Formation of Nanostructured Tungsten with Arborescent Shape due to Helium Plasma Irradiation

Takamura, S.; Ohno, Noriyasu; Nishijima, D.; Kajita, Shin

doi:10.1585/pfr.1.051

Cited by 397 publications

(265 citation statements)

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“…[1][2][3]). The main difference is in the comparison of the surface morphologies for the maximum surface temperatures in the transient experiments (Alcator C-Mod and Pilot-PSI) and the equivalent steady-state surface temperatures from other experiments.…”

Section: Comparison Of Alcator C-mod Results To Pisces and Pilot-psimentioning

confidence: 99%

“…The growth of tungsten (W) nano-tendrils (or "fuzz") has been well documented in a wide range of linear plasma devices [1][2][3]. The growth conditions used for W fuzz in these devices are elevated surface temperatures (T s = 1000-2000 K) of a bare tungsten substrate while implanting with helium ions at sufficient incident energies (E He+ > 20 eV).…”

Section: Introductionmentioning

confidence: 99%

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

Wright¹,

Brunner²,

Baldwin³

et al. 2013

Journal of Nuclear Materials

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Abstract:Growth of tungsten nano-tendrils ("fuzz") has been observed for the first time in the divertor region of a high-power density tokamak experiment. After 14 consecutive helium L-mode discharges in Alcator C-Mod, the tip of a tungsten Langmuir probe at the outer strike point was fully covered with a layer of nano-tendrils. The depth of the W fuzz layer (600 ± 150 nm) is consistent with an empirical growth formula from the PISCES experiment. Re-creating the C-Mod exposures as closely as possible in Pilot-PSI experiment can produce nearly-identical nano-tendril morphology and layer thickness at surface temperatures that agree with uncertainties with the C-Mod W probe temperature data. Helium concentrations in W fuzz layers are measured at 1-4 at.%, which is lower than expected for the observed sub-surface voids to be filled with several GPa of helium pressure. This possibly indicates that the void formation is not pressure driven.

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Section: Comparison Of Alcator C-mod Results To Pisces and Pilot-psimentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Wright¹,

Brunner²,

Baldwin³

et al. 2013

Journal of Nuclear Materials

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“…Takamura et al 2006;Kajita et al 2009;Zibrov et al 2017) resulting in formation of the blisters (figure 15, dust, 'fuzz', pinholes, protrusions, etc. (see e.g.…”

Section: Processes Related To Plasma-materials Interactionsmentioning

confidence: 99%

Physics of ultimate detachment of a tokamak divertor plasma

Krasheninnikov¹,

2017

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The basic physics of the processes playing the most important role in divertor plasma detachment is reviewed. The models used in the two-dimensional edge plasma transport codes that are widely used to address different issues of the edge plasma physics and to simulate the experimental data, as well as the numerical schemes and convergence issues, are described. The processes leading to ultimate divertor plasma detachment, the transition to and the stability of the detached regime, as well as the impact of the magnetic configuration and divertor geometry on detachment, are considered. A consistent, integral physical picture of ultimate detachment of a tokamak divertor plasma is developed.

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“…Shuichi TAKAMURA, Takanori MIYAMOTO and Noriyasu OHNO 1) Faculty of Engineering, Aichi Institute of Technology, Toyota 470-0392, Japan 1) Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan (Received 3 September 2010 / Accepted 12 September 2010)…”

Section: Deepening Of Floating Potential For Tungsten Target Plate Onmentioning

confidence: 99%

“…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%

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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Formation of Nanostructured Tungsten with Arborescent Shape due to Helium Plasma Irradiation

Cited by 397 publications

References 8 publications

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

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

Physics of ultimate detachment of a tokamak divertor plasma

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

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