Simulation of ECCD optimization for neoclassical tearing mode suppression in KSTAR

In high confinement discharges of KSTAR, onset of neoclassical tearing modes (NTMs) by helically perturbed local bootstrap current will be inevitable and significantly degrade plasma confinement. For the suppression of NTMs in KSTAR, high power, long pulse 170 GHz electron cyclotron current drive (ECCD) system will be implemented to compensate the missing bootstrap current. A ray tracing analysis using TORAY-GA is performed to optimize the location of EC wave launch and to predict the performance of the current drive. From the optimization simulation, it is found that the launch location should be at − 30 cm below the midplane to maximize the profile figure of merit which enables more efficient NTM suppression in itself. The profile figure of merits on q = 3 / 2 and 2/1 resonant surfaces are estimated to be about 13.5 MA/rho2 and 14.0 MA/rho2, respectively, in the case of optimal profiles with 1 MW ECCD unit power. The 110 GHz ECCD as an alternative method with higher current drive efficiency shows larger driven currents than 170 GHz ECCD, but the profile figure of merit is not much improved due to its broader profile.