Effect of fast electron transport on neoclassical tearing mode stabilization by electron cyclotron current drive

Recent studies have shown that the stabilization of neoclassical tearing modes (NTMs) by electron cyclotron current drive (ECCD) in ITER may be impeded when the injected electron cyclotron wave beam is broadened by plasma density fluctuations. This paper starts with the analysis of NTM stabilization by ECCD using the modified Rutherford equation (MRE). It is shown that with a wide wave deposition, the most effective approach for mode stabilization is to apply modulated ECCD early during the mode growth when the magnetic island is small, as expected. Numerical simulations based on reduced magnetohydrodynamic equations have been further carried out, and the results show the same trend as those obtained from the MRE. However, the perpendicular transport of fast electrons, taken into account in numerical simulations, is found to significantly degrade the mode stabilization by modulated ECCD. The modulated driven current required for mode stabilization is proportional to the square root of the perpendicular diffusivity of fast electrons and is increased by several times when the diffusivity reaches the anomalous transport level due to plasma turbulence. If the radial misalignment of the driven current from the resonant surface reaches similar to 4% of the plasma minor radius, the mode stabilization can be degraded to a great extent, depending on the wave deposition width and the perpendicular diffusivity of fast electrons.