Tungsten erosion by unipolar arcing in DIII-D

Abstract

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.

Dust and powder in fusion plasmas: recent developments in theory, modeling, and experiments

“…Both laboratory tests and numerical modelling indicate that surface tangential droplet ejection is favored (Siemroth et al 2019;Kaufmann et al 2017;. Multiple evidence of PFC damage by unipolar arcing in contemporary machines confirm the production of dust with characteristic sizes in the micrometer range Bykov et al 2017a;Rohde et al 2013Rohde et al , 2016. The experimental works provide estimates of the total mass loss in arcing processes which exhibits a strong dependence on the composition as well as the conditions of the eroded surfaces.…”

Inspection of W 7-X plasma-facing components after the operation phase OP1.2b: observations and first assessments

“…Material erosion due to arcing has been observed in fusion machines e.g. ASDEX Upgrade [10], DIII-D [11], JT60-U and LHD [12]. Such investigations were made after OP1.2b for the first time in W7-X.…”

Dynamics of a high-density plasma in a magnetic field

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