Locked mode dynamics prior to disruptionsin high performance JET-ILW plasmas

This contribution presents a characterization of the locked mode (LM) dynamics prior to plasma termination phase of high performance JET plasmas, i.e. baseline and hybrid scenarios. In these scenarios, which are under investigation in JET in preparation to the next DT phase, the LM appears first as a rotating tearing mode that eventually locks due to the presence of un-intended 3D magnetic fields, i.e. error fields (EFs). The tearing mode is triggered when the plasma performance is already degraded, mainly during the plasma ramp-up phase of hybrid discharges and during the plasma ramp-down phase of baseline discharges. In both cases, the plasma discharge is terminated by the activation of the Disruption Mitigation Valve (DMV) system. The mode triggering and the subsequent mode-locking mechanisms have been investigated by combing experimental results from multiple diagnostics and RFXlocking code modelling [1]. A method for avoiding disruptions induced by LM is presented based on the actual EF LM spin-up that was already observed during 2006 EF correction experiments.