The innermost flux surface of a toroidal plasma—a single closed curve where key equilibrium quantities like the safety factor and rotational transform are evaluated.
In any toroidal magnetic confinement device—tokamak, stellarator, or reversed-field pinch—the magnetic field lines trace out nested toroidal surfaces. At the very centre of those nested surfaces lies a single closed curve: the magnetic axis. It is the degenerate flux surface of zero cross-sectional area, the geometric and magnetic heart of the plasma equilibrium.1
On the magnetic axis, a field line closes on itself after a fixed number of toroidal and poloidal transits. The ratio of those transits defines the rotational transform (or, in tokamak convention, the inverse safety factor 1/q0). This on-axis value is one of the most important numbers in plasma stability: it determines proximity to low-order rational surfaces where current-driven instabilities such as sawteeth and internal kink modes are triggered.2
The magnetic axis does not sit at the geometric centre of the vacuum vessel. Finite plasma pressure pushes the axis outward toward the low-field side of the torus, a displacement known as the Shafranov shift (ΔS). The shift increases with the ratio of plasma pressure to magnetic pressure (beta) and decreases with the strength of the vertical field provided by the equilibrium coils. A large Shafranov shift compresses the flux surfaces on the outboard side, steepening gradients and affecting transport, while expanding them on the inboard side.3
Locating the magnetic axis precisely is essential for mapping all other flux surfaces. Equilibrium reconstruction codes such as EFIT solve the Grad–Shafranov equation on a poloidal grid, using external magnetic measurements and internal profile diagnostics to converge on an equilibrium. The magnetic axis appears as the extremum of the poloidal flux function ψ; its radial position R0 is a primary output of the reconstruction.4
Several diagnostics are sensitive to axis quantities. Soft X-ray emission peaks near the axis because temperature and density are highest there. Motional Stark effect (MSE) polarimetry measures the local magnetic pitch angle, from which the on-axis safety factor q0 can be inferred—critical for predicting sawtooth onset and planning current-profile tailoring with auxiliary heating.