Core MHD Studies in the Hybrid Regime of JET Operation
The Hybrid regime is an alternative to the ELMy H-mode with absent or very weak sawtooth activity and better prospects of MHD stability at high normalised kinetic pressure (bN). The main MHD activities observed in the hybrid regime are fishbones, Neoclassical Tearing Modes (NTM), internal kink modes/sawteeth and Alfvén eigenmodes. Plasmas with internal kink modes that saturate at low amplitude have good energy confinement and no impurity peaking. Also discharges with dominant fishbone activity feature good confinement, while the m/n = 3/2 NTM tends to seriously affect confinement. The 2/1 NTM, which restricts beta below the no-wall limit bN 4li in DIII-D hybrid plasmas, was observed at JET in a few cases only, in conjunction with profile modifications provoked by impurity injection or by ELM-free periods, while it was not observed in discharges with bN / 4li up to 1.2. The 2/1 NTM is dealt with. The focus of this paper is on the 3/2 NTM in JET hybrid discharges.