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.

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Nanosecond laser ablation of tungsten (W) exposed to helium plasma is investigated using optical emission spectroscopy. Submicrometer-sized holes/bubbles are formed on the surface of W when it was exposed to the helium plasma at a sufficiently high temperature (≳1500–1600K). The emissions from a virgin W (before the helium plasma irradiation) cannot be detected when the fluence is <1J∕cm2; however, the threshold fluence for the detection of neutral W emission after it was exposed to the helium plasma is ∼0.2J∕cm2. The physical mechanism of laser-induced bursting of holes/bubbles is proposed for achieving a significant reduction in ablation power threshold.

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Laser-induced blow-off from a tungsten surface that was exposed to helium plasma is investigated experimentally in the divertor simulator NAGDIS-II. The pulse width of the laser is submillisecond and is similar to the duration of type-I edge localized modes in ITER. The temporal evolution of blow-off particles, which are visualized by the electron impact excitation in the surrounding plasma, is investigated by using filter spectroscopy. We demonstrate the effect of helium irradiation damages on the tungsten ejection behavior in response to a transient heat load.

Tungsten samples were irradiated with high density helium plasma in the divertor plasma simulator NAGDIS to investigate the formation condition of the fiberform nanostructured tungsten. Experimental results indicated that the surface temperature threshold for the formation of nanostructure was approximately 1000 K when the incident ion energy was 50 eV. When the surface temperature was above 1000 K, the width of the nanostructure increased with the surface temperature. Three tungsten samples manufactured by different procedures (ultra fine grained tungsten, ITER-reference tungsten grade and powder metallurgy tungsten) were exposed to the helium plasma. The nanostructure was formed on all samples.

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