Description
The presence of 2/1 tearing modes, single or double, has been observed during the current ramp-up phase of JET-ILW hybrid pulses with hollow electron temperature profiles. These modes can lock and trigger the disruption mitigation valve. About 6% of hybrid pulses disrupted in this way during experimental campaigns performed with deuterium plasmas in preparation of deuterium-tritium JET campaigns. The analysis of these disruptive pulses showed that in some cases the MHD activity is characterized by two different saturated 2/1 magnetic islands, rotating with the same toroidal velocity, each centered on one of the two q = 2 resonant surfaces on the left and on the right of the position of the minimum of q (linked to the hollow temperature profile), with the more external island finally growing up to a large width. In other cases, when the electron temperature profile is deeply hollow, an impulsive reconnection of magnetic field lines is produced, as highlighted by sawtooth-like profile rearrangements. In all the analyzed pulses, a locked mode is observed, and the JET massive gas injection system is triggered to avoid unmitigated disruption.
Comparing pulses with and without MHD activity during the current ramp-up phase has allowed to highlight which conditions may increase the risk of developing tearing modes in the early phase of the hybrid scenario (fast current ramp-up, high current overshoot, low electron density, core impurity accumulation), thus providing a reference to describe a key piece of underlying physics crucial for the optimization of the q-profile for high performance hybrid plasmas on JET.