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Description
The new ITER baseline foresees boronizations for reliable plasma operations in the initial stages of experimental campaigns. However, boron can enhance the fuel retention in the wall and decrease the availability of ITER. Therefore, it is essential to understand the erosion and redeposition mechanisms of boron and its codeposition with hydrogen isotopes.
We performed studies of boron erosion and redeposition in deuterium plasma in the linear plasma device PSI-2. PSI-2 produces a steady-state plasma column with a diameter of ~70 mm. PSI-2 is equipped with a quartz microbalance (QMB) system, an in-situ deposition monitor, placed 175 mm away from the plasma axis. A disc of pure boron with an exposed diameter of 55 mm, fixed by a Mo mask, was used as the target. The axial distance between target and QMB was varied 5-35 cm by changing the target position. According to its solid angle, the QMB detector collects a fraction of ~10-4 - 10-5 of particles eroded from the target. The temperature of QMB was kept at RT by water cooling. Simultaneously the eroded boron was monitored by optical emission spectroscopy (OES) of the B I 2p-3s transition at 249.7 nm. The incident ion flux was 5x1022 m-2s-1 and the ion energy was varied 30 - 100 eV by biasing the target. After the exposure, the quartz detector was analysed by scanning electron microscopy (SEM) and nuclear reaction analysis (NRA).
The maximum deposition rate at QMB of 10 nm/hour was obtained for 100 eV and a target position 15-20 cm away from QMB. For 60 eV and 30 eV the deposition rate dropped by factors of 2.2 and 6, respectively. The drop of the OES signal intensity was less steep, 1.8 for 60 eV and 4.5 for 30 eV. In general, this behaviour reproduces the data available for boron sputtering from previous ion beam and OES studies.
SEM observed a layer of boron deposited on the detector plate with a thickness of 10-20 nm, in agreement with QMB. The estimated layer density is 1.1 g/cm3. NRA measured 10x1015 B/cm2 and 5x1015 D/cm2 in the boron layer. A significant D amount of 20x1015 D/cm2 was detected in the substrate. The SEM and NRA results showed that the quartz plate is not the optimal substrate for the post-mortem analyses. Additional experiments with a polished tungsten substrate are planned.