**2. Properties of W for fusion applications**

As a result of extensive research, tungsten (W) has emerged as a highly useful plasma-facing material (PFM) [1–3]. Its high melting point, high thermal conductivity, low coefficient of thermal expansion, high sputtering threshold energy, low tritium retention and low neutron activation make W a potential candidate for fusion applications [4–6]. Previously, applications of W were rare and were limited to experimental purposes only in tokamaks due to the formation of high-Z dust, which originates from materials eroding on the surfaces of the plasma-facing components. This dust has detrimental effects on the plasma parameters [7], but research has revealed the feasibility of plasma operations with W [1, 8]. The plasma impurity problem associated with W may be eliminated by ensuring that the energy of the plasma particles remains lower than the sputtering threshold (~700 eV for tritium) [9]. In fusion applications, an increase in the future utilization of W is foreseen [9, 10], as it is considered as a first wall [11] and a divertor surface [1] material for future fusion reactors, as illustrated in

**Figure 1.** (a) A schematic cross-section of tokamak and (b) a solid model illustration of divertor (commons.wikime‐

As compared to other high-temperature applications, which require good physical and mechanical properties such as high thermal conductivity, good thermal shock resistance and high-temperature strength, toughness and stiffness [12], fusion reactors impose very complex requirements on plasma-facing materials (PFMs) [1]. While PFMs are assumed to be able to sustain high mechanical, thermal and magnetic loads for prolonged periods of time [8], irradiation effects are particularly important in plasma-facing components [13]. Transmutation and ballistic damage, which alter the composition and microstructure of materials due to the interaction between the materials and the high-energy neutron flux (~14 MeV), reduce the mechanical properties of PFMs. The interaction of neutron flux with materials forms disloca‐

tion loops and clusters of transmutation products from non-equilibrium phases [14].

tritium retention and morphology variations [8].

The energetic ions and neutral atoms in the service environments of fusion reactors cause sputtering erosion in plasma-facing components [1], which is a major concern associated with these materials [8]. The free atoms can be ionized and form contaminants. These contaminants can then be deposited at various locations on the chamber wall. The erosion and re-deposition create new layers which can shorten the service lifetime of the fusion reactor by enhancing

**Figure 1**.

140 Nuclear Material Performance

dia.com).
