24P089-P
Characterization of Tungsten Surface w/wo Nano-Fiber Structure
繊維状ナノ構造He損傷タングステンの表面特性評価
Takanori Miyamoto and Shuichi Takamura
宮本隆徳, 高村秀一
Department of Electrical Engineering, Faculty of Engineering, Aichi Institute of Technology,Toyota 470-0392, Japan
愛知工業大学 〒470-0392 豊田市八草町八千草1247
One of the serious concerns for tungsten materials in fusion devices is the radiation defects caused byhelium plasma irradiation since helium is the fusion product. Fiber-formed nanostructure is thought to havea possible weakness against the plasma heat flux and may destroy the reflectivity as an optical mirror in areactor. On the other hand, characterization of tungsten surface with nano-fiber structure is not fully
studied. In this study investigation on the characteristics of nano-fiber structure surface and an interesting
method for a recovery of such tungsten surface are shown.

1. Introduction
Divertor materials in fusion devices are exposed
to high density and high heat flux plasmas.
Tungsten(W) may be employed in the ITER
because of its high thermal property, low tritium
retention and low sputtering yield. However, it isknown to be damaged by helium ion irradiation,and it is called helium defects. We have two kindsof helium defects. One is bubble/holes on thesurface [1], the other is fiber-form nanostructure on
the surface produced at relatively low temperature,less than 1500K and at the incident ion energy ofgreater than roughly 10eV [2].
The surface characteristics of nanostructured W
would change compared with the flat non-damaged
surface, especially the heat conduction. The thinfiber-form nanostructure may be easily melted byplasma heat flux and it is considered that suchnanostructured surface may enhance the probabilityof unipolar arcing than non-damage surface so thatrecovery of that surface have been tried.
On the other hand, it should be considered that
outstanding properties have been found for the wallof fusion reactor.
2. Experimental Set up
The device for the present study is called
AIT-PID (Aichi Institute of Technology – Plasma
Irradiation Device) which is equipped with threepairs of neodymium permanent magnet bars
composing a multi-cusp (azimuthal mode number :6) magnetic configuration. In addition a solenoidalwinding underneath the magnets produces a weakaxial magnetic field up to 10mT [3].
3. Characterization
Figure 1 shows the fiber-form nanostructured
tungsten made in AIT-PID where we have highdensity (~1018m-3) helium plasma with the ion
bombardment energy of 50eV and the starting
surface temperature of 1420K. The helium ion
fluence is ~1026m-2. The specimen is a cold worked
powder metallurgy tungsten (PM-W). One of theoutstanding characteristics of AIT-PID is thepresence of hot electron component (Th ~30eV, α~5%) where Th and α are the temperature and thefraction of hot electron component while the bulk
electron temperature is around 4eV.

Fig.1 W surface with nano-fiber structure.
(a) : top view (b) : cross-section view

3.1 Recovery of Tungsten Surface
The previous experiment of temperature
excursion for nanostructured tungsten in the helium
plasma [4] has already shown a shortening and afattening of originally long and thin fibers when thesurface temperature increases up to 1600K for a few
minutes with an helium ion energy of larger than
6eV. However, the holes and bubbles on the surface
are newly created or survived so that a surface
roughness may not be removed when the incident
helium ion energy is larger than 6eV. It suggeststhat the recovery would be obtained with
non-damaging He plasma whose ion energy is of lessthan 6eV by biasing to increase the electron heatload on fiber-form nanostructured surface.

Figure 2 shows some typical FE-SEM images ofrecovered tungsten surface after 60min irradiationof helium plasma with the incident 2aaenergy of lessthan 6eV and surface temperature 1800K. Theseimages show a substantial diminution of fiber roots
which remain on the surface for an incomplete
plasma annealing. Therefore, it seems that almost
complete recovery of nanostructured tungsten may
be possible by a sufficiently long irradiation ofhelium plasma with a high surface temperature.

Fig.4 Suppression of Sputteringof SEE.
Fig.2 Recovery of W surface with nano-fiberstructure. (a) : top view, (b) : cross-section view

3.2 Cooling and Suppression of Secondary Electron
Emission (SEE)
The tungsten surface temperature has beenmeasured with a radiation thermometer assuming afixed radiation emissivity, for example ε = 0.43 at
the wavelength of 0.9μm. However, the surface
morphology is substantially changed so that theemissivity approaches to almost unity due to a
blacking on the way to fiber-form nanostructured
surface formation. Figure 3 shows a cross-check forthe cooling with both a radiation thermometer and athermocouple. At the end the change in emissivity
from ε = 0.43 to 1.0 gives almost the same value forboth measurement.
Figure 3 also shows a deepening of floating
voltage during producing nano-fiber structure. The
nano-fiber structure W has a complication on thesurface as show Fig.1(b), which decreases SEE sothat we have a deepening floating voltage. Thus theplasma power flux decreases with the suppression

Fig.3 Cooling of Tungsten

3.3 Reduction of sputtering yield
The nano-fiber structure on W surface is a kindof jungle for secondary electrons not to be able togo out. This mechanism decreases also physicalsputtering yield. We have demonstrated such effects
as shown in Fig.4 where the triangular biasing
voltage is applied on the target in an argon plasma.
The intensity of tungsten atomic line (498.26nm) is
plotted as a function of time.
4. Summary
Good properties of nano-fiber structure tungsten
surface as a plasma-facing wall in fusion reactor
have been demonstrated, that is, a cooling function
coming from the increase in radiation emissivity
and the suppression of secondary electron emission,a suppressive behavior of sputtering. We can saythat nano-fiber subsurface shows the outstanding
self-defensive properties for a wall in fusion reactor.
On the other hand a recovery technique fornanostructured W is shown. Grain size dependenceof plasma annealing was found.
Acknowledgments
The work is supported by a Grant-in-aid forscientific Research (B) (20360414) from JSPS. The
authors would like to thank Prof. N. Ohno and Dr. S.
Kajita of Nagoya University for their discussions.
References[1] S. Takamura, Plasma Fusion Res. 81 (2005) 25 (inJapanese).
[2] W. Sakaguchi, S. Kajita, N. Ohno and M. Takagi,J. Nucl. Mater. 290-291 (2009) 1149.
[3] S. Takamura, T. Tsujikawa, Y. Tomida, K. Suzuki, T.
Minagawa, T. Miyamoto and N. Ohno, J. Plasma
Fusion Res. SERIES 9 (2010) 441..
[4] S. Takamura et al., J. Nucl. Mater. 415 (2011)
S100–S103.