Comparison of tokamak behaviour with tungsten and low-Z plasma facingmaterials

Abstract

Graphite wall materials are used in present day fusion devices in order tooptimize plasma core performance and to enable access to a large operationalspace. A large physics database exists for operation with these plasma facingmaterials, which also indicate their use in future devices with extended burntimes. The radiation from carbon impurities in the edge and divertor regionsstrongly helps to reduce the peak power loads on the strike areas, but carbonradiation also supports the formation of MARFE instabilities which can hinderaccess to high densities. The main concerns with graphite are associated withits strong chemical affinity to hydrogen, which leads to chemical erosion andto the formation of hydrogen-rich carbon layers. These layers can store asignificant fraction of the total tritium fuel, which might prevent the use ofthese materials in future tritium devices. High-Z plasma facing materialsare much more advantageous in this sense, but these advantages compete withthe strong poisoning of the plasma if they enter the plasma core. Newpromising experiences have been obtained with high-Z wall materials inseveral devices, about which a survey is given in this paper and which alsoaddresses open questions for future research and development work.