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Low-Z elements are in many ways well suited as plasma-facing materials in
the harsh environment of a magnetic fusion devices. For carbon in particular, its beneficial properties include large heat handling capability, and minimal surface distortions due to the lack of melting during transient heating events. Also, plasma-material interactions eventually result in traces of wall materials to be present as an impurity in the thermonuclear core plasma where Deuterium-Tritium fusion
occurs. The core plasma is found in experiments to be relatively robust to the low-Z elements since atoms are stripped of all bound electrons, and unwanted radiation losses are minimized which would otherwise negatively affect fusion power balance. However, low-Z materials have two properties that are undesirable in a fusion environment: high erosion and high hydrogenic solubility. High erosion is a concern to the wall's operational lifetime since plasma-facing surfaces can only be ~cm in thickness for reasons of heat exhaust. High solubility can lead to quasi-permanent storage of the tritium fuel in materials, in particular plasma-deposited films, which can be unacceptable to the fusion fuel cycle which must effectively self-breed tritium. The operational limits for low-Z surfaces are primarily set by the complex interplay between the plasma and the material surfaces, making accurate predictions very difficult. We will explore the challenges and recent progress in both measuring and controlling erosion and tritium fuel retention, particularly with regard to ITER, and future D-T fusion devices.
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