Flat-Top Control
Holding steady plasma conditions during the useful phase of a discharge, regulating current, shape, density, and profiles against drift.
Steady is not passive
Flat-top is where the plasma does its work - the phase held at target current and shape for fusion performance or physics study. Holding it steady is active: transport slowly reshapes profiles, the wall exchanges particles, and small instabilities appear and must be managed. Flat-top control rejects these drifts to keep the regime on target.
What is regulated
- Plasma current and shape held at reference
- Density held below the Greenwald limit with margin
- Stored energy and normalized pressure kept below the beta limit
- Current profile and q managed to suppress tearing modes
- Radiated power and divertor conditions kept in the desired regime
Profile control
The deepest flat-top challenge is controlling internal profiles - the current and pressure profiles - that govern stability. These respond slowly to heating and current drive and are only partly observable, making profile control a hard, model-based problem. Getting it right is what separates a robust flat-top from one that drifts into a neoclassical tearing mode.
Sustaining current
In long-pulse or steady-state scenarios, inductive volt-seconds run out, so flat-top must be sustained non-inductively by current drive and the self-generated bootstrap current. Controlling the balance among ohmic, driven, and bootstrap current to hold a stable profile is the central steady-state control problem.
The quiet ideal
A well-controlled flat-top looks uneventful: signals sit flat, actuators trim gently, the disruption predictor stays quiet. That calm is the product of good ramp-up, sound profile control, and margins held against every limit - not of the plasma being inherently docile.