Fast-ion enhanced modeling of neoclassical tearing modes at NSTX and DIII-D

A new framework for the solution of modified Rutherford equation including fast ions is successfully applied to interpret the tearing mode stability of two discharges NSTX #134020 and DIII-D #135861. The simulated island width growth rates are in better agreement with the measured island growth rate when the fast ions are included. While constants are multiplied to the polarization current contribution terms for NSTX #134020, no constants are necessary to match the simulated and measured island width growth rates for DIII-D #135861. The estimated island frequencies appear to provide an explanation of the different constants used in the two discharges. The gradient scale lengths suggest that the fast ion contribution can become significant in plasmas with flat thermal ion density profile and steep fast ion density profile.