ECCD power necessary for the neoclassical tearing mode stabilization in ITER

Abstract

The stabilization of the neoclassical tearing mode (NTM) by an electron cyclotron current drive (ECCD) has been numerically studied for the parameters of ITER in order to evaluate the necessary EC power for the stabilization of NTMs in ITER. The time evolution of an island width of NTM is calculated by the modified Rutherford equation. The modification of the background current density profile by the EC current located on the rational surface is calculated by the current diffusion equation and the variation of the tearing stability index Δ′ is taken into account. When the EC power is higher than a threshold value, NTMs with any island width can be fully stabilized. The dependence of the threshold power on the parameters in the modified Rutherford equation is examined. The threshold power depends significantly on the parameters of the bootstrap current term in the modified Rutherford equation. The injected EC current decreases the value of Δ′ through the background current modification, which results in the reduction of the threshold power. The effects of the peaked nature of the EC current density profile and the EC power modulation on the threshold power are investigated. When the width of the EC current density profile becomes half, the threshold power is reduced to half or less. The EC power modulation is found to be inessential for the threshold power reduction if the EC current density profile can be peaked. As regards the maximum value of the threshold power in the range of parameters, an EC power of about 25 MW is found to be sufficient for the simultaneous stabilization of both the m/n = 3/2 and 2/1 NTMs in ITER.