Neoclassical Tearing Mode Seeding by Nonlinear Three-Wave Interactions in Tokamaks

We report seed magnetic island formation by nonlinear three-wave coupling [1]. In this experiment a 2,1 seed island forms due to 4,3 and 3,2 island interaction with a 1,1 sawtooth precursor in the DIII-D ITER baseline scenario. The 2,1 island grows linearly as expected by neoclassical theory, and terminates the discharges. STRIDE shows these plasmas are classically stable and a number of ELMs and sawtooth crashes occur without seeding. Therefore, the 2,1 island is not formed by a classical current driven instability, but an NTM seed island is formed by frequency matching and nonlinearly interacting islands that satisfy the mode number resonance condition. These results are general and relevant for future reactors, as 70% of the analyzed DIII-D ITER baseline scenario plasmas are characterized by frequency matching of resonant tearing modes at the time of 2,1 seeding. As 2,1 seeds are produced in classically stable plasmas, tearing free operation may not be possible by maintaining a classically stable current profile in future reactors. This predicts new challenges for the development of stable plasma scenarios, calling for high differential rotation at q=2, active control and avoidance of high m,n islands as much as possible.