Role of fast ions in spontaneous NTM instabilities in NSTX

Neoclassical tearing modes (NTMs) are important because they can affect confinement in tokamaks with significant bootstrap current. An existence of a "seed" island is necessary for NTMs to outgrow the stabilizing effects of parallel transport and ion polarization current. Spontaneous NTMs are observed, however, that seemingly grow without an explicit seed island. We report evidence of fast ions playing a decisive role in the early growth of spontaneous NTMs, although their effect does not directly explain the formation of a seed island. The modified Rutherford equation including the nonlinear classical tearing mode index, bootstrap current drive, curvature stabilization, and the polarization current term is used to successfully interpret NSTX discharges. Here, the polarization current term is augmented by the fast ion driven "kinetic neoclassical tearing mode" term, which becomes significant when the ion poloidal Larmor radius is comparable to the magnetic island width, as is the case in NSTX. This term is essential for the simulated island width to match the measured island width. The input variables for the model, including those of fast ions, are provided self-consistently using TRANSP. Additional simulations using RDCON and M3D-C1-K reveal that in addition to classical tearing modes, fast ion pressure driven ideal modes may be a viable source of an implicit seed island for spontaneous NTMs in NSTX.