$\alpha$ contours in a toroidal annulus

Figure 21 compares the $\phi$ coordinate surface of $(\psi , \theta , \phi )$ coordinates with the $\alpha$ coordinate surface of $(\psi , \theta , \alpha )$ coordinates.

Figure 21: Comparison between isosurface of $\phi = 2 \pi / 8$ (projection of magnetic field lines onto $\phi = 2 \pi / 8$ plane) and isosurface of $\alpha = 2 \pi / 8$. The $\alpha$ isosurface is made of a family of contours of $\alpha = 2 \pi / 8$, which are all magnetic field lines. These field lines are traced by starting from a series of points on the low-field-side midplane $(\theta = 0)$ at different radial locations and the field lines are followed by a complete poloidal loop. The radial range is given by $\psi _N \in [0.4 : 0.5]$, where $\psi _N$ is the normalized poloidal magnetic flux. Magnetic field from EAST discharge #59954@3.03s.

Figure 22: The same plot as in Fig. 21 but with a larger radial range. $\psi _N \in [0.4 : 0.7]$, where $\psi _N$ is the normalized poloidal magnetic flux.