Mechanism of unipolar arcs in tokamaks

Maeno, Masaki; Ohtsuka, H.; Yamamoto, Satoshi; Yamamoto, T.; Suzuki, Norio; Fujisawa, N.; Ogiwara, Norio

doi:10.1088/0029-5515/20/11/009

Cited by 22 publications

(8 citation statements)

References 10 publications

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“…In presence of strong magnetic field parallel to the surface the arcs are known to move in the retrograde direction B× J i.e. opposite to the direction of Lorenz force on net arc current [1]. This property makes it possible to reconstruct the local B-field direction at the time when arc tracks appeared.…”

Section: Post Mortem Tile Analysismentioning

confidence: 99%

“…Unipolar arcs are electric discharges between a material surface acting as the cathode and the plasma acting as the anode. Arcing is an important local surface erosion mechanism in controlled fusion devices [1]. Arcing is routinely observed in machines with graphite first wall but owing to the high sputtering and chemical erosion of C it usually is not the dominant mechanism [2].…”

Section: Introductionmentioning

confidence: 99%

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Tungsten erosion by unipolar arcing in DIII-D

Bykov¹,

Chrobak²,

Abrams³

et al. 2017

Phys. Scr.

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Unipolar arcing was an important mechanism of metal surface erosion during the recently conducted Metal Rings Campaign in DIII-D when two toroidally continuous tile rings with 5 cm wide W-coated TZM inserts were installed in graphite tiles in the lower divertor, one on the floor and one on the shelf. Most of the arc damage occurred on the shelf ring. The total amount of W removed by arcing from the affected ∼4% of the shelf ring area was estimated ∼0.8×10 21 at., about half of the total amount of W eroded and redeposited outside the inserts (1.8±0.9)×10 21 at. The rings were exposed for a total of ∼480 discharges, an equivalent of plasma time on W surfaces (with Ip >0.5 MA) ∼10 3 s. Arcing was monitored in situ with WI (400.9 nm) filtered camera and photomultipliers and showed that: (i) arcing only occurred during ELMs and disruptions, (ii) arcing rate was much lower on the floor than on the shelf ring, and (iii) arcing had a low cut off power flux density about 2 MW/m 2. About half of arc tracks had large 10 • pitch angle and probably were produced during disruptions. Such tracks were only found on the shelf. Moderate toroidal variation of the arc track density and W erosion with nearly n=1 pattern has been measured.

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Section: Post Mortem Tile Analysismentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Tungsten erosion by unipolar arcing in DIII-D

Bykov¹,

Chrobak²,

Abrams³

et al. 2017

Phys. Scr.

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“…If the local potential becomes too high, arcing to the probe can occur [51,52]. Fern-type arcs are commonly observed on the grounded probe shafts, but these are most likely associated with disruptive instabilities.…”

Section: Heat Flux Modelsmentioning

confidence: 99%

Edge-plasma properties of the UCLA tokamaks

Zweben

Taylor

1983

Nucl. Fusion

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Probe measurements of the edge plasmas of the Macrotor and Microtor tokamaks are described. Limiter scrape-off layer thicknesses as measured with Langmuir probes are for Macrotor λ≅7–10 cm and for Microtor λ ∼ 1 cm, both values being consistent with the Bohm diffusion rate. Heat deposition measured with thermocouples attached to small probes shows an anomalously high heat flux, particularly from the electron drift direction. It is argued that the anomalous particle diffusion is most likely associated with the large edge density fluctuations, while the anomalous heat flux is most likely due to a directed high-energy electron population.

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“…In this paper we explain the asymmetry of the different sides of the limiter and give the relevant calculations for the ISX device, in which this phenomenon was observed on the rod limiter entering the plasma from the shadow of an ordinary bar limiter [ 1 ]. It may be noted that in the case of arcing induced by runaway electrons, both sides of the limiter are equivalent [7].…”

Section: Dx)mentioning

confidence: 94%