Vertical Position Control

Actively stabilizing the vertical position of an elongated plasma, which is intrinsically unstable and must be corrected faster than it grows.

Why elongated plasmas fall

Vertical elongation improves confinement and current capability, but an elongated plasma is vertically unstable: a small upward displacement produces forces that push it further up. The surrounding conducting structures slow the growth to a resistive timescale of a few milliseconds, buying enough time for active feedback - but not much. Vertical control is the most latency-sensitive of the magnetic loops.

The two-timescale picture

Passive conductors (the vacuum vessel, stabilizing plates) react instantly to oppose fast motion, converting an ideal-MHD instability into a slower resistive one. Active feedback then acts on that slower mode. The passive and active systems are designed together: more passive stabilization relaxes the demand on the active loop, and vice versa.

The fast loop

Vertical control uses dedicated fast coils driven by a high-bandwidth loop, typically running at tens of microseconds. It senses vertical displacement from magnetic measurements and applies a radial field to push the plasma back. Because it fights an exponential instability, its gain and delay budget are tight; loss of this loop is a common precursor to a disruption.

Controllability limits

There is a maximum stabilizable growth rate for a given coil, power supply, and delay. Beyond it, no controller can hold the plasma - the actuator cannot respond fast or hard enough. This defines a hard limit on usable elongation. Scenario design keeps the plasma inside the vertically stabilizable region, with margin for noise and disturbances.

Coupling and coordination

Fast vertical action perturbs shape and current, so its effect is fed forward to the slower loops to be trimmed out. Keeping vertical control on its own fast coils and clock isolates the hardest, fastest job from the more forgiving shape and current loops.