S15-4
Spectroscopy of Highly Charged Ions with an Electron Beam Ion Trap
電子ビームイオントラップによる多価電離イオンの分光測定
NAKAMURA Nobuyuki, SAKAUE Hiroyuki A., KATO Daiji, and MURAKAMI Izumi
中村信行，坂上裕之，加藤太治，村上泉
Institute for Laser Science, The University of Electro-Communications
1-5-1 Chofugaoka, Chofu, Tokyo 182-8585, Japan
電気通信大学レーザー新世代研究センター 〒182-8585 調布市調布ヶ丘1-5-1
We present spectra of highly charged heavy ions obtained with an electron beam ion trap (EBIT). In
particular, we focus on tungsten spectra, which is important for the diagnostics of the ITER plasma. Many
previously-unreported lines have been observed and identified.

1. Introduction
An electron beam ion trap (EBIT) [1] is a
powerful device to obtain the atomic data ofhighly charged ions needed for understanding andcontrolling high temperature plasmas, such as
fusion plasmas and the solar corona. It can trap
highly charged ions interacting with amonoenergetic electron beam for many hours. It
is thus regarded as a well-defined simple plasma
consisting of unidirectional monoenergetic
electrons and trapped ions with a narrow charge
state distribution. Consequently the EBIT plasma
is an unique and ideal source for high resolution
spectroscopic studies of highly charged ions.
Spectra from an EBIT are useful to survey andidentify previously unreported lines, and also toprovide benchmark for plasma models. An EBIT
can also be used as a device to study theinteractions of electrons with highly charged ions.
Such spectroscopic and collisional data can beobtained for ions over wide ranges of charge state
and atomic number; any ion of any element canpractically be studied. Interaction energy between
ions and electrons can also be varied over a wide
range, such as 100 eV to more than 100 keV.
2. Electron Beam Ion Trap (EBIT)An EBIT [1] was developed at the Lawrence
Livermore National Laboratory based on theprinciple of an electron beam ion source (EBIS) [2]developed at Joint Institute for Nuclear Research in
Dubna. An EBIT consists of a Penning-like ion trap
and a high-energy, high-density electron beam
going through the trap. Its main components are anelectron gun, a drift tube (ion trap), an electron
collector, and a superconducting magnet. The drift
tube is composed of three (or more) successive
cylindrical electrodes where a well potential isapplied for trapping ions axially. Radial ion trap isachieved by the combination of the strong axial

magnetic field produced by the magnet and thespace charge potential of the high density electron
beam compressed by the magnetic field. Highly
charged ions are produced by successive electron
impact ionization of the trapped ions. Emission ofhighly charged ions excited by the electron beam
can be studied spectroscopically through a slit
opened at the middle of the drift tube. Since thetrapped ions are produced and excited by an(quasi-)monoenergetic electron beam, an EBIT has
following advantages over plasma sources. (1) A
narrow charge state distribution can be obtainedwith a dominant charge state controlled by theelectron energy. (2) Electron energy dependent
emission processes, such as resonant excitation, canbe studied. (3) There is no Doppler shift and less
Doppler broadening. (4) Polarization of radiation
excited by a unidirectional electron beam can bestudied. We have developed a high-energy EBIT
called the Tokyo-EBIT [3] in 1995 and a compact
low-energy EBIT called CoBIT [4] in 2007. The
former can be operated with electron energies of 1
to 200 keV, and the latter can be operated withelectron energies of 0.1 to 2 keV. The
complementary use of them enables spectroscopic
studies of tungsten ions with a wide range of charge
state.
An element of interest is usually introduced
through the slit on the drift tube as a molecularbeam. Not only rare gases and molecular gases,
compounds which have a relatively high vapor
pressure can also be used. For example, for
producing tungsten ions, tungsten hexacarbonyl(W(CO)6) are used respectively.
We have several spectrometers for a wide
wavelength range: a crystal spectrometer for the
x-ray range, a grazing-incidence flat-field grating
spectrometer for the EUV to soft x-ray range, and a
Czerny-Turner spectrometer for the visible range.

3. Tungsten Spectra
Tungsten is considered to be the main impurity in
the ITER plasma, and thus spectroscopic data oftungsten ions are necessary to diagnose and control
the high temperature plasma in ITER [5]. In
particular, there is strong demand for emission lines
in the visible range in the diagnostics of the edge
plasmas [6]. Since efficient optical components,
such as mirrors, lenses, optical fibers, etc., areavailable, efficient and effective diagnostics can be
expected with the visible range. Until recently,
however, only one visible emission line [7] hasbeen reported for tungsten ions with a charge statehigher than two. Survey and identification ofpreviously unreported visible lines of tungsten ions
are thus in strong demand. An EBIT is a suitabledevice for such a purpose. As an example, tungsten
spectra obtained with CoBIT are shown in Fig. 1.
As seen in the figure, observed lines revealed strong
dependence on electron energy, i.e., they appeared
at a certain threshold energy and their intensity
became weak when the energy was further
increased. This strong dependence reflects the
charge distribution in the trap. For example, after
the electron energy was changed from 630 eV to675 eV, production of W23+ became available
because the ionization energy of W22+ is 643 eV [8].
The lines at around 409 and 432 nm, appeared at
675 eV, are thus considered to be emission lines
from W23+. When the energy was further increased
to 725 eV, which is higher than the ionizationenergy of W23+ (690 eV), the intensity of these lines
became small because the number of W23+ wasdecreased due to further ionization, and the linefrom W24+ appeared at around 419 nm. The validityof such identification based on the appearance
energy has been confirmed through several
previous experiments [9]. Consequently, the lines
indicated by arrows in the figure are assigned to be
the transition of tungsten ions shown in each
spectrum. Since transitions between different
electronic configurations in highly charged heavy
ions should fall in shorter wavelength range, suchas EUV and x-ray, transitions in the visible range
can be assigned as M1 transitions between finestructure levels. The detailed identification of thefine structure levels should be done throughcomparison with theoretical calculations. Although
it is rather difficult to calculate fine structure
splitting precisely for many electron heavy ions,some lines in Fig. 1 have been identified throughthe comparison with detail calculations [10]. Surveyof previously unreported lines is also possible withplasma sources, but observation of spectra excitedby a mono-energetic electron beam in an EBIT isquite useful for the identification of the responsible
charge state as shown here.

Fig.1. Visible spectra of highly charged tungsten ionsobserved with CoBIT. IP(Wq+) represents the ionization
potential of Wq+.

Acknowledgments
This work was performed with the support andunder the auspices of the NIFS Collaboration
Research program (NIFS09KOAJ003) and JSPS
KAKENHI Grant Number 23246165, and partly
supported by the JSPS-NRF-NSFC A3 Foresight
Program in the field of Plasma Physics (NSFC:
No.11261140328).
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