Speaker
Description
The recent decision from ITER to replace the beryllium first wall with tungsten requires boronization on the first wall to capture sufficient oxygen and minimise plasma cooling caused by high-Z plasma material interactions. The deposited layer thickness and frequency of boronizations are critical, since redeposition in remote locations leads to thick deposits, potentially causing flaking and enhanced tritium retention. These lead to concerns for safe machine operation via dust production from the deposits and tritium accumulation within them.
Upgraded Pilot-PSI (UPP) experiments were carried out on thin (0.8 $\mu$m) boron films pre-deposited on Si substrates by Pulsed Laser Deposition (PLD). The experiments aimed to distinguish chemical and physical erosion by measuring erosion rates at ion impact energies between 2.26 and 72.26 eV, both below and above the physical sputtering threshold. ITER-relevant high-flux deuterium plasmas were used to load the targets at surface temperatures of 400 K, while in-situ ion-beam analysis was used to determine the erosion via Elastic Backscattering Spectrometry (EBS) and deuterium retention via Nuclear Reaction Analysis (NRA), allowing contamination-free, time-resolved measurements in between plasma exposure. In-situ ion-beam analysis was performed using a 1.5 MeV H beam for Elastic Backscattering Spectrometry (EBS) and a 2.6 MeV $^3$He beam optimised for boron and deuterium thickness determination. During plasma exposure, Thomson Scattering (TS) measured deuterium plasma conditions of $T_e = 0.93$ eV and $n_e = 0.16\times 10^{20}$ m$^{-3}$, corresponding to a deuterium flux of $8.61\times 10^{22}$ D/m$^2$. These parameters provide ITER-relevant high-flux conditions for quantifying chemical versus physical erosion mechanisms and investigating deuterium retention.
The erosion experiments revealed an erosion yield of $(9.22 \pm 1.04)\times 10^{-4}\,\text{atoms/ion}$ at $E_{\text{ion}} = 2.26\pm0.06$ eV. Deuterium retention ($\phi$) in the boron layer was measured after 600 seconds of low impact energy ($E_{\text{ion}} = 2.26\pm0.06$ eV) deuterium plasma exposure, remained below $\phi=15\times10^{15} \text{ atoms/m}^2$ and exhibited a power-law time (t) dependence described by $\phi = 2.04\, \times \text{t}^{0.28}$. Additional outgassing measurements showed a significant (approximately 40$\%$) long-term outgassing after 60 hours in vacuum. These results indicate that substantial chemical erosion occurs below the physical sputtering threshold energy and needs to be considered in order to accurately extrapolate the expected boron layer lifetime and boronization frequency for future fusion reactors. Conversely, the deuterium retention in the boron is low, likely limited by the chemical erosion. A broader discussion of the energy and temperature dependence of the erosion and retention processes will be presented.