Description
When dominant electron heating prevails, the electron velocity distribution function deviates from being Maxwellian, developing a distinct suprathermal tail. Furthermore, impurity contamination by high-Z ions, such as tungsten (W), expected to serve as the first wall material in modern tokamaks (ITER, WEST), poses a serious challenge for the stable and efficient operation of tokamaks featuring such a first wall.
This contribution addresses issues that remain poorly documented in the literature, where non-Maxwellian electron distribution and transport effects are mostly neglected. To this end, theoretical and numerical tools are being developed to calculate the ionization equilibrium of high-Z impurities in plasmas with non-Maxwellian electron velocity distributions and moderate particle transport, as typically occurs in tokamaks with strong electron heating.
The effect of such impurities on the suprathermal electron population can be studied using a Fokker-Planck solver. However, in this case, it is necessary to modify the electron-ion collision operator to account for the influence of partially ionized high-Z elements resulting from the uncontrolled impurity influx [L. Hesslow et al., Phys. Rev. Lett. 118, 255001 (2017)]. This, in turn, requires atomic models that are sufficiently accurate yet also allow for fast and efficient calculations for all elements present in the plasma, regardless of their local ionization level.
Several simple atomic models have been proposed and implemented in the Fokker-Planck solver (LUKE code [Y. Peysson et al., Plasma Phys. Control. Fusion 58, 044008 (2016)], developed at CEA). These semi-empirical atomic models for elastic and inelastic collisions involving fast electrons and ions have been calibrated and optimized [Y. Savoye-Peysson et al., Nucl. Fusion 63, 126041 (2023), J. Walkowiak et al., At. Data Nucl. Data Tables 161, 101696 (2025)].
The outcome of this work will be the development of diagnostic tools for the experimental visualization of the spatial distribution of impurities and their spectral emissivity characteristics, as well as for the identification of particle transport and non-Maxwellian effects. This will involve the development and use of synthetic diagnostic tools, tomographic reconstruction, and X-ray spectroscopy methods [A. Jardin et al., Phys. Plasmas 31, 082514 (2024), D. Mazon et al., Rev. Sci. Instrum. 96, 063509 (2025)].
Acknowledgements. This work was partially funded by National Science Centre, Poland (NCN) grant OPUS 29 no. 2025/57/B/ST2/04168. We gratefully acknowledge Polish high-performance computing infrastructure PLGrid (HPC Centers: ACK Cyfronet AGH, CI TASK, WCSS) for providing computer facilities and support within computational grants no. PLG/2025/017971 and no. PLG/2026/019127.