Effectiveness of Electron Cyclotron Current Drive Modulation for m/n=3/2 Neoclassical Tearing Mode Suppression in KSTAR

The effectiveness of electron cyclotron current drive (ECCD) modulation is predicted for neoclassical tearing mode (NTM) suppression in Korea Superconducting Tokamak Advanced Research (KSTAR). ECCD effectiveness is calculated by improving the original current drive model proposed by Perkins. The model evaluates the current drive effectiveness by integrating the dimensionless volume-averaged Gaussian current density over the island helical flux. As improvements to the model, the influences of positional misalignment and phase difference on the current drive effectiveness are newly added. The calculated current drive effectiveness is applied to the modified Rutherford equation constructed for the KSTAR plasma and the m/n = 3/2 NTM stability is analyzed for the 170 GHz X2-mode ECCD and the 110 GHz O1-mode ECCD under a toroidal magnetic field of 3.5 T. The stability analysis verifies that the narrow current density profile of the 170 GHz X2-mode ECCD diminishes the need for ECCD modulation to reduce the minimum ECCD power required for complete stabilization of the mode. However, modulation can reduce the minimum power required for the 110 GHz O1-mode ECCD, which deposits a larger peak current density with a broader profile width. The narrowing ECCD profile is clearly shown to be more critical than increasing the peak current density to reduce the minimum required power and choosing the optimal current drive strategy.