Prompt ignition of a unipolar arc on helium irradiated tungsten

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

doi:10.1088/0029-5515/49/3/032002

Cited by 123 publications

(108 citation statements)

References 26 publications

(32 reference statements)

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“…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. Through experiments with steadystate surface temperatures it has been well documented that W nano-tendrils begin to form at ~1000 K, and as surface temperature increases, the nano-tendrils' diameter increases until at ~2000 K the individual nano-tendrils become indistinct and the surface is populated with I-10 9 large pores [10]. For the transient surface temperature exposures on Pilot-PSI, a similar transition from fine to coarse nano-tendrils is seen as the maximum surface temperature rises, finally transitioning to large surface pores at the highest surface temperatures, but the thresholds are different.…”

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

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%

“…However, we do not have measurements of the heat flux received by the W probe during these disruptions. There is also no evidence of the uni-polar arcing that was seen on nano-tendril surfaces exposed in the LHD stellerator [9] and in NAGDIS-II [10].…”

Section: Alcator C-mod Exposuresmentioning

confidence: 87%

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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“…3 Recent studies have shown that transient heat loads on the divertor, such as Edge Localized Modes (ELMs), might trigger unipolar arcing, and that fuzz significantly increases its ignition. 4 The existence of the unipolar arc has been confirmed phenomenologically by erosion craters found in numerous fusion devices: JT-60U, LHD, 5 DIII-D, 6 and ASDEX Upgrade. 7,8 Moreover, since the arcing may take place in fusion experiments, special model experiments have been conducted for a systematic study in which arcing is initiated by plasmas generated by a high-frequency discharge, 9,10 a plasma gun, 11,12 and a laser beam.…”

Section: à3mentioning

confidence: 94%

“…7,8 Moreover, since the arcing may take place in fusion experiments, special model experiments have been conducted for a systematic study in which arcing is initiated by plasmas generated by a high-frequency discharge, 9,10 a plasma gun, 11,12 and a laser beam. 4,13,14 What is missing is an investigation into the properties of (unipolar) arc-induced plasma, which requires proper diagnostics. This research uses the transient heat load of a laserpulse to trigger unipolar arcing on fuzzy tungsten and has the aim to spectroscopically characterize the arc-induced plasma.…”

Section: à3mentioning

confidence: 99%

Spectroscopic characterization and imaging of laser- and unipolar arc-induced plasmas

Aussems

Nishijima

Brandt

et al. 2014

Journal of Applied Physics

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Characteristics of plasma properties in an ablative pulsed plasma thruster Phys. Plasmas 20, 033503 (2013) Tungsten plasmas induced by unipolar arcs were investigated using optical emission spectroscopy and imaging, and compared with laser-induced tungsten plasmas. The unipolar arcs were initiated in the linear-plasma simulator PISCES-A at UCSD under fusion relevant conditions. The electron temperature and density of the unipolar arc plasmas were in the range 0.5-0.7 eV and 0.7-2.0 Â 10 20 m À3, respectively, and increased with increasing negative bias voltage, but did not correlate with the surface temperature. In comparison, the electron temperature and density of the laser-induced plasmas were in the range 0.6-1.4 eV and 7 Â 10

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“…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.…”

Section: Deepening Of Floating Potential For Tungsten Target Plate Onmentioning

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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Prompt ignition of a unipolar arc on helium irradiated tungsten

Cited by 123 publications

References 26 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

Spectroscopic characterization and imaging of laser- and unipolar arc-induced plasmas

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

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