Let , , be the corresponding cylindrical coordinate system, where and . It follows that
Finally, let us define a flux coordinate system: , , , where [5,14](Note that the , , , are the same as the corresponding quantities defined in Section 2.10.) Here, is a convenient scale major radius, the magnetic flux-surface label has units of length, and is is an angular coordinate that increases by radians for every poloidal circuit of the magnetic axis. Let correspond to the magnetic axis of the plasma, and let correspond to the last closed magnetic flux-surface. Suppose that on the outboard midplane of the plasma. It follows that below the midplane.