Matt Thompson

Probing helium nano-bubble formation in tungsten with grazing incidence small angle x-ray scattering

Helium nano-bubble formation in plasma facing materials has emerged as a major concern for the next-step fusion experiment ITER, where helium plasmas will be used during the tokamaks start-up phase. Here, we demonstrate that grazing incidence small-angle x-ray scattering is a powerful technique for the analysis of helium nano-bubble formation in tungsten. We measured helium bubbles with sizes between 1.5–2.5 nm in tungsten exposed to helium plasma at 700 °C, where a smaller number of larger bubbles were also observed. Depth distributions can be estimated by taking successive measurements across a range of x-ray incidence angles. Compared with traditional approaches in the field, such as transmission electron microscopy, this technique provides information across a much larger volume with high statistical precision, whilst also being non-destructive.

Pregnant women’s knowledge of obesity and ideal weight gain in pregnancy, and health behaviours of pregnant women and their partners

We conducted a survey among a convenience sample of 149 women and their partners (n = 136) attending antenatal services in the Canberra Hospital in 2010. Over one‐third (39%) of women and 63.6% of their partners were overweight/obese, and 69.2% of women had not received advice from their caregiver on their weight. Pregnant women and their partners’ health behaviours including smoking, and fruit and vegetable intake were strongly correlated (P ≤ 0.01). Pregnant women require targeted advice on their weight, ideal weight gain and impact of these on pregnancy.

Investigation of He–W interactions using DiMES on DIII-D

Tungsten button samples were exposed to He ELMing H-mode plasma in DIII-D using 2.3 MW of electron cyclotron heating power. Prior to the exposures, the W buttons were exposed to either He, or D, plasma in PISCES-A for 2000 s at surface temperatures of 225–850 °C to create a variety of surfaces (surface blisters, subsurface nano-bubbles, fuzz). Erosion was spectroscopically measured from each DiMES sample, with the exception of the fuzzy W samples which showed almost undetectable WI emission. Post-exposure grazing incidence small angle x-ray scattering surface analysis showed the formation of 1.5 nm diameter He bubbles in the surface of W buttons after only a single DIII-D (3 s, ~150 ELMs) discharge, similar to the bubble layer resulting from the 2000 s. exposure in PISCES-A. No surface roughening, or damage, was detected on the samples after approximately 600 ELMs with energy density between 0.04–0.1 MJ m−2.

Measuring helium bubble diameter distributions in tungsten with grazing incidence small angle x-ray scattering (GISAXS)

Grazing incidence small angle x-ray scattering was performed on tungsten samples exposed to helium plasma in the MAGPIE and Pisces-A linear plasma devices to measure the size distributions of resulting helium nano-bubbles. Nano-bubbles were fitted assuming spheroidal particles and an exponential diameter distribution. These particles had mean diameters between 0.36 and 0.62 nm. Pisces-A exposed samples showed more complex patterns, which may suggest the formation of faceted nano-bubbles or nano-scale surface structures.

Observation of a helium ion energy threshold for retention in tungsten exposed to hydrogen/helium mixture plasma

Helium retention is measured in tungsten samples exposed to mixed H/He plasma in the Magnum-PSI linear plasma device. It is observed that there is very little He retention below helium ion impact energies of 9.0 +- 1.4 eV, indicating the existence of a potential barrier which must be overcome for implantation to occur. The helium retention in samples exposed to plasma at temperatures  >1000 K is strongly correlated with nano-bubble formation measured using grazing incidence small-angle x-ray scattering. The diameters of nano-bubbles were not found to increase with increasing helium concentration, indicating that additional helium must be accommodated by increasing the bubble concentration or an increase in bubble pressure. For some samples pre-irradiation with heavy ions of 2.0 MeV energy is investigated to simulate the effects of neutron damage. It is observed that nano-bubble sizes are comparable between samples pre-irradiated with heavy-ions, and those without heavy-ion pre-irradiation.

Validation of GISAXS Model with TEM Data

Validation of GISAXS through comparisons with more widely used techniques such as TEM is important for many reasons. First and foremost, it serves as a test of the model and the assumptions which underpin it. Another aspect is that it helps build the profile of the technique within the wider fusion-materials community, by providing a clear demonstration of what it can do, and how it compares to already established techniques. In this section, a study is described where both GISAXS and TEM were performed on the same sample to measure He-induced nano-bubble diameter distributions in W, demonstrating close agreement between the two techniques. For TEM, nano-bubbles must be counted manually,1 so the number used to calculate the distribution is limited by instrument availability and the man-hours one is willing to commit to the task. The process of creating the samples, and the statistical analysis used for the TEM aspect of the study are described, followed by the GISAXS measurement conditions and some of the specific details of the model which was fitted for this sample. For comparison, a number of different nano-bubble size distribution models were tested.

Investigating Synergistic Effects on W Performance with Magnum-PSI

Understanding synergies is essential in order to better predict the performance of W based materials in future fusion experiments. In this chapter, a systematic study on the linear plasma device Magnum-PSI into the synergistic effects of H/He plasma composition, sample temperature, and radiation damage on H retention, He retention, and microstructural changes in W is described. Incident helium ion energy was found to have the most significant effect on heluium retention and the formation of sub-surface nanobubbles, while hydrogen retention is more strongly influenced by temperature.

Developing a GISAXS Model to Enable Study of Nano-bubble Formation

This chapter discusses the advantages of Grazing Incidence Small Angle X-ray Scattering (GISAXS) to measure sub-surface bubble formation in tungsten exposed to helium plasma. The GISAXS experimental requirements are first dicussed, along with a brief overview of the techniques advantages over other common techniques such as transmission electron microscopy. Next, the mathematics of X-ray propagation through materials is described, followed by an in-depth discussion of the physics of X-ray scattering from electron density fluctionations in materials. Other practical considerations, such as finite particle size distributions and fitting algorithms, and error estimation are then discussed. The detail provided in this chapter and attached references should be sufficient for anyone with a graduate-level physics background to understand the GISAXS technique.

Effect of Sample Temperature and Transient Heat Loading on Nano-bubble Growth

Temperature plays a critical role in the formation of He nano-bubbles in W. Generally, the higher the temperature the larger the bubbles which form, though there is evidence that nano-bubble sizes may not vary below a certain temperature. In this chapter, the results of a series of experiments utilising GISAXS to measure He nano-bubble formation are reported. To investigate steady-state temperature effects, separate experiments were conducted on both the NAGDIS-II and PISCES-A linear plasma devices. The influence of transient thermal loading was then studied by exposing W samples pre-loaded with He plasma in PISCES-A to an ELMy He discharge in the DIII-D tokamak. A further study was attempted where samples exposed to He plasma in MAGPIE were then subsequently annealed at 1073 K, however, surface oxidation destroyed the nano-structures which were to be studied.

Effect of He Fluence on Nano-bubble Growth

He loading and subsequent nano-structure formation is a serious concern for the development of plasma facing materials. In a large device such as iter, plasma facing surfaces will be exposed to very high plasma fluences over the life of the machine, so understanding the long-term effects of this particle exposure on material performance is essential. In order to develop a better understanding of this phenomenon the fusion community will need access to advanced techniques beyond TEM. In this chapter, a study is discussed which investigates the role of He fluence on nano-bubble growth using GISAXS, providing unprecedented precision for nano-bubble diameter measurements. This work was performed on two separate linear plasma devices: the Australian National University’s MAGnetised Plasma Interaction Experiment (MAGPIE), and the University of California, San Diego’s PISCES-A device.