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a) b) Figure 1.1: a) A

a) b) Figure 1.1: a) A cross-section of the complete ITER fusion device being designed to generate future fusion power. b) A close-up of the tokamak vessel with Plasma Facing Material (PFM). [a] During operation, the reactor wall eventually becomes radioactive, due to activation of the wall materials by neutron irradiation. In addition, there will be a small amount of tritium retention in the wall material. The amount of radioactive waste generated from a fusion reactor is relative small compared to a fission reactor. Additionally, the decay time and radiation levels of the formed isotopes in the reactor are considerable lower than those of a fission reactor. Hence, the products won’t have to be stored into deep underground bunkers for preservation and safety of the environment. Besides radioactive safety concerns, the hot plasma is also a thread to the machine, since no material can withstand direct contact with such extreme high temperatures. Although lower temperatures are measured at the vessel boundary, high heat fluxes are still there eroding the PFM (Plasma-facing Material). To increase the lifetime of the wall components during operation, a suitable PFM is needed. - 4 -

1.2 Plasma Facing Material During operation, the PFM (Plasma Facing Material) has to withstand immense temperatures and pressures and degradation from particle impact. Carbon materials have been applied in many fusion reactors for optimal performance. Carbon materials, graphite and Carbon fibre reinforced Carbon (CFC) show strong mechanical properties, good thermodynamic properties and do not melt. Therefore, carbon materials were seen as the ideological choice as PFM. Unfortunately, carbon materials have three drawbacks. 1. CFC shows strong chemical reactivity with hydrogen, leading to high erosion yields under hydrogen impact. This drastically reduces the lifetime of the wall components, which have to be replaced frequently [3, 4, 5]. 2. The possible threat of hydrogen retention, especially in the case with radioactive tritium will lead to safety concerns in the co-deposited layers. [3, 5] 3. Over time neutron damage will change the physical properties of graphite making it brittle and break up. During operations this could lead to disasters. Another concern with carbon is plasma impurity. Since carbon is a low Z element, it is easily sputtered and ionized within the plasma. The ionization of carbon by the plasma reduces the core plasma temperature. To reduce plasma impurities, radiation loss of the plasma, and hydrogen retention, a mixture of carbon and metals have been considered [4]. Low Z elements, in the case of ITER; beryllium (Z=4) and carbon (Z=6), are foreseen for PFM, since they lead to lower cooling of the core plasma via radiation. Besides lower plasma cooling after being eroded, beryllium has the advantage of being a good oxygen getter. Low Z elements are easier eroded than high Z elements, reducing the lifetime of the wall components. To increase the lifetime of the wall components, high Z elements are used. In addition they are more durable against heat loads and particle bombardment [5]. The disadvantage of high Z elements is that they are more harmful for the core plasma via radiation, leading to cooling of the plasma. - 5 -

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