ramp-up is the opening phase of a tokamak plasma pulse, during which the 📝plasma current is driven from near zero up to the full value it will hold through the steady heart of the discharge.
In everyday usage a ramp-up is any gradual increase in activity or output, and the 📝fusion sense is a direct extension of it. Every pulse in a 📝Tokamak begins with breakdown, when neutral gas ionizes into 📝plasma; ramp-up follows, as the changing magnetic field of the 📝central solenoid induces a climbing current in that plasma, typically over several seconds. The rate of the climb is a controlled compromise. Ramping too fast distorts the current profile and invites instabilities that can end the discharge in a 📝disruption; ramping too slowly spends solenoid flux — the finite inductive resource that sets how long the pulse can last — before the experiment proper begins.
Ramp-up ends when the current reaches its programmed value and the 📝flattop begins: the constant-conditions plateau where confinement and fusion performance are actually measured. Its mirror phase, ramp-down, lowers the current gradually at the end of the pulse for the same stability reasons, since an abrupt termination stresses the machine much as a disruption does.