Comparison between different types of poloidal angle

Figure 7 compares the equal-arc-poloidal angle with the straight-line poloidal angle, which shows that the resolution of the straight-line poloidal angle is not good near the low-field-side midplane.

Figure 7: The equal-arc poloidal angle (left) and the straight-line poloidal angle (right) for EAST equilibrium #38300@3.9s. The blue lines correspond to the equal-$\theta$ lines, with uniform $\theta$ interval between neighbour lines. The red lines correspond to the magnetic surfaces, which start from $\sqrt {\overline {\Psi }_t} = 0.01714$ (the innermost magnetic surface) and end at $\sqrt {\overline {\Psi }_t} = 0.9851$ (the boundary magnetic surface), and are spaced in equal increments of $\sqrt {\overline {\Psi }_t}$.