Helium irradiation on tungsten changes the surface morphology dramatically by forming a nanometre-sized fibreform structure which could bring about serious problems for fusion reactors. From the experimental results in liner divertor simulators, it is revealed that the incident ion energy and surface temperature are key parameters for the formation of the structure. It is shown that the tungsten nanostructure is easily formed when the temperature is in the range 1000–2000 K, and the incident ion energy is higher than 20 eV. Furthermore, on the basis of the helium irradiation experiments performed in the divertor simulator NAGDIS-I, the initial formation process of the nanostructure is revealed. It is shown that the nanostructure formation is related to pinholes appearing on the bulk part of the material, and then, the rough structure develops to a much finer nanostructure. The nanostructure was also observed on the molybdenum surface that was exposed to the helium plasma. It increases interest in the possibility that nanostructure formation by helium irradiation is a common phenomenon that occurs on various metals.

Black metal absorbing light can be used for the light absorber material for a solar thermophotovoltaic system, which is expected to be a photovoltaic system in the next generation. Here we show that the fiberform nanostructured tungsten formed by helium irradiation absorbs the light from all angles of interest and is virtually black for a solar spectrum, from visible to near infrared wavelength; the absorptivity of the total solar power could be 98%. It is revealed that the nanostructure is formed by a novel process, self-growth of the helium nano-bubbles.

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Morphology changes on various metals by helium irradiation are shown experimentally. It has been revealed that a whisker-like nanostructure was formed on tungsten and molybdenum surfaces by the exposure to a helium plasma. After helium irradiation to titanium and stainless steel, many dots in several hundreds of nanometers cover the surface. The morphology change may be caused by the growing process of helium bubbles in the sub-surface region.

(Nagoya univ.) Masayuki Tokitani (NIFS) In future experimental fusion devices, many in-vessel mirrors will be used for optical diagnostics. It is generally recognized that metallic mirrors are used for them, and the candidate materials for the first mirror are molybdenum, tungsten, copper, stainless steel, and rhodium. Although it has been found that neutron irradiation does not have a significant influence on the optical reflectivity, there is concern that the reflectivity decreases by the exposure to charge-exchange neutrals, which cause erosion and deposition. In the present report, we will show the effect of low-energy helium bombardment to molybdenum and rhodium from the experiments conducted in a linear plasma device.The experiments were performed in the linear divertor simulator NAGDIS (NAGoya DIvertor Simulator)-I. The specimens were exposed to the helium plasma in the NAGDIS-I. The surface temperature and incident ion energy were controlled by changing the helium flux to the specimen. The incident ion energy was controlled by electrically biasing the specimen, and was ~ 50 eV. The wavelength dependence of optical reflectivity was measured with a spectrophotometer (Nihon Bunkosya: ARV-47S) for the wavelength from 200 to 900 nm.Figure 1(a) shows the wavelength dependences of the optical reflectivity of Mo-1 and Mo-2. The irradiation temperatures for Mo-1 and Mo-2 are 1200 and 1500 K, respectively, and the helium ion fluences for Mo-1 and Mo-2 were 1.8×10 26 m -2 and 1.1×10 27 m -2 , respectively. The reflectivity considerably decreased by the surface modifications due to the helium irradiation. The optical reflectivity was less than 10% for Mo-1, whereas, for Mo-2, it decreased to almost zero. Non-uniformity of Mo-1 surface is effectively averaged for the reflectivity measurement, because the spot size in the measurement is much larger than the crystal grain. Figure 1(b) shows the reduction factor of the reflectivity for Mo-1. Here, the reduction factor was defined as the original reflectivity divided by the decreased reflectivity. At 900 nm, the reduction factor was approximately 5; it increased as decreasing the wavelength, and was 30 at 200 nm. In general, optical reflectivity is strongly altered when the size of the structure on the surface is less than several times the wavelength [1]. From Fig. 1(b), the size of the structure is less than 1 �m; moreover, there may exist much finer structure on the surface.Figure 2(a) shows the wavelength dependences of nonirradiated rhodium, helium irradiated rhodium (Rh-1), and helium irradiated molybdenum (Mo-3). The incident ion energy, surface temperature, and helium ion fluence for Rh-1 and Mo-3 were, respectively, 45 eV, 600 K, and 8×10 24 m -2 and 45 eV, 850 K, and 3.7×10 25 m -2 . The exposure to the helium plasmas also decreased the optical reflectivity significantly for both rhodium and molybdenum even at those temperatures. Figure 2(b) shows the reduction factors for Rh-1 and Mo-3. In both cases, the factor increases as decreasing the wavelength. Th...