Erosion properties of mixed tungsten and boron layers relevant for nuclear fusion devices

Abstract

Plasma-facing materials in fusion reactors are exposed to extremely harsh conditions. They have to withstand high temperatures and bom- bardment of energetic particles, escaping from the confined plasma. Tungsten has been selected a first wall material in ITER due to its favorable properties, such as high melting point and comparatively low sputter yield. One goal of ITER, is to maximize energy output. To improve plasma performance and re- duce impurities like oxygen, wall conditioning techniques such as boronization are applied [1]. Boronization then results in the deposition of a thin layer of boron on the reactor wall, which modifies the erosion properties of the wall. In a laboratory study the influence of boron on the sputter yield was investigated using thin films of varying tungsten-boron concentrations. Five samples, ranging from pure tungsten to pure boron, were irradiated with 2 keV Ar+ and sub- sequently 2 keV D+2 ions, simulating plasma con- ditions in fusion devices. Argon is commonly used as a seeding gas, while deuterium is a key component of the fusion fuel cycle. The sput- ter yield was measured using a quartz crystal microbalance (QCM), which enables the detec- tion of mass changes down to 10 pg cm→2 s→1 via eigenfrequency changes of the QCM setup [2]. The relationship between frequency change and mass change is described by the Sauerbrey equa- tion !f = →!m [3]. fQ mQ The results showed a clear trend: the total eroded mass decreased significantly with in- creasing boron content under 2 keV Ar+ irradi- ation. A similar, though less pronounced, behav- ior was observed for 2 keV D+2 irradiation. Converting the trend from sputtered mass/ion into sputtered atoms/ion, it was shown that the amount of sputtered tungsten particles decreased and the amount of boron particles increased with increasing boron concentration. The conversion was performed under the assumption that, under steady-state conditions, the ratio of the sputter yields of the components is equal to the ratio of their respective bulk concentrations. An excep- tion to the mentioned trend was the pure boron sample, where the amount of sputtered boron particles decreased again under 2 keV Ar+ irra- diation compared to the compound samples. An explanation for this trend might be the enhanced gettering property of pure boron compared to the compound samples. The trend towards lower sputtered mass and fewer sputtered tungsten particles is advanta- geous for nuclear fusion devices because it re- duces the amount of high-Z impurities, which severely impact reactor performance. References [1] J. Winter et al. J. Nucl. Mater. 162 (1989) [2] R. Stadlmayer et al. Rev. Sci. Instrum. 91 (2020) [3] G. Sauerbrey et al. Zeitschrift fu ̈r Physik 155 (1959)