Stabilization of Neoclassical Tearing Mode by Electron Cyclotron Current Drive

控制MHD不稳定性是稳定维持血浆β水平的一项重要任务。特别是，新经典撕裂模(NTM)是一种限制最大β的临界不稳定性。电子回旋电流驱动(ECCD)由于其局部驱动无感电流而被认为是一种有吸引力的稳定NTM的方案。在本研究中，我们进行了稳定m=3/n=2和m=2/n=1模的NTM的实验和它们的模拟。这里，m和n分别是极向模数和环向模数。该实验的主要目的是稳定NTM，以维持高β等离子体。利用平板模型分析了磁岛周围的电流密度分布。当EC驱动电流在O点阶段进入磁岛时，EC驱动电流密度的峰值低于磁岛外的峰值。这是因为电流沿磁岛中的磁面平均。当磁岛中心与驱动电流分布的峰值重合时，O点处的电流密度与出生电流密度相同。这一结果表明，如果EC驱动电流分布的宽度与磁岛宽度相当，则应仔细调整EC驱动电流的位置。在m/n=2/1模式下，进行了降低EC注入功率的实验。实验结果表明，随着EC功率的降低，稳定化作用减弱。NTM完全稳定在0.3 mW，功率阈值在0.3 mW到0.5 mW之间。电流分布的宽度以及EC驱动的总电流也是重要的。对于基波O模和二次谐波X模，EC驱动电流密度与自举电流密度几乎相同，EC驱动电流位置不对准的稳定作用与EC模式无关。

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.