The magnetic null where poloidal-field lines cross in an ‘X’ pattern, defining the separatrix and steering exhaust heat toward the divertor.
In a diverted tokamak, the poloidal magnetic field does not simply wrap around the plasma in concentric loops. One or more external divertor coils create a region where the poloidal field passes through zero—a magnetic null. At that point the field lines form a characteristic ‘X’ pattern, giving the feature its name. The flux surface that passes through the X-point is the separatrix: everything inside is confined plasma; everything outside is the scrape-off layer (SOL), where field lines connect to material surfaces.1
The simplest diverted geometry has a single X-point, usually at the bottom of the vessel, creating a single-null (SN) configuration. Plasma exhaust flows along the SOL, follows the open field lines through the X-point region, and strikes the divertor target plates below. A double-null (DN) configuration places a second X-point at the top, splitting exhaust between upper and lower divertors and roughly halving the heat load on each target—an advantage that comes at the cost of more complex coil control and sensitivity to small vertical displacements.2
Because the poloidal field vanishes at the X-point, the connection length—the distance a field line travels before hitting a wall—diverges logarithmically there. This long connection length allows cross-field transport to spread the exhaust over a wider area, partially relieving the intense heat-flux problem that threatens divertor materials.3
The X-point also plays a starring role in edge stability. In H-mode plasmas, type-I ELMs launch filaments of hot plasma that follow field lines through the X-point region into the divertor. The geometry of the X-point influences the ELM footprint and the resulting transient heat loads on target plates. Advanced configurations such as the snowflake divertor deliberately bring two X-points close together, further expanding the flux near the null and spreading the exhaust over a larger wetted area.4
Accurate real-time knowledge of the X-point position is essential for machine protection. If the X-point drifts too far, the strike point can move off the divertor tiles and onto unprotected first-wall surfaces, risking damage. Magnetic equilibrium reconstruction codes such as EFIT solve for the X-point location on a millisecond timescale, feeding the plasma control system that adjusts the divertor coil currents to hold it in place.