Limiters are discrete components used in tokamaks to protect the first wall surfaces against plasma instabilities and disruptions. They are located on the first wall of the tokamak at specific positions where a direct contact with the boundary of collapsing or moving plasma can be expected.
The limiters would protrude into the plasma scrape-off-layer by 30-70 mm beyond the level of plasma facing surfaces, such that in an instability mode, the plasma would touch the limiters first.
With their sacrificial role, the limiters become the components in the tokamak with the highest radiation damage and must be replaced the most frequently.
Further details on the limiter of EU-DEMO design are give in the page, Divertor and Limiter