Suppression mechanism of equilibrium magnetic islands in CFQS low-<inline-formula><tex-math id="Z-20231023161017">\begin{document}$\boldsymbol \beta$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20230546_Z-20231023161017.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="21-20230546_Z-20231023161017.png"/></alternatives></inline-formula> plasma

Abstract

Magnetic island 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 adjusting the rotational transform, and hence, affecting the generation of magnetic islands. In this study, we use an additional toroidal magnetic field to generate <i>m</i>/<i>n</i> = 5/2 magnetic islands in the low-<i>β</i> operation on the Chinese First Quasi-axisymmetric Stellarator (CFQS) so that the influence of the bootstrap current is negligible. Then, we investigate the suppression mechanism of magnetic islands in low-<i>β</i> plasma by using the HINT code. It is found that in the case of the constant current, when the current direction is positive, with the increase of current, the width of island increases. When the direction of current is reversed, the island is suppressed when the current is larger than 6 kA. The main reason is that the rotational transform is away from <i>ι</i>/2π = 0.4 rational surface and the <i>m</i>/<i>n</i>=5/2 magnetic island does not meet the resonance conditions. In the case of local current profile, the magnetic island width decreases as a result of the enhanced magnetic shear at <i>ι</i>/2π = 0.4 rational surface. Moreover, effects of the direction and the amplitude of the current on the suppression of magnetic islands are also discussed in more detail.