limiter is a material structure that defines the outer edge of a 📝fusion plasma by direct contact, used in machines that do not have a divertor.
The limiter and the 📝divertor are the two ways a 📝Tokamak answers the same question — where does the plasma end? A limiter answers it with material: a protruding rail, ring, or shaped wall section intercepts the outermost field lines, and the plasma simply stops where it touches. A divertor answers it with magnetic geometry, steering the edge 📝plasma into a separate exhaust chamber. The limiter's virtue is simplicity — no extra magnetic shaping, no dedicated chamber — and nearly all early tokamaks used one. Its defect is that the plasma's full heat and particle flux lands on a surface directly adjacent to the confined core, so atoms sputtered from the limiter enter the plasma with little to stop them, raising impurity contamination.
That defect is why the divertor displaced the limiter as machines grew more powerful, and why the high-confinement 📝H-mode regime is far harder to reach in limited plasmas. Limiters have not disappeared, though: diverted tokamaks still begin many discharges limited — the growing plasma leans on the wall or a startup limiter until the 📝X-point geometry forms — and limiter surfaces continue to protect wall sections during 📝ramp-up and ramp-down.