Studies on the suppression mechanism of equilibrium magnetic islands in CFQS low-β plasmas

Magnetic islands produced in toroidal magnetic confinement plasma has a three-dimensional helical structure because of the rotational transform, especially the equilibrium magnetic surface of the stellarator is three-dimensional helical structure. Thus, the formation and instability of the magnetic island of the Stellarator is a typical issue of the three-dimensional physics and is also one of the key topics of the physics research of the Stellarator. Magnetic islands and related tearing mode physics are major issues in stellarator. The non-inductively current drive, i.e. electron cyclotron current drive (ECCD) can be used as one of the approaches to adjust the rotational transform, and hence, affecting the generation of magnetic islands. In this study, we have applied additional toroidal magnetic field to generate m/n=5/2 magnetic islands in the low β operation on the Chinese First Quasi-axisymmetric Stellarator (CFQS) so that the influence of the bootstrap current is negligible. Then, we have investigated the suppression mechanism of magnetic islands in low β plasmas using the HINT code. It is found that in case of the constant current, when current direction is positive, with increasing current the width of islands increases. When the direction of current is reversed, the island is suppressed with the amplitude of the current >6kA. The main reason is that the rotational transform is away from iota=0.4 rational surface and the m/n=5/2 magnetic island does not meet the resonance conditions. In case of local current profile, the magnetic island width decreases as a result of the enhanced magnetic shear at iota=0.4 rational surface. Moreover, effects of the direction and the amplitude of the current on the suppression of magnetic islands have been discussed in more detail.