Soft X-Ray Spectroscopy of Rare-Earth Elements in LHD Plasmas

Abstract

Soft X-ray spectra from high Z rare-earth (lanthanide) elements have been systematically observed in optically thin, high-temperature plasmas produced in the Large Helical Device (LHD), a facility for magnetically confined fusion research. It has been demonstrated that the discrete and quasicontinuum (UTA) spectral features from highly charged lanthanide ions are observed depending on the plasma temperature. The analyses of the measured spectra are ongoing by comparisons with theoretical calculations and/or previous experimental data available. The discrete spectra recorded in high-temperature conditions are dominated by individual lines of Ge- to Ni-like ions, while prominent peaks in the narrowed UTA spectra observed in low-temperature conditions are well explained by the transitions of Ag- to Rh-like ions.

Spectra of Ga-Like to Cu-Like Praseodymium and Neodymium Ions Observed in the Large Helical Device

“…Though the electron temperature is not necessarily kept constant during one exposure time of the detector, the representative central electron temperatures at the middle of the corresponding period are indicated in Figures 1 and 2. As reported in our previous papers [19][20][21][22], the spectral feature of highly charged lanthanide ions changes from a quasi-continuum to discrete as the electron temperature rises. As the spectra in Figures 1 and 2 were observed in relatively high temperature conditions, above 0.8 keV, the quasi-continuum spectra emitted from lower ion stages were very well suppressed, and the spectra are composed only of discrete features due to higher ion stages such as Cu-, Zn-, and Ga-like ions.…”

“…The experiments were performed in the LHD, a large-scale magnetically confined torus device for fusion research, as described in our previous papers [19][20][21][22]. Accordingly, only a brief explanation is given here.…”

“…We have already carried out systematic observations of EUV spectra of lanthanide ions in the LHD in the last ten years [19][20][21][22]. Though the assignments of some of the isolated lines have been reported, most of the observed spectra are still being analyzed.…”

Detailed Analysis of Spectra from Ga-like Ions of Heavy Elements Observed in High-Temperature Plasmas

“…As mentioned in the previous papers [17,[19][20][21], discrete spectra due mainly to ions with 4s or 4p outermost electrons are observed in high-temperature conditions ( 1-3 keV). In contrast, quasi-continuum features (the so-called unresolved transition array) of ions with 4d or 4f outermost electrons are observed in low-temperature conditions (below 1 keV).…”

“…Magnetically confined torus plasmas are considered to be appropriate for producing spectroscopic data on highly charged heavy ions because of their favorable plasma parameters and the availability of reliable diagnostic tools. From this point of view, we have so far systematically recorded EUV and soft X-ray spectra of highly charged heavy ions in high-temperature plasmas produced in the Large Helical Device (LHD), a magnetically confined torus plasma device at the National Institute for Fusion Science (NIFS) [16][17][18][19][20][21][22][23]. In order to complete the missing experimental data, this article focuses on the Z-dependent properties of the isolated spectral lines of Ga-like heavy ions observed in LHD plasmas with electron temperatures of about 2-3 keV.…”