Ideal conductive wall and magnetohydrodynamic instability in Tokamak

Abstract

In order to explore the conductive wall effect of plasma magnetohydrodynamic (MHD) instability and the wall designing idea, the various forms of ideal conductive walls based on divertor equilibrium configurations in the HL-2A Tokamak and their role in suppressing kink modes are studied. The MHD instabilities and the ideal MHD operational <i>β</i> limits under free boundary or ideal wall conditions are compared. In the stability calculation, <i>n</i> = 1 kink mode is considered, which has a decisive influence on the MHD instability of Tokamak plasma. The research focuses on verifying the effectiveness of various shapes of conductive walls in suppressing internal and external kink modes, and observing the operational <i>β</i> limit changes, and discussing and analyzing related physics. It is found that an ideal conducting wall placed at a suitable distance from the plasma can effectively suppress the external kink modes. Under the condition that the average distance between the wall and the plasma surface is the same and small enough, the circular cross-section wall is not necessarily the best option. Setting an optimized polygonal conductive wall can more effectively suppress the MHD instability. It makes the ideal MHD operational <i>β</i> limit of the device, <i>β</i>_<i>N</i>, increase to 2.73, which is about 6.5% higher than that for the device with a wall assumed to be set at infinity (<inline-formula><tex-math id="Z-20230129122243-1">\begin{document}$ \sim $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20222043_Z-20230129122243-1.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="3-20222043_Z-20230129122243-1.png"/></alternatives></inline-formula>2.56). This implies that it is necessary to optimize and make a polygonal conductive wall as close as possible to the average distance from the plasma surface according to the poloidal-section shape of the elongated and shaped plasma, so as to achieve the suppression of external kink mode and increase the operational <i>β</i> limits. The physical mechanism of the stabilizing effect of the ideal wall on external kink modes is analyzed. With the development of the kink mode, when the plasma column is twisted closely to the wall, the plasma column will squeeze the magnetic field in the vacuum area, making the magnetic field line compressed and bent. At this time, the magnetic pressure and the component force of the magnetic tension in the opposite direction of the radial direction push the plasma back, thus stabilizing the kink mode. Finally, a conclusion is given.