Ramp-Up Control
Ramp-up brings a discharge from breakdown to the flat-top current while keeping the plasma stable and the current profile well behaved.
The delicate early phase
Ramp-up is the phase where plasma current is driven from near zero to its flat-top value. It is delicate because the current profile is still forming, the plasma is small and cool, and several instabilities are most easily provoked here. Ramping too fast can leave a hollow current profile that seeds tearing modes; ramping too slow wastes flux and can hit density or radiation limits.
What is controlled
- Plasma current rate of rise
- Plasma position and elongation as the shape forms
- Density buildup by fueling
- Edge safety factor to avoid low-order rational surfaces
- Internal inductance, a proxy for profile peaking
Current profile matters
Because current diffuses inward on the resistive timescale, the profile shape during ramp-up depends on the history of heating and current drive, not just the present demand. Auxiliary heating applied early keeps the plasma hot and slows current penetration, giving a broader, more stable profile. This is why ramp-up control couples tightly to heating scheduling.
Flux management
The central solenoid has a finite volt-second supply. Ramp-up must reach flat-top without exhausting it, leaving enough for the flat-top burn. Efficient ramp-up, sometimes assisted by early current drive, stretches the available flux and extends the usable pulse.
In the Kronos program
The Hyperion breeder is a spherical tokamak, so its slim central column limits solenoid flux and puts a premium on efficient ramp-up. Control designs for the machine, which is in simulation ahead of construction beginning in the second quarter of 2027, aim to establish the design current of 9.86 megaamperes with a profile that avoids seeding tearing modes at the transition to flat-top.