vertical displacement event (VDE) is a loss of vertical position control in a tokamak, in which the plasma column accelerates upward or downward until it strikes the vessel and its current collapses.
The vulnerability is designed in. Modern tokamak plasmas are elongated — stretched into a D-shaped cross-section — because elongation buys more 📝plasma current and better confinement in the same field. But an elongated 📝plasma is vertically unstable: displaced slightly up or down, the shaping fields pull it further in the same direction. Stability therefore depends on continuous feedback control, with sensors and shaping coils correcting the plasma's position in real time. A VDE is what happens when that control is lost — because the control system fails, its coils saturate, or a 📝disruption abruptly changes the plasma faster than the feedback can follow. The column moves on a millisecond timescale, scrapes against the wall, and terminates.
The distinction against a generic disruption is direction of causation: a VDE can be the cause of a disruption (position is lost first, then the plasma quenches against the wall) or its consequence (the thermal quench alters the plasma and vertical control is lost). VDEs are among the most punishing off-normal events for a 📝Tokamak, because the halo currents flowing between the displaced plasma and the vessel exert forces the structure must be built to survive.