Operational issues at high lower hybrid power density in JET: waveguide conditioning and arc detection

The power handling capability of the JET Lower Hybrid Current Drive (LHCD) system is examined using the long-term database. The limitations, in particular in H-mode plasmas, are discussed and the performance compared with other LHCD experiments using multijunctions as power dividers. Although the power density of 25 MW m−2 has been exceeded in L-mode and almost obtained in ELMy H-mode (on 1/6th of the antenna), it is concluded that the RF conditioning performed on JET does not allow us to exceed an electric field of ∼5.5 kV cm−1 which is generally not sufficient under the rather weak coupling conditions of the JET H-mode. Modelling of an arc occurring in a waveguide indicates that rather small variations of the reflected wave (amplitude and phase) may occur, rendering arc detection based on RF measurements difficult in some cases. The JET bolometry diagnostic with four chords viewing the antenna front is found to be an efficient tool to detect an arc. In L-mode plasmas, a very good correlation between the amplitude of the bolometry signals and the iron spectroscopic lines is found. In H-mode the arc detection is clearly more difficult with enhanced radiation during the ELM but is still possible when the bolometry signals are properly processed.