Speaker
Description
Plasma-facing components in fusion reactors are exposed to extreme conditions including high heat flux and bombardment by energetic particles. Tungsten, the chosen first wall material in ITER, offers favorable properties such as a high melting point and low sputter yield. To enhance plasma performance and minimize impurity influx such as oxygen, wall conditioning techniques like boronization are applied [1]. Boronization deposits a thin boron layer on the reactor wall, which modifies surface properties and its erosion behavior.
This laboratory study investigates the effect of varying boron concentrations on the sputtering behavior of tungsten-boron (W-B) thin films under ion bombardment. Five samples, ranging from pure tungsten via 3 different W:B mixtures to pure boron, were irradiated with $2\,\mathrm{keV}\,\mathrm{Ar}^+$ and subsequently $2\,\mathrm{keV}\,\mathrm{D}_2^+$ ions to simulate fusion plasma conditions. Argon serves as a seeding gas, while deuterium is a main component of the fusion fuel cycle. The elemental compositions of the samples were characterized ex situ by time-of-flight elastic recoil detection analysis (ToF-ERDA) before and after irradiation [2]. Additional high-resolution Rutherford backscattering spectrometry (HR-RBS) was performed after irradiation.
Sputter yields were quantified using a quartz crystal microbalance (QCM), capable of detecting mass changes down to $10\,\mathrm{pg\,cm^{-2}\,s^{-1}}$ through eigenfrequency shifts of the oscillating quartz [3]. The relationship between frequency and mass change is described by the Sauerbrey equation [4].
The results revealed a clear trend: the total eroded mass decreased with increasing boron content under both $\mathrm{Ar}^+$ and $\mathrm{D}_2^+$ irradiation with the effect more pronounced for $2\,\mathrm{keV}\,\mathrm{Ar}^+$ irradiation.
The measured mass loss was converted into atoms per incident ion using the known film composition from ToF-ERDA and assuming steady state conditions. Our data show that tungsten sputtering decreased and boron sputtering increased with higher boron concentrations.
These findings highlight the beneficial impact of boron enrichment on reducing high-Z impurity sources in fusion environments, supporting boronization as an effective wall conditioning technique.
[1] J. Winter $\textit{et al. J. Nucl. Mater.}$ $\textbf{162}$ 713 (1989).
[2] E. Pitthan $\textit{et al. Surf. Coat. Technol.}$ $\textbf{417}$ 127188 (2021)
[3] R. Stadlmayer $\textit{et al. Rev. Sci. Instrum.}$ $\textbf{91}$ 125104 (2020).
[4] G. Sauerbrey $\textit{Z. Phys.}$ $\textbf{155}$ 206 (1959).