Erosion of thin boron films at the linear plasma device PSI-2 during deuterium discharges: Atomic and molecular spectroscopy of boron

This study reports on deuterium plasma-boron layer interaction investigated at the linear plasma device PSI-2. In preparation, RF magnetron sputtering deposited 115 nm thick boron layers onto tungsten substrates. Exposing the samples to a deuterium plasma in PSI-2 provided the bombardment by ions. The discharge conditions in PSI-2 were chosen such that the bombardment was predominantly due to deuterons (D + ) impinging at normal incidence. This operating scenario yielded a typical ion flux density onto the sample of 4 × 10 21 m −2 s −1 . Varying the bias applied to the sample from -43 V (floating potential) to -100 V allowed for investigating the near-threshold erosion regime. The B I 2p-3s transition (249.7 nm), and the BD A-X transition (432.8 nm) provided the time-resolved (10 s) spectroscopic fingerprint of physical and chemical erosion during the plasma discharges of 70 s duration. The emission of the BD molecular band depends only weakly on the bias applied to the sample. Notably, this emission is also present at the lowest impact energies, suggesting a contribution by chemical erosion. Contrary to that, atomic boron emission follows reasonably close to the expectations by the binary collision approximation (BCA). The post-mortem layer thickness characterization reveals net sputtering yields that exceed the predictions by BCA and agree well with data from Hechtl et al. (1992). • Erosion study of ITER relevant 115 nm boron layer under deuteron bombardment. • 70 s partial erosion experiments performed to measure sputtering yields. • Sputtering yields higher than predicted by binary collision approximation (BCA). • Atomic emission intensity depends on the ion energy in line with BCA. • Molecular emission intensity is essentially independent of the ion energy.