Speaker
Description
Increased hydrogen isotope retention in materials damaged by energetic neutrons has been a major concern for the operation of fusion reactors, as it leads to the loss of precious fusion fuel. Laboratory studies on the effect of such damage on deuterium (D) retention in tungsten (W) rely primarily on self-ion damaging of the material as a surrogate for neutrons. Most such studies were performed in the sequential manner—the W samples were first self-damaged and later exposed to D to populate the defects. The first studies conducted in the simultaneous manner with low-flux D atoms and ions [1,2] revealed an important role of the synergistic effects and D retention, differing largely from the sequential case.
The new POSEIDON experimental system, consisting of the PISCES-RF linear plasma device coupled with a 3 MV ion beam accelerator, allows us to conduct simultaneous damaging and plasma exposure at fusion-relevant D fluxes. Four W samples were exposed to D plasma at 333 K to a flux of $1.8\times 10^{22}\;\mathrm{D/m^2s}$ for 3 hours, resulting in a fluence of $1.9\times 10^{26}\;\mathrm{D/m^2}$. After 1 hour of plasma exposure, a defocused 20 MeV W$^{6+}$ ion beam was used to self-damage the samples during the ongoing D plasma exposure. The damage rate was around $6\times 10^{-5}\;\mathrm{dpa/s}$, and the damaging time varied from 40 s to 2 hours, resulting in 0.002, 0.02, 0.2, and 0.35 dpa. After the damaging, the D plasma exposure continued until the desired fluence was reached.
He-3 nuclear reaction analysis (NRA) was utilized to measure D depth profiles in the simultaneously damaged and plasma-exposed samples. The depth profiles indicate that D did not diffuse throughout the entire expected thickness of the damaged region. Still, D concentration shows a 3- to 4-fold increase compared to sequentially self-damaged and plasma-exposed samples [3], despite D not yet reaching the depth of the peak damage. Preliminary thermal desorption spectroscopy (TDS) spectra show a strong increase of the low-temperature peaks and a moderate decrease of the high-temperature peaks for an overall ~50% increase in total D amount. Additional D plasma exposures will be conducted to populate all the remaining defects, and additional NRA and TDS analyses will be performed.
This work is supported by US-DOE-FES under agreement DE-SC0022528.
[1] S. Markelj et al., Nucl. Mater. Energy 12 (2017) 169
[2] S. Markelj et al., Nucl. Fusion 59 (2019) 086050
[3] T. Schwarz-Selinger, Mater. Res. Express 10 (2023) 102002