EUV Spectroscopic Analysis of Highly Charged Kr²⁴⁺ Ion for Impurity Seeding Studies in Fusion Plasmas via Electron-Impact Excitation and Collisional-Radiative Modeling

This study presents a spectroscopic investigation of Kr²⁴⁺ ion, generated via a krypton impurity seeding experiment in the Large Helical Device, aimed at supporting Extreme Ultraviolet (EUV) diagnostics of high-temperature fusion plasma. EUV spectral lines corresponding to fine-structure transitions among the 2p⁶3s², \, 2p⁶3s3p, \, 2p⁶3s3d, and 2p⁶3p² configurations were observed in the 12–25 nm wavelength range. To systematically analyze the measured emission lines, extensive relativistic atomic structure calculations were carried out over a broad configuration space, spanning more than 40 configurations, including core-excited and correlation-dominated states up to n≤ 7 and ≤ 4 . Bound-state wave functions were obtained using the relativistic many-body perturbation theory and configuration interaction method, implemented via the Flexible Atomic Code. Parallel calculations based on the relativistic multiconfiguration Dirac-Hartree-Fock method with configuration interaction were performed using the GRASP-2018 code to ensure numerical consistency. This article presents fine-structure-resolved excitation energies and transition parameters, including oscillator strengths and transition probabilities, for the relevant spectroscopic configurations up to 3 . Moreover, electron impact excitation and ionization cross-sections from the ground state ( 2p⁶3s² \, ¹S₀ ) as well as from selected excited states to higher-lying levels were calculated using the relativistic distorted wave method from the respective thresholds over a wide energy range. The corresponding Maxwellian averaged rate coefficients for excitation, de-excitation, ionization, and three-body recombination were evaluated over fusion-relevant electron temperatures. The results are presented for the prominent spectroscopic transitions up to 3 . These complete atomic and electron-collision datasets were incorporated into a suitable collisional-radiative model, accounting for the dominant population and depopulation processes, including electron impact excitation, de-excitation, ionization, radiative decay, and three-body recombination. The theoretically modeled EUV spectrum, calculated at nₑ=5.5× 10¹9 \, m⁻3 and Tₑ=578 \, eV , shows good agreement with experimental observations, validating the accuracy of the calculated atomic structure, and electron-collision parameters. The resulting atomic dataset and modeling framework enable detailed spectral analysis of highly charged Kr²⁴⁺ ion under magnetically confined fusion plasma conditions.