeking techniques to maximize in real time the RF power coupled to the plasma in the Frascati Tokamak Upgrade (FTU) experimental facility. The RF under consideration is the Lower Hybrid (LH) and the coupling issue consists in the match between an antenna structure (an array of waveguide, the grill) and a load i.e. the plasma. It can be shown that the reflection coefficient (ratio of the reflected and direct power) is a convex function of the density in front of the grill and then of the plasma-grill distance. The optimization of the coupling level reduces the reflected power thus maximizing the RF effects on the plasma and at the same time avoiding temporary power shutdown to protect the system from the excess of reflection. It is shown in the paper that the proposed extremum seeking technique achieves a satisfactory level of robustness with respect to measurement errors and well performs both in simulation and in experimental tests, thus leading to an improved effectiveness of the RF heating system.