Sub-ms laser pulse irradiation on tungsten target damaged by exposure to helium plasma

Abstract

The effects of a transient heat load on tungsten damaged by helium plasma irradiation have been investigated using a ruby laser with long pulse duration in the divertor simulator NAGDIS-II (Takamura et al 2002 Plasma Sources Sci. Technol. 11 A42). The pulse width of the ruby laser was ∼0.6 ms, which is close to that of the expected heat load accompanied by type-I edge localized modes (ELMs) in ITER operation. Helium holes/bubbles, which were formed in the surface region of powder metallurgy tungsten due to the exposure to the helium plasma, disappeared after the laser pulse irradiation to the tungsten surface with sufficient pulse energy. The results indicated that the transient heat loads similar to those expected by ELMs will mitigate damages such as bubbles and holes produced by helium irradiation. When a vacuum plasma sprayed tungsten coating on graphite was exposed to the helium plasma, the surface was covered with arborescent nanostructured tungsten containing many helium bubbles inside the structure. Melting traces were found on the surface after the laser pulses irradiated the surface even though the pulse energy was lower than that for melting bulk tungsten. A numerical temperature calculation of the sample suggested that the effective thermal conductivity near the surface dramatically decreased by several orders of magnitude due to the formation of nanostructured tungsten.

Comparison of tungsten nano-tendrils grown in Alcator C-Mod and linear plasma devices

“…demonstrating that despite what is assumed to be a strongly reduced thermal conductivity through the fuzz layer [8], these nano-tendrils can survive and even grow in steady thermal heat fluxes of up to ~40 MW/m 2 . During the 14-discharge sequence there were three full current (900 kA) disruptions, including on the final shot of the discharge sequence, which had no obvious effect on the fuzz.…”

Deepening of Floating Potential for Tungsten Target Plate on the way to Nanostructure Formation

“…A completely different idea could explain the reduction of SEE as follows: tungsten nanostructure is composed of thin skin with many helium cavities [5] so that the tungsten skin could be too thin for the primary electrons to pass through it. However, the electron range for the energy of less than 100 eV is less than 1 nm [10,11] so that the above idea is not the case.…”

“…Helium defects are the concerns when employing the tungsten for the divertor target and/or the first wall since helium is the fusion product and would be contained by around 10% of scrape-off layer plasmas in fusion devices. Recently the nano fiber-form structures have been identified on a variety of tungsten surface irradiated by helium or helium/deuterium mixture plasmas [1,2].The surface characteristics of thus formed tungsten plate would change compared with the flat non-damaged surface, for example, the radiation emissivity [3], the sputtering yield [4], the heat conduction [5], the discharge property [6] and so on. In this report the secondary electron emission (SEE) property will be discussed in relation to the floating potential which is important with respect to the impurity releases through physical sputtering and plasma heat flux.…”

“…The surface characteristics of thus formed tungsten plate would change compared with the flat non-damaged surface, for example, the radiation emissivity [3], the sputtering yield [4], the heat conduction [5], the discharge property [6] and so on. In this report the secondary electron emission (SEE) property will be discussed in relation to the floating potential which is important with respect to the impurity releases through physical sputtering and plasma heat flux.…”

Formation Condition of Fiberform Nanostructured Tungsten by Helium Plasma Exposure

“…This is because thermal vacancies could be dominating trap sites for helium atoms when material temperature is high [3]. When tungsten is used as a material in fusion devices, the nanostructure may cause serious problems because it could lead to the decrease of the optical reflectivity [6] and the thermal conductivity [7]. Moreover, the nanostructure formation enhances the tungsten impurity release from the material in response to the transient heat load accompanied by ELMs (Edge Localized Modes) and disruptions in future fusion…”