Vertical Stabilization Control
Elongated plasmas are vertically unstable and drift toward the wall unless a fast feedback loop actively holds them centered.
Why it is unstable
Shaping a plasma into a tall, elongated cross-section improves performance but makes it vertically unstable: a small upward or downward displacement produces a force in the same direction, so the plasma runs away toward the wall. The growth is exponential with a time constant set by the surrounding conducting structure, often milliseconds, so it must be actively stabilized.
The feedback loop
A fast controller measures the plasma's vertical position or velocity from magnetic sensors and drives a dedicated stabilization coil to produce a restoring radial field. The loop bandwidth must exceed the instability growth rate with margin. Because the plasma moves quickly, this is typically the fastest magnetic-control loop in the machine and demands a high-voltage, high-bandwidth power supply.
Coupling to shape control
Vertical control cannot be separated cleanly from shape and position control, since the same poloidal coils influence elongation, position, and stability. Controllers handle this by dedicating a fast inner loop to vertical stabilization and layering slower shape and position loops on top, or by treating the coil set as one multivariable system with the vertical mode given priority.
- Elongation causes an exponential vertical instability
- Growth time set by nearby conducting structure
- Fast loop drives a coil to restore position
- Highest-bandwidth magnetic loop in the machine
Loss of control
If the stabilization loop saturates or fails, the plasma drifts into the wall as a vertical displacement event, which can impose large forces on the vessel. Interlocks watch the control margin and trigger a controlled termination before the plasma is lost against the wall. In the Kronos breeder, a spherical tokamak with strong shaping and negative triangularity of -0.30, robust vertical control is essential in the design model; the machine is simulated, not built.
Vertical stabilization is the loop that never gets to rest while the plasma exists.