Ignition and Sustainment of Arcing on Nanostructured Tungsten Under Plasma Exposure

Laser-induced arc was initiated on a nanostructured “fuzz” tungsten cathode with simultaneous exposure to a stationary helium (He) plasma. Arc spots moved randomly and eroded almost all the surface area. The diameter of each arc spot was measured to be 5-10 μm. A linear relationship was observed in the arc current-voltage characteristic, which was affected by the presence of resistance through a He plasma column. The specific resistance of He plasmas was estimated from the current-voltage characteristic, which showed a good agreement with theoretical calculations. A greater potential gap should be maintained to keep a higher arc current, indicating a good sign for preventing a high-current arc in fusion devices. The condition of arc ignition was investigated in detail by changing the He plasma parameters and the sheath potential drop. The ignition and sustainment of arcs were very sensitive to both the sheath potential drop and the sheath electric field. Arcs ignited when the sheath potential drop was negatively deeper than -100 V and the sheath electric field was above 2 MV/m. The relatively low electric field threshold would result from the thermofield emission aided by heating of spots from adjacent former spots. This result indicates that the electron density and the electron temperature should be reduced in order to lower the potential drop and the sheath electric field in front of the plasma-facing material surface, which can prevent arcs from igniting in fusion devices.