06aC12
Thermal response of nanostructured tungsten
Shin Kajita, 1Gregory De Temmerman, 1Thomas Morgan, 1Thijs de Kruif,
1Stein van Eden, 2Noriyasu Ohno
1EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, JapanFOM Institute DIFFER, Dutch Institute for Fundamental Energy Research, Association
EURATOM-FOM, 3430 BE Nieuwegein, The Netherlands
2Graduate school of engineering, Nagoya University, Nagoya 464-8603, Japan

In future fusion devices, nuclear reaction
produces helium atoms. It has been recognized thatthe helium irradiation to tungsten leads to theformation of nanostructures on the surface.
Previously, it was suggested that the thermal
conductivity significantly decreased and ananomalous temperature increase occurred inresponse to transients when the nanostructures wereformed on the surface [1]. To investigate thetemperature evolution in response to transients, theexperiments were conducted in MAGNUM-PSI
device with laser and transient plasma [2] using
nanostructured tungsten samples that were preparedin the divertor simulator NAGDIS-II. The
measurement of the temperature was conductedwith a fast framing IR camera.
Figure 1 shows temporal evolution of thesurface temperature on (a,c) a pristine sample and(b,d) nanostructured sample. In response totransient plasma irradiation, the surface temperaturesignificantly increased on the nanostructured
tungsten surface compared with that on the pristine
tungsten sample. Although the surface temperature
increased only by ~100 K on pristine tungsten
sample, the SEM analysis on the nanostructured
tungsten revealed that the nanostructures melted on
the same power.
Similar experiments were conducted using
laser pulse irradiation in MAGNUM-PSI chamber.
The peak temperature in response to laser
irradiation to fuzz sample was investigated withchanging the laser power. It was revealed that thesignificant temperature increase and melting traces
were also identified for the laser irradiation similaras the pulsed plasma irradiation.
There are several possibilities to lead thephenomena; they are discussed with using thenumerical analysis of the surface temperature bysolving the thermal diffusion equation. It is foundthat the decreases in the density of tungsten andthermal diffusivity by the formation ofnanostructures could lead to the increase in thesurface temperature. The nanostructure decreases
the cohesive energy, and the melting point may bedecreased consequently. However, it was likely thatthese effects should be minor, and cannot explain
the experimental observation overall. It was likelythat nanostructure matrix formed a thermallyisolated part near the surface, and non-uniformity inthe temperature increase would have been takenplace. The locally heated part would increase thethermal radiation and increase the measurement
temperature.

Fig. 1: Temporal evolutions of the surface
temperature measured with an infrared fast camera
during pulsed plasma irradiation experiments. (a, c)shows the case for the pristine W sample, and (b, d)shows the case for nanostructured W sample.
References:[1] S. Kajita, S. Takamura, N. Ohno, Dai Nishijima, H. Iwakiri
and N. Yoshida, Nucl. Fusion 47, 1358 (2007).
[2] G.D. Temmerman, J. Zielinski, S. van Diepen, L. Marot andM. Price, Nucl. Fusion 51, 073008 (2011).