Sputtering properties of tungsten ‘fuzzy’ surfaces

Nishijima, D.; Baldwin, M.J.; Doerner, R.; Yu, J. H.

doi:10.1016/j.jnucmat.2010.12.017

Cited by 164 publications

(134 citation statements)

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“…In the following we investigate two of these possible causes. The most obvious is the formation of a distorted surface layer, as shown in several experiments (see references [1][2][3][4][5][6][7][8][9][10][11]), which is investigated as a function of fluence and temperature in section 3.2.1. The second is the dependence of the sputtering yield on the crystal orientation.…”

Section: Investigation Of Possible Mechanismsmentioning

confidence: 99%

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Tungsten erosion under combined hydrogen/helium high heat flux loading

et al. 2014

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We investigated the erosion behaviour of tungsten under irradiation with a fusion reactorrelevant H/He mixture of 94%/6% at a power density of 10 MW/m². The investigated surface temperatures range up to 2000°C and the range of the applied fluence was up to 7×10 25 m -2 . The erosion yield we observe exceeds the value expected from physical sputtering data by a factor of 2. In addition we observe an Arrhenius-like increase of the erosion yield with temperature with an activation energy of 0.04 eV.

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Section: Investigation Of Possible Mechanismsmentioning

confidence: 99%

“…The resulting morphology after irradiation with a H/He mixture is distinctly different form the morphology after irradiation with the pure species. The question how this morphology influences the erosion behaviour of tungsten has been addressed in linear plasma devices and also in tokamak edge plasmas [6,7,8].…”

Section: Introductionmentioning

confidence: 99%

Tungsten erosion under combined hydrogen/helium high heat flux loading

et al. 2014

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“…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.…”

mentioning

confidence: 99%

“…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.…”

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confidence: 99%

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

Takamura

Miyamoto

Ohno

2010

Plasma and Fusion Research

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Deepening of floating potential has been observed on the tungsten target plate immersed in high-density helium plasma with hot electron component on the way to nanostructure formation. The physical mechanism is thought to be a reduction of secondary electron emission from such a complex nano fiber-form structure on the tungsten surface. Tungsten material is very important in terms of fusion reactors, especially plasma-facing component. 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.Nano fiber-form structured tungsten surfaces have been formed in a newly developed linear plasma generator AIT-PID [3,7] as shown in Fig. 1. In this device the high density helium plasmas have been produced with a hot electron component. The plasma density exceeds 1 × 10 18 m −3 and the bulk electron temperature (T c ) is about 4 eV, while the hot electron component with the fraction of roughly 8% has a temperature (T h ) of up to 40 eV. Such a two electron temperature plasma gives a deep floating potential ensuring a high ion impact energy which is important to form nanostructured tungsten surface. The floating potential is around −40 V with respect to the vacuum chamber, and the ion incident energy to the tungsten is 45 eV considering the plasma potential of around +5 V which is unchanged during the exposure. Such a high sheath voltage can be explained by the numerical analyauthor's e-mail: takamura@aitech.ac.jp sis on the floating condition that the electron flux balances with the ion one, as shown in Fig. 2, which indicates that the value of 45 V is a little smaller than the theoretical prediction assuming a complete Maxwellian distribution for both electrons. Figure 3 shows the time evolutions of the floating potential and the tungsten surface temperature observed with a radiation thermometer with a fixed emissivity of 0.43. The drop of surface temperature comes from an increase of radiation emissivity and associated target cooling assuming a constant plasma heat flux onto the target, which was discussed in [3]. The floating potential changes rapidly from −40.0 down to −48.5 V during the period of large change in surface temperature, meaning a certain correlation with nanostructure formation.

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“…The question of how this nano structure formation may influence the erosion of such tungsten surfaces has been raised and investigated by spectroscopic erosion yield measurements in linear plasma devices using Argon impurities [4] or intense laser pulses [5] as well as in a tokamak edge plasma by exposure of pretreated tungsten surfaces in the TEXTOR device [6].…”

Section: Introductionmentioning

confidence: 99%

Erosion behavior of actively cooled tungsten under H/He high heat flux load

Maier

Greuner

Balden

et al. 2013

Journal of Nuclear Materials

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We devised a method to measure erosion in the micrometer range on actively cooled tungsten samples. The method is based on applying micrometer-sized markers onto the side faces of the samples using a focused ion beam. With this method we measured the erosion of tungsten samples exposed to a hydrogen beam with 6% helium content. At 10.5 MW/m² the samples were exposed to particle fluences between 2•10 25 m -2 and 7•10 25 m -2 . Up to 630 pulses with a pulse duration of 30 s were employed. The actively cooled samples reach an equilibrium surface temperature which was varied from about 600°C to 2000°C. We find an amount of erosion which clearly exceeds the value computed from physical sputtering yields by roughly a factor of two. This is possibly correlated with the formation of a complex nanometer-sized morphology, which was observed for all temperatures.

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Sputtering properties of tungsten ‘fuzzy’ surfaces

Cited by 164 publications

References 11 publications

Tungsten erosion under combined hydrogen/helium high heat flux loading

Tungsten erosion under combined hydrogen/helium high heat flux loading

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

Erosion behavior of actively cooled tungsten under H/He high heat flux load

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