Stabilization of Neoclassical Tearing Mode by Electron Cyclotron Current Drive

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It is an important task to control MHD instabilities for keeping plasma beta high steadily. In particular, neoclassical tearing mode (NTM) is a critical instability which limits the maximum beta. Electron cyclotron current drive (ECCD) is considered attractive scheme to stabilize the NTM since it locally drives non-inductive current. In this research, we have conducted experiments stabilizing NTM of m=3/n=2 and m=2/n=1 modes and their simulation. Here m and n is the poloidal and toroidal mode numbers, respectively. The main purpose of the experiment is to stabilize the NTM for the maintenance of high beta plasmas. The current density profile around magnetic island is analyzed by using a slab model. As the EC driven current enters into the magnetic island at the O-point phase, the peak value of EC driven current density is lower than that at the outside of magnetic island. This is because the current is averaged along the magnetic surface in the magnetic island. The current density at the O-point is found to be the same as the birth current density when the center of the magnetic island coincides with the peak of driven current profile. This result suggests that the position of EC driven current should be carefully adjusted if the width of EC driven current profile is comparable with the magnetic island width. The experiment on reducing the EC injection power for the m/n=2/1 mode is also conducted. The experimental results reveal that the stabilizing effect is weaker with the decrease of EC power. The NTM is fully stabilized up to 0.3 MW, and the power threshold is between 0.3 MW and 0.5 MW. The width of current profile is also found to be important as well as the total EC driven current. The EC driven current density is almost the same as the bootstrap current density for both the fundamental O-mode and the second harmonic X-mode, and the stabilizing effect of the misalignment of EC driven current position is independent of the EC mode.