The vertical stretching of a tokamak’s plasma cross-section, measured as the ratio of height to width — increasing confinement and stability limits but requiring active feedback control.
Elongation (κ) describes how vertically stretched a plasma cross-section is compared to a circle. A circular cross-section has κ = 1; typical tokamaks operate with κ = 1.6–2.2. Elongation is achieved by shaping the poloidal field coil currents to create a vertically stretched equilibrium.[1]
An elongated plasma is inherently unstable to vertical displacement — if the plasma shifts slightly up or down, the restoring force from the external field is insufficient, and the displacement grows exponentially. This vertical displacement event (VDE) can dump the plasma’s stored energy onto the first wall in milliseconds, causing severe damage.[2]
Active feedback control using internal saddle coils or external field coils is mandatory. The growth rate of the vertical instability increases with elongation, setting a practical upper limit on κ that depends on the speed and authority of the feedback system.[3]