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

Experimental identification of new soft X-ray spectral lines of highly charged
heavy ions through Z-dependence analysis
C. Suzuki1, F. Koike2, I. Murakami1, N. Tamura1, T. Oishi3, N. Nakamura4
1
National Institute for Fusion Science, National Institutes of Natural Sciences2
Department of Materials and Life Sciences, Sophia University
3
Department of Quantum Science and Energy Engineering, Tohoku University
4
Institute for Laser Science, The University of Electro-Communications
e-mail (speaker): csuzuki@nifs.ac.jp
Experimental data on soft X-ray spectral lines of highly
charged heavy ions are important for plasma diagnostics
relevant to nuclear fusion research and industrial lightsources. Besides the applications, such spectra are ofgreat interest in terms of basic atomic physics. Though
soft X-ray spectra from various heavy ions have beenmeasured so far in tokamaks, laser-produced plasmas
and electron beam ion traps (EBITs), the available data
are still incomplete for the elements with atomic
numbers approximately larger than 50.
In this study we investigate the atomic number (Z)dependence of soft X-ray spectra from heavy ions having
outermost N shell electrons, mainly based on theobservations in high-temperature plasmas produced in
the Large Helical Device (LHD) at the National Institutefor Fusion Science (NIFS). In particular, we focus on
isolated spectral lines from ions with one or twooutermost 4s/4p subshell electrons (i.e., Cu-, Zn-, Gaand Ge-like ions) which give rise to relatively simple
spectral structure. In LHD, a small amount of heavyelements are injected by a tracer-encapsulated solidpellet (TESPEL) [1], and the temporal evolutions of soft
X-ray spectra are recorded by multiple grazing incidence
spectrometers.
In order to complement the missing elements in the pastexperiments, we have systematically observed spectrafrom almost all heavy elements with Z of 57–74 in LHD[2–5]. As an example, spectra from cerium andpraseodymium ions are shown in Fig. 1 in which theresonance lines of Cu- and Zn-like ions are clearly seen.
The measured wavelengths are compared with the resultsof the past EBIT experiments as well as our new
experiment at Tokyo EBIT for europium. Consequently,
we could successfully identify many new lines for theprominent transitions by interpolation or extrapolation of
the Z dependence of the existing data.
Figure 2 plots the Z dependence of the wavelength ofthe (4s)1/2 – (4p+)3/2 resonance transition of Cu-like ions.
The measurements in LHD and EBITs are indicated bydiamonds and squares, respectively, together with thetheoretical values calculated with a multi-configuration
Dirac Fock code (GRASP92) by a dotted line. It isclearly shown that the measured wavelengths line up
along a smooth curve, and the calculated wavelengths
are slightly shifted to shorter side. Consequently, thelines for Z=57–59, 63, 65, 67, 69 and 71 areexperimentally identified for the first time in this study.
Similar Z dependence analyses have been carried out for
major transitions of various ion stages. Accordingly, thepresent study would give fundamental data to study
atomic physics specific to highly charged heavy ions.
References[1] S. Sudo and N. Tamura, Rev. Sci. Instrum. 83 023503(2012)[2] C. Suzuki et al, Plasma Phys. Control. Fusion 59
014009 (2017)[3] C. Suzuki et al, Atoms 6 24 (2018)[4] C. Suzuki et al, J. Phys. B: At. Mol. Opt. Phys. 54015001 (2020)[5] C. Suzuki et al, Atoms 9 46 (2021)[6] N. Nakamura et al, Phys. Scr. T73 362 (1997)[7] F. Koike et al, Phys. Rev. A 105 032802 (2022)

Figure 1. Soft X-ray spectra of cerium (top) andpraseodymium (bottom) ions in LHD plasmas. The
prominent lines from Cu-like and Zn-like ions areindicated by brackets.

Figure 2. Z dependence of the wavelengths of the (4s)1/2– (4p+)3/2 transition of Cu-like ions. The measurements in
LHD (diamonds) and EBITs (squares) are shown
together with the calculated values (dotted line).