Large electrical currents that flow through the vacuum vessel and plasma-facing structures during a vertical displacement event — the dominant source of electromagnetic forces that tokamak structures must be designed to survive.
During a vertical displacement event (VDE) in a tokamak, the plasma column drifts into contact with the first wall while still carrying a substantial toroidal plasma current. As the plasma scrapes along the wall and its cross-section shrinks, a fraction of the plasma current transfers into the thin, cool plasma layer between the hot core and the material surface — a region called the "halo." This halo current flows poloidally through the scrape-off layer, enters the wall or divertor structures, travels through the conducting vessel, and returns to the plasma, completing a circuit.[1]
The interaction of this poloidal halo current with the strong toroidal magnetic field produces large forces — predominantly vertical — on the vacuum vessel and its internal components. These electromagnetic loads are typically the most severe structural design driver for a tokamak's in-vessel components and vessel supports.
The halo current fraction (the ratio of peak halo current to the pre-disruption plasma current) varies from event to event. In existing tokamaks, measurements show halo current fractions typically between 0.2 and 0.4, with extreme events reaching 0.5 or higher. The resulting vertical force on the vessel scales as:[2]
Fz ∝ Ihalo × BT × geometric factors
For ITER, with a pre-disruption plasma current of 15 MA and a toroidal field of 5.3 T, the design-basis vertical force exceeds 80 MN. This force acts over a timescale of 10–50 milliseconds, short enough to excite structural resonances in vessel components.
Halo currents are rarely uniform around the torus. Toroidal asymmetries — often linked to the n=1 kink mode that grows during the current quench — concentrate the halo current on one side of the vessel. The degree of peaking is quantified by the toroidal peaking factor (TPF), defined as the ratio of the maximum local halo current density to the toroidally averaged value.[1]
The product of the halo current fraction and the TPF determines the peak local force on vessel structures. ITER uses a design value of Ihalo/Ip × TPF ≤ 0.72, based on a statistical envelope of data from JET, DIII-D, ASDEX Upgrade, Alcator C-Mod, and other machines.
Halo currents are measured by arrays of Rogowski coils, shunt resistors, or fibre-optic current sensors installed in the divertor structure, first-wall tiles, and vessel supports. In JET, instrumented divertor tiles provided some of the first detailed maps of halo current distribution and its toroidal asymmetry, establishing the empirical database that underpins ITER's structural design.[2]
Halo current loads affect nearly every structural element inside the vessel:
Blanket modules must withstand both the net vertical force and the moment from toroidal asymmetry. Their attachment bolts and flexible supports are sized primarily by VDE loads, not by steady-state thermal or pressure loads.
Divertor cassettes experience the most intense halo current densities because they are closest to the plasma contact point during a downward VDE. The cassette body, rails, and locking mechanisms must tolerate repeated VDE loading over the device lifetime.
Vacuum vessel and ports carry the halo current through their walls and transmit the resulting forces to the external support structure (gravity supports, lateral keys). The vessel's toroidal electrical resistance — deliberately kept low to slow VDE growth — also determines the fraction of halo current that flows through the vessel versus through internal components.[3]
Reducing halo current loads requires either preventing the VDE entirely (through disruption avoidance and early mitigation) or reducing the plasma current before wall contact occurs. Shattered pellet injection and massive gas injection both aim to trigger a radiative thermal quench and an early, fast current quench that completes before the plasma has drifted far enough to produce large halo currents. Achieving this consistently is one of the critical performance requirements for ITER's disruption-mitigation system.[3]