Ramp-Up Control
The early, fragile phase where current and shape are established while avoiding the instabilities that thrive at low current and density.
A vulnerable phase
Ramp-up builds the plasma current from breakdown toward flat-top while forming the diverted shape. It is one of the most disruption-prone phases: current is low, density is low, and the plasma cannot shield error fields by rotation. Many locked-mode and low-q disruptions originate here, so ramp-up control is conservative by design.
Managing the current profile
How fast current is ramped sets the internal current profile and the evolving safety factor q. Ramp too fast and the edge current can drive kink instabilities as q passes through low-order rational values; ramp too slow and volt-seconds are wasted. Ramp-up control shapes the current rate to keep q on a safe path while spending flux efficiently.
Coordinated actions
- Establish and hold vertical stability as elongation is built
- Apply error-field correction while density is low
- Form the diverted boundary shape gradually
- Introduce heating at the right time to control the current diffusion
- Track density up in step with current to stay below the Greenwald limit
The q-profile handoff
The current profile set during ramp-up largely determines the flat-top q profile, because current diffusion is slow. Getting the ramp right is therefore an investment in flat-top stability - a good ramp-up hands a well-conditioned plasma to the flat-top controller, while a poor one bakes in a mode that surfaces later.
Why it is scripted and watched
Ramp-up follows a pre-validated trajectory, but with active feedback and disruption prediction running, because the phase is fragile enough that small deviations matter. It is a phase where avoidance logic is especially likely to intervene, softening the ramp if the plasma shows early warning.