7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Study of EUV spectrum from Kr25+ ion in Kr gas puffing experiment at the large
helical device using a collisional radiative plasma model
Shivam Gupta1, Tetsutarou Oishi2, and Izumi Murakami1, 31National Institute for Fusion Science, National Institutes of Natural Sciences, Toki, Japan
2Department of Quantum Science and Energy Engineering, Tohoku University, Sendai, Japan
3Graduate Institute for Advanced Studies, SOKENDAI, Toki, Japane-mail (speaker): shivam.gupta@nifs.ac.jp

High-temperature fusion plasma spectroscopy is anactive area of research for exploring various plasma
diagnostics in the context of magnetic confinement fusion
devices. Large tokamak and stellarator devices like ITER,DEMO, LHD, Wendelstein 7-X, JET, and others aredesigned to incorporate inert gases as external impurities,
which are introduced into the system to serve as coolant
gases [1-5].
In this study, we delve into the line emissions
originating from Na-like Kr25+ ion within the Extreme
Ultraviolet (EUV) range of 14-23 nm. These emissions
were observed during a Kr gas impurity seedingexperiment conducted at the Large Helical Device (LHD).
In the LHD plasma spectrum, we have identified emission
lines associated with the Kr25+ ion in the EUV wavelength
range, specifically at wavelengths of 22.00, 17.89, 16.51,
15.99 and 14.08 nm respective to 2p63p(2P1/2)-2p63s(2S1/2),
2p63p(2P3/2)-2p63s(2S1/2),
2p63d(2D3/2)-2p63p(2P3/2),
2p63d(2D5/2)-2p63p(2P3/2), and 2p63d(2D3/2)-2p63p(2P1/2)
transitions, respectively.
To provide a comprehensive understanding of theseobservations, we have developed a collisional radiativemodel. This model generates a synthetic spectrum for K25+
ion, allowing us to validate the atomic ion and electron
collision calculations against experimental measurement.
In developing a plasma model, we have incorporated vital
electron impact excitation processes, including theirreverse processes, and computed essential atomic
structure data and electron collision parameters using
various relativistic methods.
Our approach involved conducting Relativistic
Multiconfiguration Dirac-Hartree-Fock (RMCDHF) andRelativistic Configuration Interaction (RMCDHF-RCI)
calculations utilizing the General-Purpose AtomicStructure Package (GRASP 2018) [6]. Additionally, we
employed the relativistic Many-Body Perturbation(RMBPT) and RCI methods within the Flexible AtomicCode (FAC) [7]. These calculations yielded transitionenergies, oscillator strengths, transition rates, andwavelengths for the various fine structure transitions. A
thorough comparison of these parameters across different
methodologies and with previous results demonstrates
consistent and favorable agreement.
Furthermore, we calculated Electron Impact
Excitation (EIE) cross-sections for fine structuretransitions, considering both ground-to-excited state andamong the excited states transitions using the Relativistic
Distorted Wave (RDW) method [8]. These cross-section
results span a range of incident electron energies, from theexcitation threshold up to 21 keV. We determined
excitation rates for transitions from the ground states as afunction of electron temperature, extending up to 600 eV.
To incorporate these processes appropriately, we
integrated the excitation and de-excitation rates derived
from the cross-section data into the collisional radiativemodel. In this model, the rate balance equations weresimultaneously solved to determine the distribution ofstate populations for the considered fine structure levels.
To validate our findings, we compared the emission lines
from Kr25+ ion at wavelengths of 22.00, 17.89, 16.51,
15.99 and 14.08 nm measured in the LHD experimentwith the collisional radiative model spectrum. Our
comparative analysis demonstrates a strong overall
agreement between the experimental and theoreticalspectra. This comparison underscores the reliability of ouratomic ion structure, electron collision, and collisionalradiative model calculations for Kr25+ ion. These findings
can be utilized in the development of advanced plasma
models, enabling comprehensive investigations of fusion
plasma behavior in Kr seeding experiments acrossdifferent tokamak devices in the future.
Acknowledgements: The authors acknowledge Dr.
Motoshi Goto, Dr. Yasuko Kawamoto, Dr. Tomoko
Kawate, and the LHD experiment group for theirassistance in the LHD experiment.
References[1] A Kallenbach et al, Plasma Phys. Control. Fusion 55,124041 (2013).
[2] C. Dong el al, Plasma and Fusion Research 6, 2402078(2011).
[3] F. Effenberg et al, Nucl. Fusion 59, 106020 (2019).
[4] K. Mukai et al, Nucl. Fusion 61, 126018 (2021).
[5] T. Oishi et al, Plasma and Fusion Research 16,2402006 (2021).
[6] F. Fischer et al, Comput. Phys. Commun. 237, 184–
187 (2019).
[7] M. F. Gu, Can. J. Phys. 86, 675 (2008).
[8] S. Gupta, et al, J. Quant. Spectrosc. Radiat. Transf. 253,106992 (2020).