# Linear Plasma Device for the Study of Plasma-Surface Interactions
## 4 Results and Discussion  
In Figure 3, SEM images of tungsten samples before (Figure 3a) and after exposure to helium plasma at temperatures of T = 500 \({}^{\circ}\)C (Figure 3b), T = 900 \({}^{\circ}\)C (Figure 3c), and T = 1300 \({}^{\circ}\)C (Figure 3d) are presented. The research results indicate that exposure to helium plasma leads to changes in the surface morphology of tungsten. Relief areas are formed on the surface due to the sputtering by helium ions, as well as the accumulation of helium ions in the near-surface layer, which leads to the formation of blisters. In the case of tungsten samples irradiated at T = 500 \({}^{\circ}\)C (Figure 3b), etching areas are observed on the surface due to the sputtering by helium ions. For tungsten samples irradiated at T = 900 \({}^{\circ}\)C and T = 1300 \({}^{\circ}\)C, a highly relieved morphology forms on the surface. Blisters and eroded areas on the surface can be observed. The formation of blisters is attributed to the creation of high-pressure regions within the lattice due to the accumulation of helium atoms, causing localized expansion of the material and resulting in blister formation on the surface. The X-ray structural phase analysis showed that there is no change in the phase composition of tungsten after exposure to helium plasma (Figure 4). From the diffractogram, we can observe a change in the shape of the diffraction peaks. Broadening of the diffraction  
Figure 3: SEM images of tungsten sample surface before (**a**) and after helium plasma irradiation at T = 500 \({}^{\circ}\)C (**b**), T = 900 \({}^{\circ}\)C (**c**) and T = 1300 \({}^{\circ}\)C (**d**). peaks (110) and (211) is observed on the diffractogram. This indicates the emergence of mechanical stresses after exposure at T = 1300 \({}^{\circ}\)C. Mechanical stresses in tungsten during exposure to helium plasma can arise due to several factors. When helium ions are implanted into the material, they can create lattice defects such as vacancies and interstitials. Accumulation of these defects can lead to the generation of internal stresses in the material. Additionally, this may be associated with the formation of bubbles. Helium ions have a high affinity for forming helium bubbles (or voids) within the material as they accumulate. These bubbles can exert pressure on the surrounding material, leading to swelling and the emergence of mechanical stresses. The hardness and elastic modulus of the tungsten samples were measured before and after irradiation with helium plasma (Figure 5). After irradiation at T = 500 \({}^{\circ}\)C, an increase in hardness by 10% and a slight decrease in elastic modulus was observed. The increase in hardness may be attributed to the development of mechanical stress due to helium accumulation in the near-surface layer of tungsten. It is known that the accumulation of helium in the near-surface layer leads to compressive stresses which reduce the elastic modulus and increase the hardness of the metal. After irradiation at T = 900 \({}^{\circ}\)C and T = 1300 \({}^{\circ}\)C, hardness and elastic modulus decrease depending on the temperature. This might be associated with structural changes, increased roughness, and an increase in the volume fraction of voids. The erosion of tungsten samples was assessed by weighing the samples before and after irradiation. Surface roughness was also evaluated. Figure 6a shows the dependence of surface roughness and mass loss of tungsten on the irradiation temperature of helium plasma. From Figure 6a, it can be observed that surface roughness increases after irradiation. The increase in roughness is associated with surface sputtering by helium ions, as well as the formation of blisters. The graph indicates that the surface roughness of tungsten increases with increasing irradiation temperature. As revealed by microstructural analysis, defects such as etching pits and blisters increase with temperature. These changes are reflected in the surface roughness.