Influence of the Initial Defect Structure on Helium Trapping in Tungsten under Ion Implantation

The trapping of helium in tungsten irradiated with He+ ions with an energy of 3 keV and fluence of 10(19)-10(22) He/m(2) at room temperature was studied by thermal desorption spectroscopy and scanning electron microscopy. Both as-prepared and recrystallized (at 2000 K for 30 min prior to irradiation) tungsten foils with a thickness of 50 m were used. It was found that the initial structure of tungsten affects both the dynamics of helium accumulation and the size of defects formed in the process of irradiation and subsequent heating. At low irradiation fluences, helium desorption proceeds primarily at 2000-2500 K in recrystallized tungsten and at 1100-1900 K in as-prepared tungsten samples. At high fluences (higher than 10(21) He/m(2)), a considerable amount of helium is released at low temperatures (starting from 400 K), but a significant fraction of it remains in the samples even after heating to maximum temperatures. Analysis of cross-section of the samples performed after thermal desorption revealed pores 10-75 nm in diameter. The largest pores were formed in the samples that were recrystallized prior to irradiation.