Onset of Tearing Modes in JET Advanced Scenarios
One main limitation to the performance of high-bN tokamak regimes arises from the development of tearing modes that short-circuit radial transport by forming magnetic islands. The observation of such modes in linearly stable plasmas led to the concept of Neoclassical Tearing Modes (NTM), which are driven by the perturbed bootstrap current if an external event produces a seed island overcoming a metastability threshold. Magnetic islands that form in consequence of the development of long-lasting kink modes are considered in this paper. Kink modes that grow on long time-scales (typically 0.2s from birth to saturation) have been systematically studied at JET since they constitute the main obstacle to the prolongation of high bN phases at high confinement in the steady-state advanced regime. The mode topology transforms from kink to tearing after a period that will be referred to as island latency. Attention will be focused on a case with long latency period, which is particularly challenging in view of the comparison with the rapid seed island formation that is observed at large sawtooth crashes.