Measuring helium nano-bubble formation in tungsten with grazing-incidence small angle X-ray scattering
The behaviour of helium in tungsten is an important concern for the fusion materials community. Under helium plasma exposure, small nano-scale bubbles form beneath the material surface as helium precipitates from the tungsten matrix. Under certain conditions this can lead to the subsequent formation of a surface “nano-fuzz”, though the mechanisms of this process are not presently understood. Here, grazing incidence small angle X-ray scattering (GISAXS) is presented as a powerful addition to the field of fusion materials. With GISAXS, one can measure the x-ray scattering from nano-scale features throughout a relatively large volume, allowing information about full nano-bubble size distributions to be obtained from a simple, non-destructive measurement. Where it typically takes days or weeks to prepare a sample and study it under TEM, GISAXS measurements can be performed in a matter of minutes, and the data analysis performed autonomously by a computer in hours.
A GISAXS pattern fitting model was first developed, and then validated via comparison between GISAXS and TEM measurements of helium induced nano-bubble formation in tungsten exposed to a helium discharge in the large helical device. Under these conditions, nano-bubbles were found to follow an approximately exponential diameter distribution, with a mean nano-bubble diameters and computed for GISAXS and TEM, respectively. Finally, synergistic effects between plasma composition and sample temperature are explored to determine which factors are most relevant for hydrogen and helium retention. Here, evidence has been found that helium ions from the plasma require a minimum energy of ~9 eV in order to be implanted into tungsten. This was the dominant factor governing helium retention in this experiment. On the other hand, sample temperature is the dominant factor for hydrogen retention.