Electronic states of highly charged ions show magnified fine-structure, Lamb shift, and hyperfine effects making them sensitive probes of bound-state quantum electrodynamics and nuclear physics. Being also impervious to external perturbations renders them ideal candidates for precision spectroscopy and accurate clocks that could test physics beyond the standard model. This review discusses how a variety of ion species and transitions may optimally be used to target such new applications, and presents routes to handle them in the laboratory.

Highly charged ions: Optical clocks and applications in fundamental physics

We present spectra of highly charged Fe ions in the extreme ultraviolet range observed using an electron beam ion trap equipped with a flat-field grazing incidence spectrometer. The density dependence of line intensity ratios is investigated for several density-sensitive lines of Fe XIII, XIV, and XV. Unlike previous studies where the electron density was estimated from theoretical considerations, here it is derived from electron beam profile measurements. The experimental data are compared with model calculations.

https://iopscience.iop.org/article/10.1088/0004-637X/739/1/17/pdf

Intensity Ratio of Density-sensitive Lines in Fe Ions Observed with a Well-defined Laboratory Plasma

Quantitative tungsten study with reliable atomic modeling is important for successful achievement of ITER and fusion reactors. We have developed tungsten atomic modeling for understanding the tungsten behavior in fusion plasmas. The modeling is applied to the analysis of tungsten spectra observed from plasmas of the large helical device (LHD) with tungsten pellet injection. We found that extreme ultraviolet (EUV) emission of W24+ to W33+ ions at 1.5–3.5 nm are sensitive to electron temperature and useful to examine the tungsten behavior in edge plasmas. We can reproduce measured EUV spectra at 1.5–3.5 nm by calculated spectra with the tungsten atomic model and obtain charge state distributions of tungsten ions in LHD plasmas at different temperatures around 1 keV. Our model is applied to calculate the unresolved transition array (UTA) seen at 4.5–7 nm tungsten spectra. We analyze the effect of configuration interaction on population kinetics related to the UTA structure in detail and find the importance of two-electron-one-photon transitions between 4p54dn+1– 4p64dn−14f. Radiation power rate of tungsten due to line emissions is also estimated with the model and is consistent with other models within factor 2.

Development of quantitative atomic modeling for tungsten transport study using LHD plasma with tungsten pellet injection

In this paper, a new compact low energy electron beam ion trap, SH-PermEBIT, is reported. This electron beam ion trap (EBIT) can operate in the electron energy range of 60-5000 eV, with a current density of up to 100 A/cm(2). The low energy limit of this machine sets the record among the reported works so far. The magnetic field in the central drift tube region of this EBIT is around 0.5 T, produced by permanent magnets and soft iron. The design of this EBIT allows adjustment of the electron gun's axial position in the fringe field of the central magnetic field. This turned out to be very important for optimizing the magnetic field in the region of the electron gun and particularly important for low electron beam energy operation, since the magnetic field strength is not tunable with permanent magnets. In this work, transmission of the electron beam as well as the upper limit of the electron beam width under several conditions are measured. Spectral results from test operation of this EBIT at the electron energies of 60, 315, 2800, and 4100 eV are also reported.

A very low energy compact electron beam ion trap for spectroscopic research in Shanghai.

We report on the identification of the optimum plasma conditions for a laser-produced plasma source for efficient coupling with multilayer mirrors at 6.x nm for beyond extreme ultraviolet lithography. A small shift to lower energies of the peak emission for Nd:YAG laser-produced gadolinium plasmas was observed with increasing laser power density. Charge-defined emission spectra were observed in electron beam ion trap (EBIT) studies and the charge states responsible identified by use of the flexible atomic code (FAC). The EBIT spectra displayed a larger systematic shift of the peak wavelength of intense emission at 6.x nm to longer wavelengths with increasing ionic charge. This combination of spectra enabled the key ion stage to be confirmed as Gd18+, over a range of laser power densities, with contributions from Gd17+ and Gd19+ responsible for the slight shift to longer wavelengths in the laser-plasma spectra. The FAC calculation also identified the origin of observed out-of-band emission and the charge states responsible.

Tuning extreme ultraviolet emission for optimum coupling with multilayer mirrors for future lithography through control of ionic charge states

A compact electron beam ion trap (EBIT) has been constructed for spectroscopic studies of moderate charge state ions. The electron beam energy range of the present EBIT is 100–1000eV, for which it is rather difficult to operate an ordinary EBIT which used to be designed for operation with higher electron energy (∼10keV or more). To cut down the running costs, a superconducting wire with a high critical temperature is used for the central magnet so that it can be operated without liquid helium. The performance of the compact EBIT has been investigated through visible spectroscopy of highly charged krypton and iron ions.

Compact electron beam ion trap for spectroscopy of moderate charge state ions.

The visible transition of a multiply charged Ba ion was observed by using an electron beam ion trap. From the dependence of the line intensity on electron beam energy, it is identified as a magnetic dipole transition between ground-state fine-structure levels (4d9 2D5/2–2D3/2) of Rh-like Ba. The wavelength determined from the present experiment is compared with the existing theoretical values as well as the indirect experimental value estimated from the wavelength difference between previously reported VUV transitions 4d85p–4d9 2D5/2 and 2D3/2.

https://iopscience.iop.org/article/10.1088/0031-8949/2011/T144/014011/pdf

Hiroyuki Kikuchi

Papers

Compact electron beam ion trap for spectroscopy of moderate charge state ions.

Visible spectroscopy of Rh-like ions