Energetic particle effect on neoclassical tearing mode stability in NSTX*

The impact of energetic particles on neoclassical tearing mode (NTM) stability is modeled and compared to experiments in the spherical tokamak NSTX. It is shown that energetic particles can be an important destabilizing mechanism for neoclassical tearing modes, in that they allow small magnetic islands to overcome the polarization current stabilization effect, and the magnetic island growth may be damped with the loss of energetic particles due to orbit stochasticization. These results are obtained using the energetic particle and magnetic island parameters determined self-consistently by TRANSP simulations augmented by the “kick model” for energetic particle transport by instabilities [Podestà et al., Plasma Phys. Control. Fusion 59 095008 (2017)]. Inclusion of energetic particle effect improves the agreement between measured and predicted island width time evolution and may provide new insight on neoclassical tearing mode onset and growth. A new model is being implemented and tested in TRANSP for analysis and prediction of NTM stability in time-dependent simulations.