Impact of arcing on carbon and tungsten: from the observations in JT-60U, LHD and NAGDIS-II

Abstract

Arcing is a long-standing plasma-surface interaction issue, and the issue is currently being revived. This paper assesses the impact of arcing in fusion devices based on the observations in JT-60U, the Large Helical Device (LHD) and the linear divertor simulator NAGDIS-II. To investigate the first initiation process of arcing, field emission currents from several tungsten samples are measured. It is shown that the field emission current increases significantly after tungsten is exposed to helium plasmas. A postmortem analysis of JT-60U tiles reveals that arcing phenomena occurred on carbon baffle plates inside the vacuum vessel in JT-60U. From the observation of the arc trails recorded on the baffle plate, the amount of eroded materials is discussed. The arcing seems to occur frequently on the inner baffles rather than the outer baffles. From LHD, it is shown that the arcing can be initiated on nanostructured tungsten even without transient events. The erosion of tungsten by arcing will become an important issue in a fusion reactor, where helium fluence is significantly increased. From the experiments in NAGDIS-II, it is shown that arcing can be initiated even without transient heat load when the target voltage is low enough, e.g. −500 V. Frequent initiation of arcing annihilates the nanostructure growth due to helium plasma irradiation on the surface.

Observation of Arc Trails with Significant Damage due to Glow Discharge Wall Conditioning in the Large Helical Device

“…It should be noted that no other significant arc trails were observed on the surface made of SS in the FIDA diagnostic system and pre-installed materials including SS of first walls. There were some reports indicating the evidence that the LHD GDC initiated arcing on ECH and ECE mirrors [3,5]. However, the arcing damage observed in the present study shows a larger area than the previous reports during the brief GDC of 43 hours in total.…”

Spectroscopic characterization and imaging of laser- and unipolar arc-induced plasmas

“…3 Recent studies have shown that transient heat loads on the divertor, such as Edge Localized Modes (ELMs), might trigger unipolar arcing, and that fuzz significantly increases its ignition. 4 The existence of the unipolar arc has been confirmed phenomenologically by erosion craters found in numerous fusion devices: JT-60U, LHD, 5 DIII-D, 6 and ASDEX Upgrade. 7,8 Moreover, since the arcing may take place in fusion experiments, special model experiments have been conducted for a systematic study in which arcing is initiated by plasmas generated by a high-frequency discharge, 9,10 a plasma gun, 11,12 and a laser beam.…”

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

“…On the other hand, the presence of the fuzz raises the possibility of another issue, ignition of unipolar arcing. So far, the arcing in fusion‐relevant devices has been reported in: JT‐60U, LHD, ASDEX‐U, and DIII‐D . 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 .…”