22P174-B
Optical emission spectroscopy of sputtering atoms on tungsten surfaces underion irradiation
イオン照射タングステン表面上でのスパッタリング粒子の発光分光Kenji Motohashi1), Daiji Kato2), Takahiro Kenmotsu3), Yasuhiro Sakai4), Hiroyuki A. Sakaue2),
Keisuke Nogami4), and Kenji Furuya5)
本橋健次1), 加藤太治2)，剣持貴弘3)，酒井康弘4)，坂上裕之2)，野上慶祐4)，古屋謙治5)1) Department of Biomedical Engineering, Toyo University, Kawagoe, Saitama 350-8585, Japan
東洋大学理工学部生体医工学科 〒350-8585 埼玉県川越市鯨井2100

2) National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-63) Department of Biomedical Engineering, Doshisha University, Kyotanabe, Kyoto 610-0394, Japan
同志社大学生命医科学部医工学科 〒610-0394 京都府京田辺多々羅都谷1-34) Department of Physics, Toho University, Funabashi, Chiba 274-8510, Japan
東邦大学理学部物理学科 〒274-8510 千葉県船橋市三山2-2-15) Department of Molecular and Material Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
九州大学大学院総合理工学研究院エネルギー物質科学部門 〒816-8580 福岡県春日市春日公園6-1
Visible spectroscopy of the sputtering atoms on tungsten surfaces bombarded by Kr+ ions (30−60 keV) wasperformed. Many W lines were observed in the wavelength range of 300–600nm. The emission intensity of400.9nm line (WI: 6p 7P4 → 6s 7S3) was measured as a function of the distance from the surface. The mean
velocity of the W*(6p 7P4) atoms was obtained by analyzing the exponentially decaying curves. As a result,the mean velocity in direction normal to the surface was found to be 5.6±1.7 km/s. There was noremarkable change of the velocity among different projectile energies.

1. Introduction
Tungsten has been considered to be a goodcandidate of wall materials of fusion reactors
because of the highest melting point, low sputtering
yield, and low hydrogen absorption rate. However,high emission probability of radiation from theexcited atoms emitted from the wall surface due tointeraction with various energetic particles is one of
the weak points because it makes plasma
temperature low. Thus, it is very important to know
the kinematics of the excited atoms produced in thesputtering processes. Optical measurement is one of
the most powerful methods of high-temperatureplasma diagnosis. We measured the mean velocity
of excited tungsten atoms in direction normal tosurface by observing ion induced light emission.
2. Experimental apparatus and procedures
The experiment was carried out at a beam line ofmedium current ion implanter (ULVAC
IM-200MH-FB) in National Institute for Fusion
Science (NIFS). A schematic illustration of theexperimental apparatus is shown in Fig. 1. Kr+ ion
beam extracted from a Freeman ion source wasintroduced into the collision chamber after
analyzing the mass to charge ratio (m/e) by amagnet. A polycrystalline tungsten surface set on amovable stage was installed in the collisionchamber. An electrode with a hole of 5 mmdiameter applied −100V to the surface was also
installed at 30mm forward the target to retard
secondary electrons. The ion beam (~30μA) enterednormally onto the tungsten surface. The stage canbe moved in direction parallel to the ion beam axis
within the range of 50mm.
After passing through a quartz window and theultra violet (UV) condenser lens, the light from theexcited atoms was focused onto the entrance slit ofa monochromator equipped with a charge coupleddevice (CCD). The optical axis crosses with the ion
beam axis at right angle, since it was set in direction
parallel to the surface. We measured the lightintensity as a function of the distance from thesurface by moving the stage linearly.
The pressure of the chamber maintained below
7×10-5 Pa by a turbo molecular pump during theFig.1. Schematic illustration of the experimental
apparatus

3. Results and discussion
Figure 2 shows a typical high resolution
wavelength scanning of emitted light under Kr+(33keV) irradiation. We observed a number of Wlines due to sputtering and several Kr ones due toback scattering. The most intense single peak of
400.9nm labeled with “A” in Fig. 2 is assigned toWI 5d5(6S)6p(7P4) → 5d5(6S)6s(7S3). There aremany lines of similar transitions corresponding to
6p→6s with different total angular momentum. We
measured the ion-beam current dependence of thephoton emission of the line “A”. Clear linear
dependence observed in the measurement confirmsthat no secondary process affects the photonemission.

Intensity (Arb.units)

Kr+ (33 keV) → W

5 675 675d ( S)6p ( P3) → 5d ( S)6s ( S3)
5 675 6

distribution [3], though there may be a slightincrease around 55 keV. The average of all <vz>values becomes 5.6±1.7 km/s. This velocity
corresponds to the kinetic energy of ~30eV which
was given to the sputtered W atoms in direction
normal to the surface. This value is very close to
one obtained by the analysis of a Doppler
brodening of Al line (4s 2S1/2 → 3p 2P3/2: 396.2nm)
in 45keV Arq+ (q=1-9) irradiation on Al and Al2O3surfaces [4].
100Photon intensity (arb. units)measurements, though it reached <1×10-5 Pa whenion beam does not enter onto the tungsten surface.

45 keV

10-1zC = 0.32mm
zC = 1.6mm

10-20

17A
654655 6

45Fig.3. Photon-emission intensity of the 400.9nm
line (labeled with “A” in Fig. 2) as a function ofthe distance from the tungsten surface.

2 55d 6s( D)6p ( P2) → 5d 6s ( D)43-3Distance from the surface; z (m) [10 ]5d ( S)6p ( P4) → 5d ( S)6s ( S3)4275d 6s( D)6p ( F2) → 5d ( S)6s ( S3)

10Kr+→W
399

400401402403404405406407408409

Wavelength (nm)Fig.2. A typical result of high resolution wavelength
scanning of emitted light under Kr+ (33keV)
irradiation. The solid and broken arrows indicate W
and Kr lines, respectively. The most intense single
line of WI (400.9nm) is labeled with “A”.

Figure 3 shows the photon-emission intensity of
the 400.9nm line in 45keV projectile energy as afunction of the distance from the surface z. The
curve was well fitted with two exponentialfunctions with a characteristic distance zC. Theobserved photon intensity I(z) is approximately
written as I(z) ≈ I0exp(−z/zC), where I0 is photon
intensity at z = 0 (surface) [1]. The distance zC
equals to the product of the mean velocity
component normal to the surface <vz> and the life
time of the excited state of W*(6p 7P4) atoms τ.
Thus, the <vz> value can be obtained by the fittingprocedure when we assume τ = 59.4ns [2] for thesteeper decaying curve with zC = 0.32mm.
Figure 4 shows the <vz> as a function ofprojectile energy. No remarkable change is found as
expected simply from Thompson-Sigmund velocity<vz> (km/s)

8398

5.6±1.7 km/s

64
2
020

30

40
50
E (keV)

6070Fig.4. <vz> of W*(6p 7P4) atoms as a function of the
projectile energy

Acknowledgments
This work was carried out with the support andunder the auspices of the NIFS Collaboration
Research program (NIFS09KBAF002). This workwas partly supported by the Grant-in-aid for
Scientific Research (A) (23246165).
References[1] M. Suchanska, Prog. Surf. Sci. 54 (1997) 165.
[2] E. A. Den Hartog et. al., J. Opt. Soc. Am. B4,48(1987).
[3] P. Sigmund, Rev. Roum. Phys. 184 (1964) 384.
[4] K. Motohashi et. al., Phys. Scr. T73 (1997) 329.