(Science: radiobiology) The cold, dense plasma formed outside the last closed flux surface during a vertical displacement event. The large currents which flow through this plasma stop the displacement and transfer the force to the vacuum vessel. If care is not taken in design, the halo currents can be large enough to threaten the structural integrity of the vacuum vessel or in-vessel components. Whereas the centre of a tokamak plasma is too hot for material probes to survive, probes (such as magnetic-field coils) can sometimes be placed in the halo, and can measure things such as the halo current.
See: vertical instability.