Ramp-Down Control

Ramp-down safely unwinds a discharge from flat-top to termination, avoiding the disruptions that a careless shutdown can trigger.

Not just the reverse of ramp-up

Ramp-down is often more dangerous than ramp-up. As current falls, the plasma shrinks and the current profile can become peaked, driving internal inductance up. If the vertical control cannot keep pace with the changing shape, the plasma can lose position control and disrupt. Many machine disruptions historically occur during termination.

Coordinated reduction

A safe ramp-down reduces current, heating, and density together in a coordinated way. Heating is usually lowered before or with the current so the plasma does not remain hot and dense at low current, which raises the risk of exceeding the density limit relative to current. Density is pumped down to keep the discharge away from radiative collapse.

Controlling internal inductance

The peaking of the current profile, measured by internal inductance, is the key variable. Applying off-axis heating or current drive during ramp-down broadens the profile and keeps inductance in a controllable band. Without this, the vertical stability margin can shrink faster than the ramp reduces the demand on the vertical controller.

Vertical margin during shutdown

Because elongation and vertical instability are linked, ramp-down often reduces elongation as current falls, trading shape performance for stability margin. The controller must sequence these changes so the plasma always stays within the reach of the vertical loop.

In the Kronos program

For the spherical Hyperion breeder, vertical control is the binding constraint, so its ramp-down profiles are designed to shed elongation and current together while broadening the profile. These sequences are validated in the flight simulator so that termination is routine rather than a source of off-normal events once hardware operation begins.