Identification of external kink modes in JET

The 'outer mode' is one of the MHD modes that limits the fusion performance of the hot ion H mode discharges in JET. It has previously been proposed that the outer mode is a nonlinearly saturated external kink mode. This was based on the localization of the perturbation close to the edge as observed in soft X ray (SXR), electron temperature and electron density measurements. In addition, MHD stability calculations showed. that the plasma edge is close to the ideal external kink stability boundary at the time when the outer mode is observed. The SXR data of the outer mode are compared with predictions based on the mode structure of the ideal n = 1 external kink mode. Excellent agreement is found, confirming the identification of the outer mode as an external kink mode.