Optimal strategy for RF stabilization of NTMs in ITER

Recent findings concerning the effect of the test blanket modules on NTM locking in ITER, as well as recent projections for the broadening of the RF deposition profile due to edge density fluctuations, call into question the planned strategy of RF stabilization of NTMs while the islands are rotating. We find that, given these recent projections, it becomes advantageous to opt instead for a locked mode stabilization strategy, where the mode is made to lock at small width in front of the RF wave launcher and can then be stabilized efficiently. This strategy is made possible by the recent recognition that the H-mode is preserved for a momentum confinement time after locking, and that RF stabilization on a shorter time scale than this preserves the H-mode. Our findings emerge from a model for the temporal evolution of the magnetic island width and rotation frequency that includes the effects of the bootstrap, RF-driven and polarisation currents, the viscous drag and the effects of both the error field and the resistive wall on the island's growth and rotation. These effects all play an important role in the evolution of magnetic islands, making their inclusion critical to evaluate and compare the locked mode and rotating mode stabilisation strategies. This alternative strategy requires no changes to ITER's design, and would allow for more robust stabilization of NTMs with lower peak and averaged power requirements. These would directly translate to increased fusion gain, more availability of RF power for other needs, and lower disruptivity.