ICRF heating experiments in TFR using a low-field-side antenna

Abstract

The paper describes the JET-TFR antenna designed for launching ICRF waves in TFR from the low magnetic field side of the torus, and the main experimental observations on plasma-wave interaction. Compared with a first design of a similar antenna, this antenna design includes new features, such as carbon lateral protections and a thick Faraday shield similar to the one which will be mounted on the JET antennas. Experiment shows that such a shield does not limit the coupling efficiency of the magnetosonic wave in the plasma, the loading resistance being comparable with the one predicted by theory assuming an ideal Faraday shield. With this system, the power coupled reached 600 kW. In the minority regime and for a central electron density around 1014cm−3, the ion central heating efficiency reaches 0.6–0.7 eV·kW−1 for an RF power up to 350 kW and 0.5 eV·kW−1 at 600 kW. As expected for a low-field-side antenna, no appreciable heating is observed in the mode conversion regime. Spectroscopic measurements show that, in all regimes, the RF pulse introduces a significant amount of both high-Z and low-Z impurities; at the end of a 300 kW/100 ms RF pulse, the total power radiated by the metallic impurities reaches 125–200 kW. The mechanisms responsible for such a generation of metal in the plasma remain unclear, but it can be said that a direct plasma-wave interaction in the scrape-off layer of the tokamak discharge is probably a determining factor.