Topology of toroidal helical fields in non-circular cross-sectional tokamaks
Zha Xue-Jun (查学军)ab, Zhu Si-Zheng (朱思铮)b, Yu Qing-Quanb, Wang Yan (王燕)c
a State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China; b Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China; c Department of Physics, Shanghai University, Shanghai 200436, China
Abstract The ordinary differential magnetic field line equations are solved numerically; the tokamak magnetic structure is studied on Hefei Tokamak-7 Upgrade (HT-7U) when the equilibrium field with a monotonic $q$-profile is perturbed by a helical magnetic field. We find that a single mode ($m,n$) helical perturbation can cause the formation of islands on rational surfaces with $q=m/n$ and $q=(m\pm 1, \pm 2, \pm 3,\ldots) /n$ due to the toroidicity and plasma shape (i.e. elongation and triangularity), while there are many undestroyed magnetic surfaces called Kolmogorov--Arnold--Moser (KAM) barriers on irrational surfaces. The islands on the same rational surface do not have the same size. When the ratio between the perturbing magnetic field $\tilde {B}_r (r)$ and the toroidal magnetic field amplitude $B_{\phi 0} $ is large enough, the magnetic island chains on different rational surfaces will overlap and chaotic orbits appear in the overlapping area, and the magnetic field becomes stochastic. It is remarkable that the stochastic layer appears first in the plasma edge region.
Received: 25 January 2005
Revised: 26 May 2005
Accepted manuscript online:
Altmetric calculates a score based on the online attention an article receives. Each coloured thread in the circle represents a different type of online attention. The number in the centre is the Altmetric score. Social media and mainstream news media are the main sources that calculate the score. Reference managers such as Mendeley are also tracked but do not contribute to the score. Older articles often score higher because they have had more time to get noticed. To account for this, Altmetric has included the context data for other articles of a similar age.
