flattop is the steady central phase of a plasma pulse — the plateau after 📝ramp-up when 📝plasma current and magnetic field are held constant and the discharge does its real work.
The name describes the shape of a graph: plotted against time, the current in a 📝Tokamak pulse rises through ramp-up, runs flat across the top, and falls through ramp-down, and the flat middle is the part physicists point to when they quote a machine's performance. Confinement times, temperatures, and 📝fusion power figures all describe flattop conditions, because only there are the plasma's current profile and magnetic geometry in the steady state the measurements assume.
Flattop is distinguished from the pulse as a whole: the total discharge includes the transient phases on either side, while the flattop is the fraction of it that produces usable physics — or, in a power plant, usable heat. Stretching the flattop is therefore a design goal in its own right, limited in a conventional tokamak by the finite flux of the 📝central solenoid that sustains the current inductively. 📝SPARC is designed to hold flattop for roughly ten seconds at full field and current — long enough for the 📝plasma to reach the equilibrium conditions its net-energy demonstration requires.