Comparison of tokamak behaviour with tungsten and low-Z plasma facing materials

Graphite wall materials are used in present day fusion devices in order to optimize plasma core performance and to enable access to a large operationa l space. A large physics database exists for operation with these plasma fa cing materials, which also indicate their use in future devices with extend ed burn times. The radiation from carbon impurities in the edge and diverto r regions strongly helps to reduce the peak power loads on the strike areas , but carbon radiation also supports the formation of MARFE instabilities w hich can hinder access to high densities. The main concerns with graphite a re associated with its strong chemical affinity to hydrogen, which leads to chemical erosion and to the formation of hydrogen-rich carbon layers. Thes e layers can store a significant fraction of the total tritium fuel, which might prevent the use of these materials in future tritium devices. High-Z plasma facing materials are much more advantageous in this sense, but these advantages compete with the strong poisoning of the plasma if they enter t he plasma core. New promising experiences have been obtained with high-Z wa ll materials in several devices, about which a survey is given in this pape r and which also addresses open questions for future research and developme nt work.