Locked modes and plasma rotation in JET plasmas

In magnetic fusion devices, locked modes can result from the slowing down of neoclassical tearing modes, a class of non ideal MHD instabilities that break and reconnect the magnetic field lines, or they can be induced by externally applied magnetic field perturbations. In both the cases, a magnetic island forms and heat and particle flux towards the walls occurs, frequently leading to a disruption. This event needs to be avoid in disruption adverse devices, such as ITER and DEMO. Over the years, the locked mode triggering mechanism has been studied in various experiments. In this thesis project, an analysis on locked mode dynamics in the Joint European Torus (JET) device is carried out, when external 3D fields have been applied by means of Error Field Correction Coils in a low rotation plasma regime (ITER relevant regime). Data have been collected in 2023, provided with rotation measurements performed both during and after the compass scan technique, a method for error field identification, while varying the plasma density. During the first part of the project, the role of plasma density in affecting the rotation profile is assessed and the rotation braking due to the locked mode onset is analyzed. In the second part, the investigation of the role of neutral beam injection in avoiding the locked mode destabilization is investigated and this result is complimented with momentum balance modelling.