Page 6 - Outer Divertor Damage Characterization from Deuterium Plasma Bombardment in Graphene-Coated Tungsten

OUTER DIVERTOR DAMAGE CHARACTERIZATION FROM DEUTERIUM PLASMA BOMBARDMENT · NAVARRO et al. 547
bonds to preserve its structural stability. While we do see
the rise in defect production, as more defects are introduced
in the membrane (beyond a damage ratio ID/IG > 3), it
becomes more difficult for the carbon atoms to form the
sp2 bonds of graphene and starts to form more sp3 (single
bonds). Amorphization causes the ~1585 cm−1 G-band to
shift all the way down to 1550 cm−1 once the graphene has
regions at least 1 mm apart to find a global defect ratio across the film. This is done by integrating the area under the D-band (centered around 1350 cm−1 and the G-band (centered around 1585 cm−1). Figure 6 shows ID/IG for the graphene-coated sample before and after irradiation. As presented in Fig. 4, the G-band did not shift down in wavenumber, and the ratio of the 2D-band to the G-band remains similar before and after exposures. A significant red shift of the G-band (e.g., from 1600 to 1550 cm−1) and a significant decrease in the 2D-band (centered at 2700 cm−1) intensity (I2D/IG ≪ 1) would imply that the graphene membrane fails due to a large number of defects and subsequent amorphization. As no significant changes in either band were detected at the fluences in this study, the graphene membrane is still below its failure limit.
III.C. Scanning Electron Microscopy Results for Deuterium Exposures
Scanning electron microscopy analysis provides an insight into surface morphologies and impurity deposition on the surface of these samples (Fig. 7). No helium-based morphologies develop on the surface of either tungsten
Fig. 6. Graphene damage ratio growth for sample W6 after C-2W exposure to a deuterium plasma. Although the damage ratio increased, the membrane remains below its failure threshold.
surface. The grains are visible on both samples prior to the irradiations, but once the exposures are completed, a thick layer of impurities covers the grains, and any information on the tungsten surface damage is difficult to diagnose. There is, however, a striking difference in the surfaces of the two samples after the deuterium exposure, demonstrating that graphene-covered surfaces are collecting less impurities than a bare tungsten surface; this effect was consistent across the entire sample. This collection of other materials on the surface led to a mass gain of the samples. Comparison of weights before and after the exposures showed that the bare tungsten collected twice as much mass of impurities as the graphene-coated sample (Fig. 8). We believe this result is caused by the low surface energy of graphene. Plasma vacuum vessels comprise structural materials different from tungsten (such as stainless steel in the case of the C-2W divertor and titanium), each having larger sputtering yields and secondary electron emission coefficients. The addition of the graphene membrane can prevent these materials from collecting in regions of large plasma fluxes, which could contribute in lowering the electron emission as well as resputtering and traveling along magnetic field lines and into the plasma, reducing the core temperature.
X-ray photoelectron spectroscopy was used to
attempt to identify the impurities collected on the two
W surfaces. Other than volatile light elements, we
observed traces of materials present in the vacuum vessel
as well as the divertor. Graphene has an extremely low
surface energy32,33 compared to metals found in the
34–38
surface of bare tungsten.
IV. CONCLUSIONS
Initial characterization of graphene-coated samples in the C-2W divertor shows that coating a tungsten surface with a graphene membrane reduces the amount of impurities collected on the surface possibly because the low surface energy of graphene makes it difficult for impurities to adhere to its surface, as opposed to the higher concentration of impurities collected on bare tungsten. The energy analyzer data are able to provide crucial information on the Maxwellian deuterium plasma exposures, demonstrating large fluctuations in ion energies at different
been completely amorphized.
Raman data were averaged across ten separate
24,29–31
The current divertor is made of stainless steel (iron and chromium), and titanium is used as an oxygen getter. The higher surface energy of metals makes it favorable for creating new bonds, which can explain the larger collection of other metals on the
C-2W device.
FUSION SCIENCE AND TECHNOLOGY · VOLUME 75 · AUGUST 2019