The behaviour of the (2, 1) tearing mode in lower hybrid current drive (LHCD) discharges in ASDEX is examined. Due to the change in the current profile j(r), this mode can be destabilized during LHCD. This often leads to mode locking and a disruption, thereby limiting the amount of LH power that can be transferred to the plasma during current drive. A statistical analysis for two different current drive spectra shows that this power limit is dependent on both plasma current Ip and line-averaged electron density nc. The authors discuss the possibilities of disruption control and show how tailoring of the LHCD spectra might avoid the occurrence of the (2, 1) mode.
Profile control experiments in JET using off-axis lower hybrid current drive