Speaker
Description
Boronization plays a fundamental role in impurity control and plasma conditioning in fusion devices. With tungsten now implemented as the primary plasma-facing material in ITER, new constraints have emerged regarding impurity transport, plasma stability, and hydrogen isotope retention. One of the main motivations for using boron is the high affinity for oxygen and other impurities, a requirement essential to meet for ITER operational regimes: minimizing impurity release during plasma start-up and enabling accurate monitoring of tritium inventory on the plasma-facing components [1]. During plasma operation, ions and other plasma species bombard the boronized surfaces, leading to sputtering, erosion, and redeposition of boron, tungsten, and hydrogen isotopes. These interactions result in the gradual formation of boron–deuterium and boron–tungsten composite layers, which significantly influence impurity retention and overall plasma performance. However, there are not many studies in the literature that describe hydrogen isotopes behaviour in boron structures in crystalline and amorphous form, which makes H isotopes retention and release energies very hard to predict.
Hydrogen transport in boron structures is investigated using ab initio methods based on density functional theory (DFT) and the SIESTA package, for both crystalline and amorphous structures. An energy-landscape-based approach, Landscape Flooding Algorithm (LFA), is constructed and applied to identify saddle points and determine the most probable pathways of hydrogen within the boron network. In addition, hydrogen transport is modeled as a stochastic Markov process, where transition probabilities between metastable sites are governed by the computed energy barriers. The time-dependent probability distributions of hydrogen positions within the material is found by solving the Master equastions. This multi-scale approach describes the influence of local structural variations on diffusion behavior and gives informations on hydrogen retention and migration in boron-rich layers formed during plasma operations. Diffusion coefficients are further computed by calculating the mean squared displacement (MSD) based on time-probabilities.
[1] A. Bortolon, V. Rohde, R. Maingi, E. Wolfrum, R. Dux, A. Herrmann, R. Lunsford, R. McDermott, A. Nagy, A. Kallenbach, et al., Real-time wall conditioning by controlled injection of boron and boron nitride powder in full tungsten wall asdex upgrade, Nuclear Materials and Energy 19 (2019) 384–389.
Acknowledgement
This work was supported by a grant of the Romanian Ministry of Education and Research, CNCS-UEFISCDI, project number PN-IV-P2-2.1-TE-2023-1140, within PNCDI IV.