Arcing on tungsten subjected to helium and transients: ignition conditions and erosion rates

Abstract

Arcing is a long standing plasma-surface interaction issue in nuclear fusion research, and the issue has been revived recently from new points of view. In this study, arcing under fusion relevant conditions is demonstrated in a linear plasma device using a pulsed laser to mimic the transient heat load. Ignition conditions of arcing were investigated systematically by changing the laser power, plasma conditions, surface nature and so on. Moreover, the cross-sectional view of arc trail was observed by transmission electron microscope, and the erosion of nanostructured tungsten due to arcing was estimated. It was found that more than half of the nanostructured tungsten still remained after an arc spot run on the nanostructure.

Ignition and erosion of materials by arcing in fusion‐relevant conditions

“…Arc ignition is harmful to fusion devices because it can be a cause of erosion of PFC and an impurity source to a core plasma . As the first laboratory experiment was conducted in 2009, much subsequent research has characterized several properties of the arcing, including ignition and duration conditions . Arc ignition was particularly evident on W‐fuzz surfaces when the target potential against the plasma potential was below −60 V, whereas arcs did not ignite when the target potential was higher than −60 V .…”

“…Therefore, an unknown factor may exist at the point of discrepancy between the minimum target potential and the potential fall required in Refs. .…”

“…Erosion by arcing originates from cathode electron emissions, thus erosion properties by arcing are evaluated using two units, erosion rate (mg/s) and erosion per charge (mg/C) . In addition to evaluating the ease of ignition on fuzzy surfaces, erosion per charge has been evaluated as ∼1 mg/C, which was 10 times greater than the typical net ion erosion for a vacuum arc . A recent study has revealed that erosion rates were enhanced by increasing the arc current .…”

“…[13] As the first laboratory experiment was conducted in 2009, [18] much subsequent research has characterized several properties of the arcing, including ignition and duration conditions. [19][20][21] Arc ignition was particularly evident on W-fuzz surfaces when the target potential against the plasma potential was below −60 V, whereas arcs did not ignite when the target potential was higher than −60 V. [19,20] This indicates that a fall of target potential against a plasma potential is a key factor for arc ignition. Previous research has shown that the minimum target potential required for arc ignition on W is −16 V [22] and that there is a discrepancy to the measured potential fall.…”

Observation of reduction of secondary electron emission from helium ion impact due to plasma-generated nanostructured tungsten fuzz

“…Also, metal fuzz is observed to have very high infra-red emissivity [15] and significantly (up to 10×) reduced sputtering yield from ion impact [16]. On the negative side, fuzzy metal under plasma exposure can be more susceptible to arcing than smooth metal under some conditions, possibly due to increased field emission from fuzz tips [17,18].…”