MHD stability of optimized shear discharges in JET

The main limitation to the performance of JET optimized shear (OS) discharg es is due to MHD instabilities, mostly in the form of a disruptive limit. T he structure of the MHD mode observed as a precursor to the disruption, as measured from SXR and ECE diagnostics, shows a global ideal MHD mode. The m easured mode structure is in good agreement with the calculated mode struct ure of the pressure driven kink mode. The disruptions occur at relatively l ow normalized beta (1 < beta(N) < 2), in good agreement with calculated ide al MHD stability limits for the n = 1 pressure driven kink mode. These low limits are mainly due to the extreme peaking factor of the pressure profile s. Other MHD instabilities observed in JET OS discharges include, usually b enign, chirping modes. These modes, which occur in bursts during which the frequency changes, have the same mode structure as the disruption precursor but are driven unstable by fast particles.