Control of radiation-enhanced neoclassical tearing modes by electron cyclotron current drive in tokamak plasmas

The control of plasma-radiation-enhanced neoclassical tearing modes (NTMs) by electron cyclotron current drive (ECCD) in tokamak plasmas is numerically investigated based on a set of reduced magnetohydrodynamics equations. It is found that the plasma radiation can effectively enhance the instability of NTMs through accelerating the growth of the magnetic island and make the NTMs difficult to be controlled by ECCD, in comparison with the case without plasma radiation. The plasma radiation can reduce the pressure and increase the perturbed bootstrap current, leading to the increase of the magnetic island width and growth rate. During the ECCD control, the evolution of magnetic island width experiences a resurging growth after the inclusion of plasma radiation, leading to the occurrence of phase reversal of the magnetic island. The radiation-enhanced effect can be more evident under large bootstrap current than that under low bootstrap current, making the island more susceptible to the resurging growth during the ECCD control. Unexpectedly, small radial misalignment of ECCD during NTMs control display an enhanced suppressing effect, which is analyzed in detail. Numerical results show that the nonlinear control of radiation-enhanced NTMs poses significant challenges in comparison with the cases without plasma radiation. And the enhancement effect via plasma radiation exhibits a progressively trend over temporal evolution. Consequently, preemptive activation of ECCD systems coupled with elevated input power levels is recommended to effectively suppress plasma-radiation-enhanced NTMs in experimental configurations.