Simulation experiments on unipolar arcs

Stampa, A.; Kruger, Hayes

doi:10.1088/0022-3727/16/11/016

Cited by 17 publications

(6 citation statements)

References 11 publications

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“…The DiMES surface has even higher than the floor ring arcing threshold at ∼3 MW/m 2 . This is probably because the DiMES sample did not experience disruptions and thus avoided surface contamination which could lead to increased arcing rates as in [18]. This argument is supported by the observation that in a series of discharges following disruptions the arcing rate increased on the floor ring to 0.4 cm −2 at q ⊥ =3.6 MW/m 2 and on the shelf to 0.3 cm −2 at q ⊥ =2.6 MW/m 2 , see Fig.…”

Section: In Situ Arc Detectionmentioning

confidence: 99%

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: In Situ Arc Detectionmentioning

confidence: 99%

Tungsten erosion by unipolar arcing in DIII-D

Bykov¹,

Chrobak²,

Abrams³

et al. 2017

Phys. Scr.

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“…The plate potential increases, approaching that of the plasma, to ensure that the excess electron current returns to the surface of the plate. A unipolar arc has been extensively studied in attempts at direct observation of this unique phenomenon [5][6][7]. The necessary condition for forming a unipolar arc, given by Robson and Thonemann, is presented as a solid curve for a helium plasma with tungsten plate (figure 1) [8], where A is the area of the plate exposed to the plasma, n e the plasma density and T e the electron temperature.…”

Section: Introductionmentioning

confidence: 99%

Motion of unipolar arc spots ignited on a nanostructured tungsten surface

Kajita

Takamura

Ohno

2011

Plasma Phys. Control. Fusion

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Unipolar arc is a unique phenomenon, which many people have been trying to investigate experimentally and theoretically. In this paper, we show that unipolar arcing is promptly initiated on tungsten in response to a pulsed heat load when a nanostructure is formed on the surface by exposure to a helium plasma. The behavior of the arc spot is investigated from the observation with a fast framing camera and from the arc trail recorded on the material, and comparisons are made between the observation and numerical modeling. It is revealed that the arc trail has a self-affine fractal nature under magnetized conditions, and it significantly alters with the magnetic field strength.

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“…Moreover, since the arcing takes place accidentally in fusion experiments, special model experiments have been conducted for a systematic study, using plasmas generated by a high-frequency discharge [8,9], a laser beam [10] and a plasma gun [11]. However, the observation of a comprehensive unipolar arc, using a single electrode set in a stationary plasma, has never been reported in laboratory experiments, probably because the ignition of arcing requires a totally different condition from that for sustaining stationary arcing which was shown by R&T. In fact, in [9], it is reported that arcing on a clean surface could be completely suppressed even when the product An e T 1/2 e , where A is the electrode area and n e and T e are the electron density and temperature, respectively, was 100 times larger than R&T's threshold value, which corresponds to the necessary condition for stationary arcing, obtained from the following relation [2]:…”

Section: Introductionmentioning

confidence: 99%

Prompt ignition of a unipolar arc on helium irradiated tungsten

2009

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A fibreform nanostructured layer is formed on a tungsten surface by helium plasma bombardment. The helium fluence was of the order of 1026 m−2, and the surface temperature and incident ion energy during helium irradiation were, respectively, 1900 K and 75 eV. By irradiating a laser pulse to the surface in the plasma, a unipolar arc, which many people have tried to verify in well-defined experiments, is promptly initiated and continued for a much longer time than the laser pulse width. The laser pulse width (∼0.6 ms) and power (∼5 MJ m−2) are similar to the heat load accompanied by type-I edge localized modes (ELMs) in ITER. The unipolar arc is verified from an increase in the floating potential, a moving arc spot detected by a fast camera and arcing traces on the surface. This result suggests that the nanostructure on the tungsten surface formed by the bombardment of helium, which is a fusion product, could significantly change the ignition property of arcing, and ELMs become a trigger of unipolar arcing, which would be a great impurity source in fusion devices.

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Simulation experiments on unipolar arcs

Cited by 17 publications

References 11 publications

Tungsten erosion by unipolar arcing in DIII-D

Tungsten erosion by unipolar arcing in DIII-D

Motion of unipolar arc spots ignited on a nanostructured tungsten surface

Prompt ignition of a unipolar arc on helium irradiated tungsten

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