Vertical Stabilization
Elongated plasmas are vertically unstable and drift toward the wall unless a fast feedback loop actively holds them in place.
The instability
Shaping a plasma to be tall and narrow, called elongation, improves confinement and raises the achievable current, but it makes the plasma vertically unstable. A small upward or downward displacement grows exponentially because the shaping field that stretches the plasma also pushes it further once it moves off center. Left alone, the plasma drifts into the wall and disrupts.
The fast loop
Vertical stabilization is the fastest feedback loop in a tokamak. Magnetic sensors measure vertical position; a controller drives a dedicated set of coils to push the plasma back. Because the instability grows on a millisecond scale, the loop must run at kilohertz rates with a very short latency budget. It is the loop that most constrains the whole control system's timing.
Growth rate and margin
The instability's growth rate rises with elongation, so more aggressive shaping demands a faster loop and stronger coils. Designers trade shape performance against the stability margin the vertical system can provide. The controllable elongation is limited by how fast and how hard the vertical coils can respond, not by physics alone.
Failure consequences
Loss of vertical control is a leading cause of disruptions, often producing a vertical displacement event where the plasma moves into the wall and dumps its current into the structure. This is why the fallback ladder keeps vertical control alive to the last possible rung and why its coils and sensors are treated as protection-critical.
In the Kronos program
The Hyperion breeder is a spherical tokamak, a low-aspect-ratio device that is naturally elongated, so vertical stabilization is its single most demanding control loop. It runs on dedicated real-time hardware with the shortest latency in the stack, and its performance sets the machine's achievable shape. The design is validated in simulation before hardware operation.