Neoclassical tearing mode seeding by Alfvén eigenmode coupling in MAST-U

We present evidence of tearing mode seeding by nonlinear three-wave coupling of Alfvén eigenmodes in a tokamak plasma. Here, a m,n = 2,1 magnetic island appears to be seeded in a MAST-U discharge by coupling of (Formula presented) (Formula presented) and (Formula presented) (Formula presented) Alfvén eigenmodes, which is demonstrated by increased squared normalised bicoherence calculated from magnetic probes preceding tearing mode onset. This observation offers a new explanation for tearing modes that would be otherwise addressed as spontaneous, and indicates three-wave coupling can seed 2,1 magnetic islands in plasmas without pre-existing (Formula presented) (Formula presented) tearing modes. Further, this mechanism has the potential to be more prevalent in burning plasmas expected in future fusion devices.