Tungsten erosion by unipolar arcing in DIII-D

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 × 10²¹ at., about half of the total amount of W eroded and redeposited outside the inserts (1.8 ± 0.9) × 10²¹ at. The rings were exposed for a total of ∼480 discharges, an equivalent of plasma time onWsurfaces (with I_p > 0.5 MA) ∼10³ 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.