Improved Confinement in JET Hybrid Discharges

Over the past 10 years a new scenario has emerged on the mid-size tokamaks ASDEX-Upgrade and DIII-D which combines a higher q95 operation with improved confinement compared to the H98,y2 scaling law called hybrid scenario or improved H-mode. Could this kind of scenario be reproduced on ITER, new possibilities would arise, i.e. high Q operation at reduced IP. Hybrid scenario plasmas can have significantly improved confinement above the standard H98,y2 H-mode scaling but the physics basis remains somewhat unclear. At JET similar discharges have been carried out before the 2008/9 campaigns and most of the characteristics i.e. the stability against Neoclassical Tearing Modes (NTM) have been reproduced but the confinement was not improved significantly over the H98,y2 scaling. Recent JET experiments with a low triangularity magnetic configuration at low density focused on modification of the q-profile. First, the central part of the q-profile has been modified by early NBI heating, early off-axis ICRH heating and different current ramp rates.
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