## 4 Discussion  
### Hydrogen Plasma-Induced Blistering  
Deuterium blister formation in tungsten samples may be induced by low-energy, high-flux deuterium irradiation [12]. The samples are immersed in the high-density deuterium plasma environment, and the deuterium atoms diffuse into W samples [13]. It has been reported that impurities, dislocations, vacancies, grain boundaries, and other crystal defects in the irradiation region are important trap sites for deuterium atoms. With regard to the formation of hydrogen isotope blisters, [PERSON] et al. [14] reported a microscopic vacancy trapping mechanism based on first-principles calculations. Intrinsic defects such as vacancies in the tungsten material are the controlling factors for blister formation. Deuterium atoms prefer to be trapped by vacancies. Since these trap sites capture the deuterium atoms [13], the deuterium-vacancy clusters recombine and diffuse gradually into the tungsten material. It is reported that the concentration of deuterium in the near-surface layer during irradiation far exceeds that which the material can accommodate [15]. If the flux of implanted deuterium is greater than the rate at which deuterium diffuses out of the implanted area, a local supersaturation of deuterium appears [15]. The stress caused by local supersaturation of deuterium causes more deuterium atoms to reach the trap sites and recombine, leading to the establishment of a stress field.  
In our experiments, with the irradiation fluence increasing, the blisters become increasingly obvious. The surface morphology changes indicate that the incident fluence has a large influence on the formation of the blisters. The ion energy (100 eV) is much lower than the sputtering and damage threshold of tungsten under deuterium bombardment [15]. Although [PERSON] et al. demonstrated that there was no sputtering at a low energy of 100 eV, they found that blisters were formed using 100 eV/10\({}^{21}\) m\({}^{-2}\) s\({}^{-1}\) D plasma [16]. Blistering behavior occurs at the tungsten surface, even if the ion energy is too low to create displacement damage such as vacancies [15]. When more hydrogen atoms reach the lattice positions of tungsten, tungsten atoms are ejected from the lattice positions. Since the solubility of deuterium in tungsten is very low, the tungsten atoms diffuse to release the high stress, causing blistering [17]. Additional vacancies are created during this process [18], which trap more deuterium and provide extra conditions for deuterium accumulation. The hydrogen atoms aggregate and form blisters in tungsten [19].  
Compared with those of W bulk, the size and density of the blisters on the surface of W film are smaller and lower, respectively, which may be due to the large volume fraction of grain boundaries of W film. The average grain size of W bulk is much larger than that of W film. The grain boundary volume fraction increases sharply if the grain size is smaller than 100 nm [20]. It has been reported that the high density of grain boundaries in W film enhances the threshold for hydrogen atoms to reach a supersaturated state, thus  
Figure 7: D retention in W bulks and W films at different exposure fluences.  
inhibiting blister formation [21]. Therefore, hydrogen blister formation in W film is reduced in comparison with pure W bulk under the same conditions.