Mechanisms responsible for topographical changes in PLT stainless-steel and graphite limiters

PLT limiters used during Ohmic and neutral-beam heated discharges were melted and eroded. For the steel limiters, the pattern of large (20 sq cm) melted areas was 0.1-1.0-mm ripples in a regular array. The various types of damage can be explained by the power and momentum fluxes of bulk plasma and beam ions and by disruptions. The plasma scrape-off distance inferred from the extent of the damage ranges from 5 to 30 mm and is consistent with a radially outward drift velocity of 20,000 mm/sec. A new model of the plasma scrape-off is presented which explains the pattern and extent of the damage.