During the course of the International Tokamak Reactor studies, four interrelated issues for impurity control have been identified:
- pumping capability
- sputtering erosion
- heat removal
Recent plasma physics experiments and calculations indicate that the divertor is likely to operate with presheath plasma temperatures of only 10 to 20 eV. Surface sputtering erosion should be virtually eliminated under these conditions resulting in extended sputtering lifetimes for the divertor collector plates. The reduced sputtering means that the collector plates can be designed with only a thin layer of protection material facing the plasma, which should lead to reduced thermal stresses, greater heat load capability, and longer lifetimes. To assess the advantages and disadvantages of this type of divertor system, engineering trade-off studies have been performed. The studies include analyses of the materials, fabrication, thermomechanical behavior, disruptions, erosion/redeposition, tritium permeation, and lifetime. It is concluded that the divertor collector plates, which are composed of a copper alloy heat sink with a tungsten cladding, should operate satisfactorily for a period ≳2 yr.