flux expansion is the widening of the spacing between 📝magnetic flux surfaces as they approach a 📝divertor target, so that a fixed exhaust power arrives spread over a larger wetted area.
The effect is purely geometric and has two sources. The first is poloidal: approaching the 📝X-point the poloidal magnetic field weakens toward zero, so surfaces separated by a millimetre at the outboard midplane fan apart by a centimetre or more near the target. The second is the major radius: the 📝toroidal field falls as one over R, so carrying the strike point outward to larger radius — the principle behind the Super-X and other advanced divertor configurations — expands the flux bundle further. Neither source removes energy from the exhaust channel. What reaches the surface is the parallel heat flux reduced by the expansion factor and by the sine of the shallow angle at which field lines strike the tile; the total power is unchanged. That distinguishes flux expansion from 📝divertor detachment, which dissipates exhaust power into radiation and neutrals rather than redistributing it.
Flux expansion also should not be confused with the 📝scrape-off layer power width, the few-millimetre channel set by upstream transport near the 📝separatrix. Flux expansion multiplies that width at the target but leaves the upstream physics untouched, so a machine with a very narrow power width still needs radiative dissipation on top of geometry. Real machines set flux expansion by coil currents and measure the result routinely, which makes it one of the few exhaust levers demonstrated on present devices rather than inferred. For 📝ARC the geometry is still a projection: UEDGE modelling of a long-legged, flux-expanding divertor by Wigram and colleagues (2019, DOI 10.1088/1741-4326/ab394f) found passively stable detached solutions accommodating 80–108 megawatts of exhaust at an assumed power width of 0.4 millimetres — a simulation result for a machine not yet built.