Ramp-Down Control
Bringing current and stored energy down on a controlled path - a phase as disruption-prone as ramp-up if handled carelessly.
Landing the plasma
Ramp-down reduces plasma current and stored energy to end the discharge safely. It is deceptively risky: as current falls, the Greenwald density limit falls with it, the shape must be maintained with shrinking current, and internal inductance can rise and threaten vertical control. A careless ramp-down disrupts as readily as a bad ramp-up.
The falling density limit
Because the Greenwald limit scales with plasma current, a density that was safe at flat-top can exceed the limit as current drops. Ramp-down control must reduce fueling and pump density down in step with the current, or the plasma walks into a density-limit disruption on its way out.
Vertical control under rising inductance
Current ramp-down often raises the internal inductance, which increases the vertical instability growth rate and stresses the vertical-control loop just as the plasma is being reshaped. Ramp-down trajectories are designed to keep inductance and elongation on a path the vertical loop can still hold.
Preserving margins to the end
- Reduce density with current to respect the Greenwald limit
- Keep q and current profile away from disruptive resonances
- Maintain shape and vertical stability as authority shrinks
- Hold heating long enough to control profile evolution, then reduce it
- Reserve enough control authority to complete the landing
A designed maneuver
Ramp-down is a pre-optimized trajectory, not an afterthought of turning things off. It receives the same feedback, prediction, and avoidance attention as any other phase, because the plasma is still fully capable of disrupting until the current is safely down.