## 4 Discussion
### Fluence Dependence of Deuterium Retention  
Low temperature peaks of W films at around 500 K are observed in the TDS image in Figure 6b, which are caused by deuterium trapped in intrinsic defects related to dislocations, grain boundaries, and mono-vacancies [1; 24; 25]. According to the simulation, based on a trap energy of 0.8-1.2 eV, the presence of this deuterium peak position could be attributed to the trapping of deuterium at dislocations, grain boundaries, and single vacancies [26; 27]. The high temperature peaks at 720 K are generally ascribed to the trapping from deuterium agglomerated in vacancy clusters [23; 28; 29]. As the fluence increases, the peaks become much higher, which indicates that deuterium diffuses to a deeper position. The high temperature desorption peak of 720 K in the low-fluence (1.30 \(\times\) 10\({}^{25}\) D/m\({}^{2}\)) W film is not obvious, whereas in the high-fluence (5.18 \(\times\) 10\({}^{25}\) D/m\({}^{2}\)) W film this desorption peak is relatively clear. The appearance of the TDS peaks at 720 K is ascribed to a large number of vacancy-type defects generated by deuterium plasma irradiation [1]. In the case of low deuterium fluence (1.30 \(\times\) 10\({}^{25}\) D/m\({}^{2}\)), only a few defects can be induced by deuterium plasma; this is not conducive for the formation of vacancy clusters, and thus the peak at around 720 K is not obvious. As the irradiation fluence increases, single vacancies aggregate to form clusters of vacancies. As can be seen in Figure 5b, with the increasing of the fluence up to 5.18 \(\times\) 10\({}^{25}\) D/m\({}^{2}\), the peak at around 720 K transforms into a more obvious desorption peak, which confirms that the agglomeration of vacancies in clusters takes place when the fluence is increased. There are several reasons for the defects caused by deuterium plasma irradiation below the displacement threshold: plastic deformation by deuterium supersaturation [30] and the creation of vacancies [31]. The deuterium-induced stress that we mentioned above may cause single-vacancy movement and growth [32]. In addition, the duration of high-fluence irradiation in this experiment was 8 h; the longer duration allows the injected deuterium to diffuse into a larger depth, which is conducive to the diffusion of mono-vacancies to form vacancy clusters. The density of defects induced by plasma exposure increases with increasing duration and fluence. The low temperature desorption peak of W bulk at 550 K exposed to low-fluence irradiation is not obvious, whereas in the high-fluence case this desorption peak is relatively clear (see Figure 6a). The low temperature peak at 550 K is attributed to the trapping of deuterium in the intrinsic defects. The W bulks (purity: 99.95%) used in this experiment were purchased commercially and have a higher relative density (99%) than the tungsten films (the relative density of 90%); thus, the intrinsic defects inside the W bulks are relatively few compared with the W films. With the increase in the fluence, the peaks at 550 K become obvious, which is due to the vacancies that are produced with the increase in fluence. Except for a small peak at 550 K, a main desorption peak appears at a higher temperature of 850 K. Vacancy defects created by low-energy deuterium are the main reason for the appearance of the high desorption peak at 850 K.  
As compared to the low-fluence (1.30 \(\times\) 10\({}^{25}\) D/m\({}^{2}\)) case, the intensity of each desorption peak becomes much higher for the high-fluence (5.18 \(\times\) 10\({}^{25}\) D/m\({}^{2}\)) exposure, which is considered to be due to the deuterium atoms diffusing into a deeper depth of material. For both the W bulks and the W films, there is a clear increase in overall retention as a function of plasma fluence/exposure time. The same deuterium release tendency is found, in which the deuterium retention rises with increasing fluences for W samples [24,32]. Irradiation studies by [PERSON] et al.