S5-5
Complex interactions between high flux plasma and materials
高熱流プラズマ－固体材料複合相互作用
Shin Kajita
梶田信
EcoTopia Science Institute, Nagoya University Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
名古屋大学エコトピア科学研究所 〒464-8603 名古屋市千種区不老町
Complex interactions occur between plasmas and materials in the divertor condition of nuclear fusion
reactor. Morphology changes occur on surface of tungsten, which will be used for divertor materials, by theplasma irradiation. One of the serious issues for divertor materials is the interaction with transients
accompanied by edge localized modes (ELMs). The morphology change by the plasma irradiation
essentially alters the interaction between the material and transients.

1. Introduction
In ITER, divertor material will be fully
tungsten from the start-up phase. In nuclear
fusion reactors, helium atoms are produced by anuclear reaction process, and helium plasma
irradiation of tungsten leads to the surface
morphology changes. It is known that thefibreform nanostructures are formed on thesurface [1].
As a consequence of the morphology changes,
various material properties are altered such as
optical reflectance, secondary electron emission,field electron emission, sputtering yield, energy
reflection
coefficientandthermophysicalproperties. Materials will be exposed to transientsheat load; the type-I edge localized modes(ELMs) in ITER will provide the heat load of 0.5
MJm-2 for less than 1 ms even in mitigated cases.
Considering the interaction between material andELMs, the above-mentioned variations could leadto serious problems.
One is the initiation of arcing [2]. Whennanostructures are formed on the surface, it wasfound that arcing could be much easily initiated on
the surface. Another issue is an enhancement oftungsten release by melting and vaporization [3]. In
this study, we presents the interaction between theHe irradiated tungsten and transients.
2. Nanostructure formation
Figure 1 shows a cross sectional TEM
micrograph of He irradiated tungsten. The
experiments were performed in the linear plasma
device NAGDIS (Nagoya Divertor Simulator)-II.
The electron density was on the order of 1018–1019
m3 and the temperature was ~ 5 eV. The surface
temperature was 1400 K and the incident ion

energy was 50 eV. Growth of helium bubbles
beneath the nanostructured layer is an important
process to form the nanostructures. Swelling ofmaterial by absorbing He particles during theplasma irradiation is likely to be of importance as
well.

Fig.1: Cross sectional TEM micrographs ofhelium irradiated tungsten with the helium fluenceof 5.5x1025 m2.
3. Initiation of free running arcing
Demonstration of transients heat load tomaterials in plasmas was conducted in the divertor
simulator NAGDIS-II using laser pulses for pulsed
heat source. A fast framing camera was used toobserve the behavior of arc spots. Figure 2 showsconsecutive CCD images of the emission WI fromarc spots triggered by a laser pulse. The angle
between the magnetic field and the sample was 45
degree. The frame rate was 125 000 fps (frame persecond), and the corresponding time interval
between the image is 8 s. The direction isdetermined by the ratio of the axial to transverse
magnetic field strength. When the magnetic field isparallel to the surface, the spot moves to retrograde,
namely −j ×B, direction. In Fig. 2, the direction isconsistent with the one which is determined by theso-called acute angle rule or Robson angle [4].
When arcing is initiated on W, tungsten is released
from the surface. The amount of erosion wasestimated to be ~10 mg/s for single arc spot [5], andit could increase when spots forms grouping [6].

Fig. 2: Consecutive CCD images of the emission
WI. The angle between the magnetic field and thesample was 45 degree. The frame rate was 125 000
fps (frame per second), and the corresponding timeinterval between the image is 8 s.
4. Thermal response of nanostructured W
The thermal response of nanostructured tungsten,which was fabricated in the linear divertor
simulator NAGDIS-II, was investigated using
pulsed plasma in the MAGNUM-PSI device and byusing high powered laser pulses.
Figure 3(a) and (b) shows the temporal evolutionsof the surface temperature measured with an IR fastcamera during pulsed plasma irradiation
experiments for pristine and nanostructured W
samples, respectively. The pulse energy was 430 J
and the frame rate of the camera was 13 kHz. The
surface temperature started to increase after theplasma irradiation and reached ∼700–800 K. In
response to the pulse plasma irradiation, the surface
temperature promptly jumped. On the pristine
sample, the increase was less than 100 K, while on
the nanostructured sample, the increase was muchgreater and was 400–500 K. It was also interesting
to note that the surface temperature was sufficiently
less than the melting point even though melting
traces were formed on the surface after exposing to
the pulsed heat loads. It was likely thatnanostructure matrix formed a thermally isolatedpart near the surface, and non-uniformity in thetemperature increase would have been taken place.
That is, it was suggested that the averaged surface
temperature was less than the observed temperature;
the locally heated part would increase the thermalradiation and increase the measurement temperature.
When nanostructured tungsten was exposed to
type-I ELMs in ITER, the present experimentssuggested that the vapourization will occur locallyeven in mitigated cases.

Fig. 3: Temporal evolutions of the surface
temperature measured with an IR fast camera
during pulsed plasma irradiation experiments. (a)and (b) show the cases for pristine andnanostructured W samples, respectively.
References[1] S. Takamura, N. Ohno, D. Nishijima and S. Kajita
Plasma Fusion Res. 1 (2006) 051.
[2] S. Kajita, S. Takamura and N. Ohno, Nucl. Fusion,49 (2009) 032002.
[3] S Kajita, G. Temmerman, T. Morgan, S. van Eden, T.
de Kruif, and N. Ohno, Nucl. Fusion 52 (2014)
033005.
[4] Cathodic Arcs: From Fractal Spots to Energetic
Condensation (Springer, New York 2008).
[5] S. Kajita, N. Ohno, N. Yoshida, R. Yoshihara, S.
Takamura, Plasma Phys. Control. Fusion, 52 (2012)
035009.
[6] D. Hwangbo, S. Kajita, S. Barengolts, M.
Tsventoukh, N. Ohno, Results in Physics 4 (2014)33-39.