J. Plasma Fusion Res. SERIES, Vol.7 (2006) 31-34

EUV Spectra from Xe10+ Ions Measured from LHD
OBARA Tomoyuki, KATO Takako1 , KATO Daiji1 , MORE Richard1 , SATO Kuninori1 ,FUNABA Hisamichi1 , MORITA Shigeru1 , NISHIMURA Hiroaki2 , NISHIHARA Katsunobu2 ,SASAKI Akira3 , YAMAMOTO Norimasa4 and SAFRONOVA Ulyana5
Graduate University for Advanced Studies, Kanagawa 240-0193, Japan
1 National Institute for Fusion Science, Toki 509-5292, Japan
2 Institute of Laser Engineering, Osaka University, Osaka 565-0871, Japan
3 KANSAI Research Establishment, Japan Atomic Energy Research Institute, Japan
4 Department of Physics, Rikkyo University, Tokyo 171-8501, Japan
5 University of Nevada, Reno, USA（Received: 28 October 2004 / Accepted: 29 September 2005）

Abstract
We have observed EUV spectra from Xenon ions in Large Helical Device (LHD) in the wavelength rangeof 10 − 17 nm using a high resolution spectrometer. We compare the spectra with theoretical calculations andidentify the spectral lines near 13 nm for the 4d8 − 4d7 5p transitions of Xe10+ . The observed intensity ratios
for prominent lines show the plasma conditions for diﬀerent shots.
Keywords:EUV spectrum, Xenon ion, low temperature plasma, atomic data, atomic code
1．Introduction
Extreme-ultraviolet (EUV) light sources from compact plasmas are now intensively studied for the next
generation of lithography. The emission from multicharged Xe ions has strong peaks near 11 and 13 nm
and the emission near 13 nm is attributed to transitionsin Xe10+ . Better knowledge of this emission is important for EUV sources and for optimization of a 13.5 nm
EUV source.
Recently EUV spectra from Xenon ions in LHD havebeen measured in the wavelength range 10 − 17 nm.
We analyze the spectral lines near 13 nm. We compare
the spectra with theoretical calculations and identify thespectral lines from Xe10+ ions.
We can make a bench mark test of computer codes
using the observed spectral lines. This is important because the theory has not been extensively tested for suchhigh-Z low charge ions.
We will study plasma conditions which give the best
EUV emission and will make a collisional radiativemodel for high-Z many-electron ions.

Generally Xe emission is weak in the beginning of theplasma and strong during or after radiation collapse. A
2 m grazing incidence multichannel spectrometer SOXMOS [1] with 600 grooves/mm grating was used formeasurement. The measured line-width (FWHM) isδλ = 0.023 nm for the 13.2830 nm Fe XXIII line.
We used the spectra before Xe gas puﬃng for wavelength calibration. Spectra with and without Xe gaspuﬃng are shown in Fig. 1. Iron Fe XXIII 13.283,
Fe XXI 12.873, Fe XXII 13.577, C VI 13.49, Cr XXI
14.987 nm lines were used as references. We also madea wavelength calibration using the theoretical formula
given in Ref. [1] with parameters of the incident anddiﬀraction angles, the central wavelength. However thiscalibration curve does not reproduce the observed reference lines. Therefore we used a calibration by leastsquare fit to several measured reference lines which are
shown in Fig. 1.
After wavelength calibration, we compare the observed spectra for diﬀerent shots plotted on the same
wavelength scale. In Fig. 2, we show the observed spectra measured from two diﬀerent shots in the wavelengthscale together with the spectrum without Xe gas. The
strong peaks numbered 1, 2, . . . are considered to belines from Xe ions, based on the comparison to spectra
obtained without Xe gas puﬀ, except line no.4 which

2．Experimental measurement
Xe gas was puﬀed into LHD plasma in NIFS andthe EUV spectra from Xe ions were measured every
1 sec. The electron temperature at the center of LHD
is about 3 keV while the periphery is about 10 − 30 eV.

C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchCorresponding author’s e-mail: obara@nifs.ac.jp

31

EUV Spectra from Xe10+ Ions Measured from LHD

Fig. 3 Schematic energy level diagram for Xe10+ calcluted by Hullac [8].

Fig. 1 The observed spectra with xeon (dotted line) andwithout xenon (solid line).

For a many electron system like Xe10+ ions,configuration interaction by electron correlation isimportant.
Configurations taken into account inour calculations are 4s2 4p6 4d8 (lower state) and4s2 4p6 4d7 5p1 (upper state) for MCDF code [5],4s2 4p6 4d8 , 4s2 4p4 4d10 (lower state) and 4s2 4p6 4d7 5p1 ,4s2 4p6 4d7 4f1 , 4s2 4p5 4d9 (upper state) for Cowan
code and 4s2 4p6 4d8 , 4s2 4p6 4d7 4f1 , 4s2 4p6 4d7 5s1 ,4s2 4p6 4d7 5p1 , 4s2 4p6 4d7 5d1 , 4s2 4p6 4d7 5f1 for Hullac
code. The Fritsche’s MCDF code [6] includes morethan 100 configurations.
We obtain many lines from the results of theoretical codes. Both wavelengths and transition probabilities
Ar can be compared. We also compared our measured
spectra with Churilov [2,3].
Since it is diﬃcult to compare the atomic data in detail for each line, we made a convoluted spectra witha Gaussian profile assuming the integrated line intensity is equal to theoretical value gAr (s−1 ) where g is
the statistical weight of the upper level i and A r is thetransition probability from the upper level i to the lower
level j. The MCDF calculations include 421 lines andFig. 2 The observed spectra with xenon ions and without
xenon in the wavelength scale.

appears to be blended with a Fe XXIII line.
3．Theoretical calculations
Since the electron configuration for Xe10+ is complicated, we cannot find accurate theoretical atomic data.
Recently Churilov et al. [2, 3] in NIST measured Xe10+
spectra and gave a list of prominent lines. Sasaki reports spectra calculated by a collisional radiative model
based on atomic data from the Hullac code [4] .
We calculated atomic data with three diﬀerent
atomic codes; MCDF (Multiconfiguration Dirac-Fock)by Y. Ki. Kim [5] and by Fritzche [6], Cowan (Multiconfiguration Hartee-Fock) code [7] in relativistic modeand the Hullac code [8]. The results are compared eachother. The schematic energy level diagram calculatedby Hullac code for Xe10+ is shown Fig. 3. The isolated levels under 4d7 5f below 100 eV appear when thelevels 4d8 , 4d7 4f1 , 4d7 5s1 , 4d7 5p1 , 4d7 5d1 , 4d7 5f1 and4p5 4d9 are taken into account as input data for Hullaccode. They do not appear when only 4d8 and 4d7 5f1 areconsidered as input data. The dotted line indicates theionization potential 229.02 ± 0.20(eV) [3].

Fig. 4 Theoretical convoluted spectra for Xe10+ ions by
MCDF [6], Cowan [7], Hullac [8] code and Ref. [3].

32EUV Spectra from Xe10+ Ions Measured from LHD

589 lines respectively for the codes of reference [5] and[6]. The Cowan code calculation includes 195 lines.
As seen in Fig. 4, the total convoluted spectra look
similar and the strong peaks coincide with each other.
However the detailed structure are quite diﬀerent.

tra. One of the reasons is that the observed lines areemitted by excitation by electron impact or recombination. In order to analyze the intensities we need tomake a collisional radiative model which includes allthese processes. Another reason is the diﬀerence in theoretical atomic data. We have identified the strong observed Xe10+ lines. Some of the strong lines are identified with the transitions between singlets although theground state is the triplet 4p6 4d8 3 F4 . This suggests thelines are emitted through cascade or recombination. Recombination emission is also supported by the fact thatthe emission is strong later phase of the plasma.
The comparison of observed lines with the use of thelist of lines by Ref. [3] is shown in Table 1. The valueof ∆λ in Table1 indicate the wavelength diﬀerence between our observed line and Churilov et al. [3]. We
did not list the data by the MCDF and the Hullac codes,
because LS coupling scheme is not easy to make correspondence for each level with jj coupling scheme.

4．Comparison of observed spectra with theoretical calculations and identifications
We also compared the observed spectra with theoretical convoluted spectra based on the data by MCDFand Cowan codes as well as Churilov et al. [3]. The
agreements are not very good. However we can identify
the strong lines with an error of 0.01 nm (0.1 A). The
comparison of observed spectra with theoretical calculations near 13 nm is shown in Fig. 5. We also compare
our observed spectra with the data by Churilov et al. [3]in Fig. 6.
The observed lines (number 1 and 8) at 13.071 and13.820 nm are always strong but the theoretical valueof gAr is not large. The lines with the large theoretical gAr values are not always strong in measured spec-Fig. 5 The observed spectrum (solid line) and the theoretical spectra by MCDF (dot-dashed line) andCowan code (dashed line).

5．The contribution of highly charged ions
Since the temperature in the center of the plasma is
about 2−3 keV, highly ionized Xe ions (more than 10+)are expected. We have to confirm that the observed lines
are emitted from Xe10+ in low temperature plasma. Previously highly ionized Xe ion spectra were measured
in TFR Tokamak plasma [9] and W7-AS plasmas [10].
They identified the lines from Xe19+ − Xe25+ ions. We
want to know which ions are the source of the observedlines. In order to know the intensity of the lines produced in plasma, we have estimated the eﬀective emission rate coeﬃcients using Mewe’s empirical formula[11] for the lines Xe10+ 4d − 5p and Xe26+ 4d − 4f transitions both of which emit lines near 13 nm. The eﬀective excitation rate coeﬃcients for two lines are shownin Fig. 7. The rate coeﬃcients for the line of Xe26+ issmaller than that of Xe10+ by two orders of magnitude.

Fig. 6 The observed spectra (solid line) and the spectraby Churilov et al. [3] (dotted line).

Fig. 7 The effective excitation rate coefficients for Xe10+(4d – 5p) and Xe26+ (4d - 4f) lines.

33

EUV Spectra from Xe10+ Ions Measured from LHD

Table 1 Comparison of observed lines (Fig. 2) with the lines listed in Churilov (2004).

- mark test of theoretical data for wavelengths of Xeions. We are making a model for intensities of high Z
ions with many electrons. For diﬀerent shots, the intensity ratios of several strong lines are diﬀerent. The
diﬀerent intensity ratios suggest diﬀerent conditions inplasmas. We can study the optimal condition for Xe
ion emissions from the intensity ratios. Strong emission
is measured during radiation collapse or after radiation
collapse. We will study this diﬀerence in the future witha collisional radiative model. We also would like to
study the continuum emission near 13 nm observed in
the spectra which might be produced by free- free, freebound and two photon emissions.

Fig. 8 Time evolution of the lines near 13 nm for shot
#42801.

Acknowledgements
The authors are grateful to Dr.Y. Ralchenko for his
useful discussions and comments.

Therefore the density of Xe26+ ions should be larger
than those of Xe10+ by two orders of magnitude if we
assume the same electron densities.
In order to know the contribution of highly charged
ions we studied the time behavior of line intensities forthe shot (#42801) where four spectra are obtained during 4 sec. The plasma collapse at 3.6 sec for this shot
and the temperature begins to decrease at 3 sec. Therefore the temperature at 4 sec is considered lower than at
the previous time. We plotted the time behavior of theline intensities in Fig. 8. As is seen in Fig. 8, almost all
the line intensities increase except no.4 which is considered to be a line from Fe XXIII. This is evidence thatmost of the lines are emitted from low charge ions like
Xe10+ . However we can not exclude the possibility oflines of highly charged ions. We are investigating thisquestion. The strongest line No. 14 in Figs. 1 and 2
might be the resonance line of Xe24+ ions.

References[1] J.L. Schwob et al., Rev. Sci. Instrum. 58, 1601
(1987).
[2] S. Churilov, Y.N. Joshi and J. Reader, Opt. Lett.
28, 1478 (2003).
[3] S. Churilov, Y.N. Joshi, J. Reader and R.R.
Kildiyarova, Phys. Scr. 70, 126 (2004), E.B. Saloman, J. Phys. Chem. Ref. Data 33, 765 (2004).
[4] A. Sasaki, J. Plasma Fusion Res. 79, 219 (2003).
[5] Y. K. Kim and M.E. Rudd, Phys. Rev. A 50, 3954
(1994).
[6] S. Fritzsche, Private communication (2004), J.
Electron. Spectrosc. Relat. Phenom. 114-116,1155 (2001).
[7] R.D. Cowan, The Theory of Atomic Structureand Spectra (Univ. of California Press, Berkeley,1981).
[8] A.Bar-Shalom, M. Klapisch and J. Oreg, Phys.
Rev. A 38, 1773 (1988).
[9] C. Breton et al., Phys. Scr. 37, 33 (1988).
[10] H.H. Hacker et al., Appl. Phys. B 73, 59 (2001).
[11] R. Mewe, Astron. Astrophys. 20, 215 (1972).

6．Summary
EUV Xe ion spectra from LHD near 13 nm are measured. Theoretical calculations for Xe10+ were performed and compared with observed spectra. Severalstrong lines are identified, although the agreements are
not good for some lines. LHD spectra can give a bench34