Maximization of the lower hybrid power coupling in the Frascati Tokamak Upgrade via extremum seeking

This paper reports on the experimental results arising from the implementation of extremum seeking techniques to maximize in real time the radio-frequency (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 waveguides, the grill) and a load, i.e. the plasma. 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. Since it is unviable to inject a dither signal in the plasma control system, the implemented extremum seeking relies on existing disturbances in the experimental system, thus following a somewhat novel paradigm. First, simulation results are given based on a model of the closed-loop and then the experimental tests which confirmed the effectiveness of the proposed approach are shown.