The mechanism of tungsten (W) blistering under deuterium (D) plasma exposure is still under investigation. To clearly demonstrate the microstructure and nucleation mechanism of blistering on W exposed to D plasma, special recrystallized W disc samples were prepared and electropolished with back-thinned method for TEM observation. D plasma exposure (2.0  ×  1026 D m−2, 573 K) brought it numerous intra-granular blisters and protrusions on W with typical orientation dependence. TEM observation revealed the intra-granular blister microstructure that substantial dislocations were generated on the center and edge of blister via severe plastic deformation of blister cap. Dislocation tangles formed by dislocations from blisters were revealed and supposed to be the nucleation of intra-granular blister. By using the g · b  =  0 and g · b  ×  u  =  0 criterion, 〈0 0 1〉 dislocations with edge component were identified which were generated by 〈1 1 1〉 edge dislocation interaction in dislocation tangles. A {0 0 1}〈0 0 1〉 edge dislocation nucleating and blistering mechanism based on previous works is proposed, and by applying which blistering in recrystallized W could be well explained. Dedicated experiments demonstrated that intra-granular blister formation depends on local dislocation density which validated the mechanism.

Doppler broadening spectroscopy in the positron annihilation technique (DBS-PA) has been employed to investigate the defect properties in argon-damaged tungsten exposed to low-energy and high flux deuterium plasma. Argon ion irradiations with energy 500 keV are performed for tungsten samples with various levels of damage. The remarkable increment of the S parameter in DBS-PA indicates the introduction of vacancy-type defects in argon irradiated tungsten. An increase of ion fluence results in a continuous increase of the S parameter until saturation. Unexpectedly, a much higher fluence leads to a decrease of the S parameter in the near surface, and the (S,W) slope changes greatly. This should be associated with the formation of argon-vacancy complexes in the near surface produced by the excessive implanted argon ions. With deuterium plasma exposure, a significant decrease of the S parameter occurs in the pre-irradiated tungsten, suggesting the sharp reduction of the number and density of the vacancy-type defects. The thermal desorption spectroscopy results demonstrate that the argon-damaged tungsten, compared to the pristine one, exhibits an enhanced low-temperature desorption peak and an additional and broad high-temperature desorption peak, which indicates that deuterium atoms are trapped in both low-energy and high-energy sites. All these observations directly indicate the deuterium occupation of irradiation-induced vacancy defects in damaged tungsten, which is responsible for the remarkable increase of the deuterium retention in comparison with the pristine one.

In ITER and future tokamaks, recrystallization has been identified as an important issue which may reduce the strength of tungsten plasma-facing component and deteriorate its thermal shock resistance. In this study, isothermal annealing of un-exposed and helium-exposed tungsten was performed to investigate the effect of helium plasma exposure on the recrystallization kinetics. Rolled tungsten samples with a helium plasma fluence of 1.4 × 10 26 m −2 were annealed at temperatures ranging from 1273 K to 1973 K for 1 h. It was found that helium plasma exposure influence both the recrystallization stage (for 1423 K < T < 1573 K) and the grain growth stage (T > 1523 K). The results suggested that the retarding effect is caused by the impediment of high-angle grain boundaries migration by helium clusters and bubbles. Retarded recrystallization was observed at a depth up to a few micrometers beneath the surface. Present results demonstrate that helium plasma exposure plays an important role when qualifying the tungsten divertor performance under heat loading conditions.

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Surface morphology and deuterium retention in tungsten exposed at surface temperature of ~550 K to mixed deuterium-neon plasmas of different neon concentrations are investigated. It is found that the addition of neon up to 20% mitigates blistering on the surface. Cross-section view of the surface shows the formation of pores near the surface in the depth less than 100 nm. Deuterium depth profile is featured by an enhanced deuterium concentration within a depth of 16 nm but a mitigated penetration in depth larger than 1 µm. Deuterium retention is reduced by up to a factor of four. It is suggested the open pores formed in the surface serves as an escaping channel, mitigates deuterium penetration towards bulk and retention in the bulk.

To investigate the effect of blistering on hydrogen isotope (HI) retention, a series of deuterium plasma exposures were performed using recrystallized tungsten samples at 500 K with high fluences up to 1.0 × 10 28 ions m −2 in the linear plasma device STEP. An increase of blister density and deuterium retention was observed with increasing plasma fluence. Based on the simulation of the thermal desorption spectra using TMAP, defects with different detrapping energies are found to be located at a depth of tens of microns, which coincides with the depth of the grain boundaries (GBs) close to the surface. The defect characterizations using transmission electron microscopy and positron annihilation Doppler broadening identified the defects as dislocation type and vacancy type, which were created by blistering. It is suggested that these defects can diffuse deep into the material, and the interaction between the diffusion of the defects and GBs causes a peculiar deuterium desorption spectrum over plasma fluences. Additionally, these blister-induced defects are the main source of deuterium retention. Regarding the effect of the blister-induced defects on deuterium retention, a blister-dominated retention mechanism is proposed to describe HI retention in conditions when blistering is severe as in this study. This investigation provides a new insight into the effect of blistering on retention and the modelling of retention in a tokamak edge plasma environment.

Effects of iron ion pre-irradiation with energy of 1 MeV on deuterium retention and blistering in tungsten has been investigated after exposure to low-energy (40 eV) and high flux (~1022 D m−2 s−1) deuterium plasma under high fluence of 1  ×  1026 D m−2 with various surface temperatures (450 K, 550 K and 750 K). The undamaged and pre-damaged tungsten show a similar temperature dependence of deuterium-induced blistering, as well as deuterium retention in the near surface (within 300 nm). Deuterium concentration in the near surface decreases with increasing the surface temperature in deuterium plasma exposure. For both cases, the most serious blistering is present at the surface temperature of 550 K. Due to the creation of pre-damage the size of surface blister becomes much larger and the near-surface deuterium concentration becomes higher. Analysis of blistering-related cavities in the pre-damaged and undamaged tungsten indicates that in pre-damaged tungsten deuterium aggregates in the place with a much larger depth. Based on the comparison of the pre-damage induced increment of deuterium retention within the near surface region (damage layer) and in the bulk, it reveals that the pre-damage enhances deuterium retention in the bulk more strongly than that in the near-surface region, suggesting that the deuterium flux diffusing into the bulk of pre-damaged tungsten is largely improved.

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