Proceedings of ITC/ISHW2007

Damage evolution and particle retention in metals bombarded byneutral helium particles at the first wall positions in LHD
M. Tokitani, N. Yoshida a, Y. Ohtawa b, K. Tokunaga a, T. Fujiwara a, M. Miyamoto c, N. Ashikawa,S. Masuzaki, M. Shoji, M. Kobayashi, A. Sagara, N. Noda, H. Yamada, A. Komori,
LHD experimental group, S. Nagata d, B. Tsuchiya dNational Institute for Fusion Science, 322-6, Toki, Gifu 509-5292, Japan
Research Institute for Applied Mechanics, Kyushu University, Kasuga, Fukuoka 816-8580, Japanb
Interdisciplinary Graduate School of Engineering Sciences, Kyusyu University, Kasuga, Fukuoka 816-8580, Japan
cDepartment of Material Science, Shimane University, Matsue, Shimane 690-8504, JapandInstitute for Materials Research, Tohoku University, Sendai, Miyagi 980-8577, Japana(Received day month year / Accepted day month year should be centered 10-point type)Specimens of W, Mo and stainless steel were inserted into the first wall equivalent position by using movablematerial probe system in LHD, and then, exposed to the several sequential helium discharges. After the exposure,very dense helium bubbles and dislocation loops were confirmed in all specimens from transmission electron
microscopy (TEM) observation. It means that the energy of incidence helium particles is sufficiently higher thanthat of the minimum energy for creating the knock-on damage (Emin) in all specimens (e.g. W; Emin=0.53keV).
Majority of such incidence helium particles to the specimens are energetic neutrals created by charge-exchange(CX) collisions [1]. It is known that helium atoms once injected into metals cause more intensive radiation damages
than hydrogen atoms, because they have a strong interaction with lattice defects such as vacancies or dislocation
loops. Understanding of the effects of charge exchanged helium particles (CX-helium) to the first wall is importantfor not only the elucidation of materials degradation but also realization of the high performance plasma operations
in future fusion devices.
Keywords: Large Helical Device (LHD), Helium, Charge-exchanged neutrals, Microscopic damage, Transmission
Electron Microscopy (TEM)

1. Introduction
The first wall materials of fusion reactors will sufferheavy bombardment of helium particles generated by D-T
fusion reaction. Majority of such incidence helium
particles are energetic neutrals (CX-neutrals) created bycharge-exchange collisions [1]. Helium injection into
metals causes serious radiation damages. Strong
irradiation effects of helium have been observed in manykinds of metals such as tungsten, molybdenum andstainless steel [2-6].
The Large Helical Device (LHD) is the largest
heliotron-type device with super conducting magneticcoils [7]. The first wall panels and divertor plates are
made of stainless steel and isotropic graphite, respectively.
Stainless steel is the major material in LHD and thegraphite area is only about 5 % of the total plasma facing
area. It was reported that for helium plasma discharge
experiments in the LHD, about half of the inlet helium
was trapped in the wall, even after a long helium glow
discharge cleaning. Therefore, understanding of theeffects of charge exchanged helium (CX-helium) atoms
author’s e-mail:tokitani.masayuki@LHD.nifs.ac.jp

to the first wall is important for not only the elucidationof materials degradation but also the plasma operations.
In the case of steady state operations, behavior ofincidence CX-helium atoms to the first wall is much
important.
In the present study, microscopic damages in metals
generated by CX-helium bombardment were studied, andthen, incident flux and energy of CX-helium wereevaluated.
Furthermore, to investigate the effect ofbombardment of CX-helium on optical reflectivity of Mo
mirror, change of optical reflectivity was measured bymeans of spectrophotometer. Mo is one of the potential
candidate of first mirror for plasma diagnosis in future
fusion devices [ 8 ]. Effects of the hydrogen isotope
irradiation on optical properties have been studied
extensively [9-11] but not much for the helium irradiation.
However, our fundamental study on helium irradiation
effects on Mo mirror indicated that radiation damages by
helium atoms cause serious degradation of opticalreflectivity [12].

Proceedings of ITC/ISHW2007
monitored by the thermocouples placed just beneath thespecimens. It stayed almost constant near room
temperature.

2. Experimental procedures
2.1. Material irradiation experiments in LHD
Specimens of Mo and stainless steels mounted on
the probe head were placed at the first wall equivalent
position by using movable material probe system which
installed in LHD, and then, exposed to the NBI + ICRF
heated helium discharges. This material probe system
makes possible to select desirable plasma discharges andirradiation position by using shutter and verticalmovement mechanism. Fig. 1 shows a schematic view ofthe experimental set up in this study.

3. Results and discussions
3.1. Evaluation of microscopic damage by TEMobservation

PlasmaProbe headmove(A) 0cm(B) -5cm(C) -25cm

2.2. Material analysis
After the exposure, microscopic damage of thespecimens was observed by means of transmission
electron microscopy (TEM) and then, incidence flux andenergy of CX-helium atoms were estimated by comparing
with the in-situ TEM observation results under heliumion-beam irradiation experimental results [13].
Optical reflectivity of Mo mirror before and after theexposure was measured with a spectrophotometer(JASCO, V-670) for the wave length between 190 and2500 nm.

94.5cmFig. 1 Schematic view of the experimental set up on
movable material probe system in LHD.
Total discharge time at each irradiation position which isidentified by the distance measured from the first wall
equivalent position and typical plasma parameterssummarized in Table 1. For convenience these positions
are denoted as (A), (B) and (C). (B) and (C) are thepositions in 4.5L port as illustrated in Fig. 1. The
temperature of the probe head during exposure wasFig. 2 shows TEM images of (a)-Mo and (b)-stainlesssteel exposed to the NBI+ICRF heated helium discharges
for the case of (A), (C) and (A)+(B)+(C). (A)+(B)+(C)means the specimen exposed at all positions. The upper
series of micrographs of (a) and (b) are bright field
images with large deviation parameter s. White dot
images are helium bubbles. The lower series of (a) and(b) are dark field images with small deviation parameter s,
which fits for observation of defects with strong lattice
distortion such as dislocation loops and helium platelets.
Defects with a strong white image, mainly dislocationloops. Although total irradiation time was only 349
s~1138 s, considerably large amounts of helium bubblesof about 1 to 2 nm and dislocation loops of about ~20 nm
were formed in both specimen and all cases. Evolution of
the microscopic damage was already saturated at the caseof both (A) and (C). Thus, since the creation rate ofdefects in all specimens was very high, unfortunately, thedifference of the damage rate could not identify as afunction of irradiation time and position. This means thatthe damage is caused by CX-helium atoms which are notaffected by magnetic field, and their energy and flux are
sufficiently higher to create these defects. In general,
radiation induced secondary defects are formed as
aggregates of point defects produced by knock-on
processes. Since the threshold energies of helium fordisplacement damage in Mo and stainless steel are about

Distance fromfirst wall position

Total dischargetime

Ion temp. Ti (keV)(Plasma core)

Electron density(x1019m-3)

（A)

0 cm

408 s

0.5~2.0

0.25~3.6(B)

-5 cm

349 s

0.2~2.0

0.25~2.25(C)-25 cm352 s

0.5~2.0

0.25~3.6(A)+(B)+(C)1138 s

0.1~2.5

0.23~3.85

Table 1 Total discharge time at each irradiation distance from the first wall position as illustrated in Fig. 1 andtypical plasma parameters. 0 cm corresponds to the first wall equivalent position. (Direction of -25 cm isfarthest from plasma)Proceedings of ITC/ISHW2007

It was tried to estimate the flux and energy ofCX-helium atoms to the first walls by comparing withsystematic in-situ helium ion irradiation experiments [13].
The microstructural evolution of stainless steel at room
temperature under irradiation with 2 keV-He+ ions isshown in Fig. 3. Some of the images are re-produced datafrom Ref. [13]. The upper series of micrographs arebright field images with large deviation parameter s. The
lower series are dark field images with small deviation
parameter s. When Fig. 3 compared with Fig. 2-(b), sizeand density of helium bubbles and dislocation loops ofthe all cases of Fig. 2-(b) is corresponds to 2~5×1021
He/m2. Therefore, the estimated flux and incidence
energy is 2×1018~1×1019 He/m2s and about 1~2 keV,respectively. As mentioned above, majority of such
incidence helium particles are energetic neutrals. Thisestimation is relatively rough. However, it is important(A) @ 0cm(C) @ -25cm(A)+(B)+(C)DF Image (s~0)

BF Image (s>>0)

(a)

10nm

5x1021 He/m2

1x1022 He/m2

BF Image (s>>0)

3.2. Estimation of the flux and energy of CX-helium
atoms

2x1021 He/m2

DF Image (s~0)

0.23 keV and 0.10 keV, respectively. The size and densityof defects between (A) and (C) were very similar in both
specimens. It is indicates that the flux of CX-helium atom
does not decrease much even in the port.

10nm

Fig.3 Microstructural evolution in stainless steel during
helium ion irradiation (~1×1022 He/m2) at room
temperature with energies of 2 keV. BF images atlarge deviation parameter s (upper series). White
dot contrast in DF images shows dislocation
loops (lower series).
the case of only NBI heated helium discharges, very
similar damages were confirmed after exposed to only~100 s discharges in Mo and stainless steels specimens(not shown here). In the case of 2 keV-He+, it was reported
that at a dose level of lower than ~1020 He/m2s, injected
helium atoms are trapped by vacancy-helium complex
and helium bubbles and finally filled with them. Above
this dose level, additional trapping of helium atoms byhelium bubbles become difficult [13].
Three-dimensional simulation of CX-neutral flux tothe plasma facing components (whole torus wall ~300m2) during hydrogen discharge case in LHD as a functionof line averaged electron density is under going now byusing EMC3-EIRENE code (The calculation in helium
discharge case is also under going). It is expected that
more detailed analysis will be performed by comparing
simulation results with experimental results.

3.3. Change of optical reflectivity(A) @ 0cm(C) @ -25cm(A)+(B)+(C)DF Image (s~0)

BF Image (s>>0)

(b)

10nm

Fig. 2 TEM images of (a)-Mo and (b)-stainless steel
specimens after exposed to ICRF heated helium
discharges, BF images at large deviation
parameter s (upper series). White dot contrast inDF images shows dislocation loops (lowerseries).
for elucidation of the CX-particle load to first walls. AlsoFig. 4 shows the optical reflectivity of Mo specimen
after exposed to NBI+ICRF heated helium discharges. This
spectrum is from the specimen exposed to all position,(A)+(B)+(C) in table 1.
Reflectivity of virgin specimen was also plotted. It isclear that reduction of reflectivity has already occurred at
the exposure time of only 1138 s. In particular, reductionof reflectivity about 500 nm or less is remarkable.
Referring to our previous study [12], it is considered thatreduction of optical reflectivity is due to the multiple
scattering of light by the dense helium bubbles in thesub-surface region. In comparison with hydrogen
irradiation, remarkable degradation of the opticalreflectivity induced by hydrogen ions irradiation withsimilar energy occurs at the fluence above 1025ions/m2 [9].
We should note that effect of helium bombardment isthree orders of magnitude higher than that of hydrogen
bombardment. This result is important for the design andProceedings of ITC/ISHW2007
operation of plasma diagnostics using first mirror.

100

Refrection rate [%]90

Virgin specimen

8070

After exposure(A)+(B)+(C)

60
50
4030
20

Before exp.
After exp.

100500

1000

1500

2000

2500

Wave length [nm]Fig.4 Optical reflectivity of Mo specimen after exposedto ICRF heated helium discharges. Solid line
shows (A)+(B)+(C) in table 1. Dashed line shows
virgin specimen.

4. Summary
Microscopic damages in metals bombarded byCX-helium atoms in LHD were observed by TEM, andthe incident flux and energy of CX-helium particles to thefirst wall was discussed by comparing the microscopicdamage properties with in-situ helium ion-beamirradiation experiment in laboratory. Dislocation loops
and dense fine helium bubbles were formed in Mo andstainless steel specimens exposed only 349 s~1138 s tohelium plasma discharges. It is considered that they areformed mainly by the bombardment of CX- helium atoms.
The estimated flux and incident energy from size anddensity of the defects is the order of 2×1018~1×1019
He/m2s, and about 1~2keV, respectively.
Optical reflectivity of Mo specimen after exposed to
NBI+ICRF heated helium discharges was measured byspectrophotometer. Reduction of reflectivity has alreadyoccurred at the exposure time of only 1138s. It is consider
that reduction of optical reflectivity is due to the multiple
scattering of light by the dense helium bubbles in the
sub-surface region.
Generation of such high flux and high energy
CX-helium atoms is serious problem for not only thedeterioration of the first wall materials but also plasma
diagnosis using metallic mirrors. In addition, phenomena
in plasma confinement devices such as synergistic effectsof helium bombardment and re-deposition will beinvestigated.

References[1] R.J. Goldston, P.H. Rutherford, 1996 Introduction to Plasma
Phys. p156
[2] H. Iwakiri et al., J. Nucl. Mater. 283-287 (2000) 1134
[3] N. Yoshida Y. Hirooka et a., J. Nucl. Mater. 258-263[4] K. Ono et al., J. Nucl. Mater. 283-287 (2000) 210[5] D. Nishijima et al., J. Nucl. Mater. 313-316 (2003) 97[6] D. Nishijima et al., J. Nucl. Mater. 337-339 (2005) 927[7] O. Motojima et al., 2003 Nucl. Fusion 43 1674
[8] A.E. Costley et al., Fusion Eng. Des. 55 (2001) 331[9] V.S. Voitsenya et al., J. Nucl. Mater. 258-263 (1998) 1919
[10] V.S. Voitsenya et al., J. Nucl. Mater. 290-293 (2001) 336[11] T. Sugie et al., J. Nucl. Mater. 329-333 (2004) 1481
[12] A. Ebihara et al., J. Nucl. Mater. 363-365 (2007) 1195[13] M. Tokitani et al., J. Nucl. Mater. 329–333 (2004) 761